Condensed Matter Seminar, Duke University

Regular time and place: Thursdays at 11:30am in Physics 298.
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Upcoming events

2025/04/29

Weijian Chen  (NC State University),   Unusual weekday: Tuesday! "Exceptional points in open quantum system dynamics" Open systems with loss or gain, described by effective non-Hermitian Hamiltonians, have attracted significant attention in the last two decades. Such systems generally possess complex eigenvalues and nonorthogonal eigenstates. Their degeneracies are known as exceptional points (EPs), where both the eigenvalues and the corresponding eigenstates are degenerate. This peculiar behavior has led to many nonintuitive phenomena and applications in classical platforms, such as EP sensors and chiral state transfer via encircling an EP. Recently, there has been growing interest in harnessing non-Hermiticities and EPs for quantum applications. Open quantum systems provide a natural platform to explore the relevant physics. In this talk, I will first discuss the realizations of EPs in a circuit QED setup with dissipation engineering. Then I will discuss the experiments on chiral state transfer via dynamically encircling an EP and the approach to mitigate decoherence effect. Finally, I will discuss EPs in composite quantum systems, which offer a novel method for forming higher-order EPs. Our theory shows EP-induced disentanglement and accelerated entanglement generation in proximity to EPs. Host: Gleb Finkelstein

Past events

2024/11/07		Valentina Martelli  (University of Sao Paulo) "Exploring thermal transport in complex oxides" Heat transport investigations can reveal information about phonon dynamics and collective transport regimes in an insulator. In this talk, I will discuss the insights and implications of the features observed in the thermal transport of two representative perovskite-type complex oxides, SrTiO3 and BaBiO3. The first compound has gathered fundamental research efforts across different topics, such as superconductivity, thermoelectricity, and ferroelectricity [1-2]. Thermal transport in single crystalline SrTiO3 revealed the presence of the Poiseuille flow of phonons at low temperatures and a high-temperature diffusivity approaching the Planckian limit [3]. The findings pointed to the role of the lattice degrees of freedom and specific phonon modes in setting and manipulating the heat conduction profile. The second one, BaBiO3, is the parent compound of the first high-Tc superconductor without transition metal ions. Its undoped insulating phase has an origin yet to be understood. Our very recent investigations of thermal conductivity in high-quality single crystals of BaBiO3 revealed an efficient suppression of heat transfer at high temperatures and a glass-like behaviour at low temperatures, suggesting an exceptionally high anharmonicity which could be at the base of phonon-mediated emerging phenomena [4]. [1]  X. Lin, Z. Zhu, B. Fauque, K. Behnia, PRX, 3, 021002 (2013) [2]  D. Bäuerle, D. Wagner, M. Wöhlecke, B. Dorner, H. Kraxenberger, Z. Phys. B. Condens. Matter 38, 335 (1980) [3]  V. Martelli, J. Larrea Jimenez, M. Continentino, E. Baggio-Saitovitch, and K. Behnia, PRL 120, 125901 (2018) [4]  Henriques et al. unpublished. Host: Divine Kumah
2024/10/17		John Goold  (Trinity College Dublin),   Unusual time & place: 2:00pm, 324 Gross Hall! "The thermodynamics of the quantum Mpemba effect" We investigate the quantum Mpemba effect from the perspective of nonequilibrium quantum thermodynamics by studying relaxation dynamics of quantum systems coupled to a Markovian heat bath, which are described by Davies maps. Starting from a state with coherences in the energy eigenbasis, we demonstrate that an exponential speedup to equilibrium will always occur if the state is transformed to a diagonal state in the energy eigenbasis, provided that the spectral gap of the generator is defined by a complex eigenvalue. When the transformed state has a higher nonequilibrium free energy, we argue using thermodynamic reasoning that this is a genuine quantum Mpemba effect. Furthermore, we show how a unitary transformation on an initial state can always be constructed to yield the effect and demonstrate our findings by studying the dynamics of both the nonequilibrium free energy and the irreversible entropy production in single and multiqubit examples. I will also discuss recent results in information geometry which explores the effect from a geometrical standpoint. [1]  M. Moroder, O. Culhane, K. Zawadzki, J. Goold, PRL 133, 140404 (2024) [2]  L. P. Bettmann, J. Goold, arXiv:2409.06083 (2024) Host: Iman Marvian
2024/10/17		Jeffrey G. Rau  (University of Windsor) "Altermagnetism: A symmetry-based perspective" Altermagnets, a new class of magnetic systems that combine characteristics of both conventional ferromagnets and antiferromagnets, have recently attracted significant attention. These collinear compensated magnets exhibit spin-split bands, even in the absence of spin-orbit coupling, offering exciting prospects for applications in antiferromagnetic spintronics due to their lack of stray fields, low damping and fast switching times. In this talk, I will introduce altermagnets from a symmetry-based perspective using ideas from Landau theory. Starting from the nonrelativistic limit, this Landau theory goes beyond a conventional magnetic analysis by including spin-space symmetries, providing a simple framework for understanding the key features of this family of materials. We find a set of multipolar secondary order parameters connecting existing ideas about the spin symmetries of these systems, their order parameters, and the effect of nonzero spin-orbit coupling. We account for several features of candidate altermagnets such as MnF2, MnTe, and CuF2 that go beyond symmetry alone, relating the order parameter to key observables such as magnetization, anomalous Hall conductivity, and magnetoelastic and magneto-optical probes. Finally, I will discuss how our framework can be generalized to a broader class of magnetic systems in the zero spin-orbit coupling limit, as well as potentially to noncolinear magnetic structures. Host: Sara Haravifard
2024/03/28		Marc Bockrath  (Ohio State University) "Correlated electrons in twisted bilayer graphene: superconductivity to fractional-filling insulating states" The group velocity vF of the electrons in a flat band superconductor is extremely slow, resulting in quenched kinetic energy. Superconductivity thus appears impossible, as conventional BCS theory implies a vanishing superfluid stiffness, coherence length, and critical current. Using twisted bilayer graphene (tBLG), we explore the profound effect very small vF in a superconducting Dirac flat band system. Non-linear transport studies, we measure vF via the Schwinger effect, yielding an extremely slow vF~1000m/s for filling fraction between -1/2 and -3/4 of the Moire superlattice. This same velocity yields a new limiting mechanism for the superconducting critical current, with analogies to a relativistic superfluid. We estimate the superfluid stiffness, which determines the electrodynamic response of the superconductor, showing that it is not dominated by the kinetic energy, but by the interaction-driven superconducting gap, consistent with recent theories on quantum geometric contributions. We study the BCS to Bose-Einstein condensation (BEC) crossover via coherence length measurements, finding an unprecedented ratio of the superconducting transition temperature to the Fermi temperature exceeding unity, illustrating how this can arise for very strong coupling superconductivity in ultra-flat Dirac bands. Moreover, a canonical example of geometric frustration is a triangular lattice with antiferromagnetic nearest neighbor spin coupling, which can lead to phases such as spin ices and spin liquids. Much less studied is geometric frustration based on orbital coupling. In our recent work, we have shown that incompressible states form at 1/3 fractional filling factors in twisted bilayer graphene at angles larger than the magic one that are strongly dominant over integer fillings. These results are in agreement with a strong-coupling theory based on Coulomb interactions between electrons occupying three-lobed Wannier orbitals, leading to symmetry-broken phases with distinct charge, spin, and valley order. Host: Gleb Finkelstein
2024/02/22		Steve Winter  (Wake Forest University) "Hamiltonians for linear and nonlinear responses in quantum magnets" Quantum materials represent a broad class of systems whose experimental response relies on uniquely quantum aspects such as entanglement, Berry phases, and electronic correlations. Modeling of such materials presents challenges related to a variety complex behaviours that manifest at different energy scales. In this field, first-principles approaches often provide a vital bridge between experiments and theoretical models. In this talk, I will introduce our numerical strategies for systematically building low-energy models with local charge, spin, and orbital degrees of freedom of arbitrary complexity. I will discuss the insights that these methods have yielded for layered vdW materials, in which spin-orbit coupling induces strongly anisotropic and competing magnetic interactions. I will also discuss preliminary work on extending these methods to treat (i) spin-lattice coupling, and (ii) dynamical effective Hamiltonians for modeling non-linear responses in quantum materials, and the breadth of opportunities for non-linear spectroscopy. Host: Sara Haravifard
2024/02/13		Shailesh Chandrasekharan  (Duke University),   Unusual time: Tuesday, 3:30pm! "Asymptotic freedom via qubit regularization: a new RG perspective" Asymptotically free quantum field theories (AFQFT) emerge non-perturbatively in the continuum despite the infinities that are plagued in perturbative calculations. Wilson's RG provides a framework to understand this feature of AFQFT starting from a lattice regularization, which however begins with the assumption that the local Hilbert space on the lattice is infinite dimensional so that the UV fixed point is preserved. However, recent effort in formulating these theories on a quantum computer forces one to look for lattice theories with a finite local Hilbert space. We refer to this as the "qubit regularization" of a QFT. Does the finite local Hilbert space necessarily destroy the UV fixed point, or can it be recovered via RG? Recent results suggest that the latter is true at least in some cases, where the AFQFT arises at a quantum critical point in a theory with a finite local Hilbert space. However, the RG flow is quite exotic in these cases. We will use the well-known BKT transition as an example to show this non-trivial RG flow. [1]  S. Maiti, D. Banerjee, S. Chandrasekharan, M.K. Marinkovic, PRL 132, 041601 (2024)
2024/01/22		Alexander M. Finkel'stein  (Weizmann Institute, Israel, and Texas A&M University),   Unusual time: Monday, 10:30am! "Spin-wave tunable qubit controlled by AC magnonic crystal" Wave computations rely on superposition but don't require entanglement. To utilize spin waves for fast computing and communications, one needs a method of controlling the spin waves. (For the wave computations, there should be many waves involved.) A feasible way to manipulate spin-wave propagation is through a magnonic crystal. A magnonic crystal is a periodic spatial modulation, which is analogues to the Bragg mirror in optics. We extend the idea to an AC-magnonic crystal, and show how it could be used for generating and controlling many tunable "qubits" formed by pairs of mutually scattering spin waves. One can also utilize an AC magnonic crystal for the manipulation of the "qubits" via single-qubit gates of different kinds, thereby, opening new possibilities in spin-wave computing. Host: Harold Baranger
2023/11/09		Sergei Urazhdin  (Emory University) "Orbital liquid in ultrathin magnetic films" The electron's orbital moment has recently emerged as an important degree of freedom which can be generated, transported, and used to control magnetic systems [1]. However, its role in magnetism remains poorly understood. I will discuss surprising experimental observations revealing a crucial role of the electron's orbital moment in ferromagnetism, and elucidating a previously unrecognized connection between magnetism and unconventional superconductivity. Our magnetoelectronic measurements of heterostructures based on ultrathin transition metal ferromagnets revealed two separate magnetic order parameters: one associated with spin ordering at the Curie point Tc1, and another "anomalous" order parameter with a critical point Tc2 about 50K above the Curie temperature. Remarkably, magneto-optical measurements are not sensitive to the anomalous contribution, suggesting that the origin of the latter is qualitatively different from spin magnetism. X-ray magnetic circular dichroism (XMCD) measurements show that the "anomalous" order parameter is associated with incipient orbital ferromagnetism whose signatures vanish below Tc2 without the onset of ferromagnetic orbital ordering. Electric current applied to micropatterned structures in this regime reveals that orbital magnetism is "hidden" but does not disappear below Tc2. I will show that these anomalous behaviors are captured by a simple Hubbard model of orbital correlations among nearest neighbor sites in an ultrathin ferromagnetic layer, leading to the conclusion that orbital moments form an orbital liquid [2] - a long-range correlated orbital state that lacks ordering due to the geometric orbital frustration, analogous to quantum spin liquids formed by frustrated spins and believed to hold the key to high-temperature superconductivity [3]. In the studied orbital liquid, orbital moments are ferromagnetically coupled, which would be impossible for spin liquid due to spin conservation. I will discuss the implications of these results for our understanding of the mechanisms of magnetism and for the emerging field of orbitronics. [1]  D. Go, D. Jo, H.W. Lee, M. Kläui, Y. Mokrousov, EPL 135, 37001 (2021) [2]  S. Ivanov, J. Peacock, S. Urazhdin, Phys. Rev. Mater. 7, 01440 (2023) [3]  P.W. Anderson, Science 235, 1196 (1987) Host: Alexander Kozhanov
2023/11/02		Kemp Plumb  (Brown University) "Order from-and-by-disorder: spin and orbital dynamics on the highly frustrated FCC lattice" Abstract: Magnetism in transition metal material derives from the collective action of both the electronic spin and orbital angular momentum. When these degrees of freedom are entangled through a relativistic spin-orbit-coupling, quantum fluctuations can be enhanced, engendering diverse quantum phases of matter. The most celebrated example is the Kitaev quantum spin liquid for j=1/2 Kramers doublets on the honeycomb lattice. However, spin-orbital fluctuations can lead to many other possibilities, including multi-polar order, and spin orbitals liquids. In this talk, I will discuss specific examples of spin and orbital dynamics of two model face centered cubic antiferromagnets: GaTa4Se8, that realizes an orbitally active j=3/2 model; and K2IrCl6 that realizes a j=1/2 antiferromagnet. GaTa4Se8 is a cluster Mott insulating lacunar spinel, where electrons occupy molecular orbitals that retain spin and orbital angular momentum. I will discuss neutron scattering measurements on this material that reveal a dynamical spin-orbital state preceding an order-disorder type spin-orbital ordering transition. Spin-orbit coupling overcomes the Jahn-Teller mechanism in GaTa4Se8 and results in a novel spin-orbital valence bond ground state. K2IrCl6 is a highly frustrated antiferromagnet with dominant Heisenberg and Kitaev interactions. I will present inelastic neutron scattering measurements on this compound that show a striking dichotomy of the static and dynamic correlations. Static correlations resemble a classically ordered state, but the dynamic correlations reveal a dominance of quantum fluctuations and a quantum order-by-disorder mechanism. These results showcase the power of neutron scattering to reveal spin and orbital dynamics in quantum materials. Host: Sara Haravifard
2023/10/05		Ankit Disa  (Cornell University) "Designing non-equilibrium functionalities in quantum materials using light" Quantum materials are broadly distinguished by their unique macroscopic phases, which emerge as a result of electronic interactions across many length scales. These emergent phases lead to functionalities with enormous technological potential, such as high-temperature superconductivity and multiferroicity, but devising ways to manipulate their quantum behavior "on demand" for practical applications remains a major challenge. In this talk, I will describe a methodology to engineer materials properties dynamically with ultrashort light pulses, which results in non-equilibrium states that often cannot be achieved otherwise. We use intense, tailored, THz-frequency excitation to drive ions in the crystal lattice to large amplitudes and exploit nonlinearities to steer the various degrees of freedom of the system. This approach provides a knob to tune electronic and magnetic interactions, break symmetries, and unlock new phases. I will highlight some recent experiments demonstrating the ability to optically control, enhance, and induce magnetism and ferroelectricity in complex oxides. Interestingly, the ultrafast dynamics of these driven systems gives way, in some cases, to metastable light-induced states persisting for "ultralong" times, which form even well above the equilibrium ordering temperature. Finally, I will touch upon our current efforts of integrating atomic layer materials synthesis to enable the rational design of non-equilibrium functionalities for next-generation quantum and ultrafast technologies. Host: Divine Kumah
2023/09/07		John McGreevy  (UC San Diego),   Unusual place: Zoom! "Various Dimensions of Entanglement Bootstrap" The basic principle underlying the Entanglement Bootstrap program is that all universal data about a state of matter is encoded in a local region of a single representative wavefunction. I'll introduce the application of this principle to extract the universal data from liquid topological states in 2+1 and 3+1 dimensions. Then I'll explain how it can be extended to study conformal field theories in 1+1d, and topological states with gapless boundary in 2+1 dimensions. [1]  J.L. Huang, J. McGreevy, B. Shi, arXiv:2112.08398 (2021) [2]  B. Shi, J.L. Huang, J. McGreevy, arXiv:2301.07119 (2023) [3]  T.C. Lin, J. McGreevy, arXiv:2303.05444 (2023)
2023/08/24		Jukka Vayrynen  (Purdue) "Extrinsic and intrinsic superconducting diode effects" The critical current of a superconducting wire can be non-reciprocal, i.e., dependent on current direction, when inversion and time-reversal symmetry are broken. This so-called superconducting diode effect has gained renewed attention in recent years, due to the possibility that the effect may arise from the interplay of spin-orbit coupling and Zeeman effect in a uniform superconductor. I will discuss the superconducting diode effect originating from two different mechanisms, studied in recent preprints [1-2]. The first one is an extrinsic one arising from the geometry of the setup rather than intrinsic properties. It is however relevant for uniform superconducting heterostructures due to the formation of interfacial diamagnetic currents and Josephson vortices [1]. In the second part of the talk, I will discuss an intrinsic mechanism that is relevant for generic quasi-one-dimensional superconducting system where the critical current is determined by Cooper pair depairing. By introducing a minimal model, we find the key ingredients to obtain intrinsic superconducting diode effect. The model can be microscopically derived as a low-energy limit of a Rashba spin-orbit coupled superconductor in a Zeeman field. The results quantify how system parameters such as spin-orbit coupling and quantum confinement affect the strength of the diode effect and provide a complementary description to previous Ginzburg-Landau theories of the effect. [1]  A. Sundaresh, J.L. Vayrynen, Y. Lyanda-Geller, L.P. Rokhinson, arXiv:2207.03633 (2022) [2]  T. de Picoli, Z. Blood, Y. Lyanda-Geller, J.L. Vayrynen, arXiv:2302.04277 (2023) Host: Gleb Finkelstein
2023/06/09		Ethan Arnault  (MIT),   Unusual time: Friday, 1:15pm! "Single-photon detection using a graphene-Josephson junction calorimeter" Single photon detectors (SPDs) are an enabling technology for quantum networks and quantum state readout. However, current implementations of SPDs often operate in narrow bandwidths, are limited in the tradeoff between the quantum efficiency and dark count, and many lack the ability to resolve photon number. The orthogonal approach of single-photon calorimetry based on graphene material properties can serve as an interesting option because it has 1) no band gap allowing for light absorption from optical down to microwave frequencies, 2) a vanishing density of states providing a low electronic heat capacity and 3) short electron-electron interactions (~100 fs). This makes graphene an ideal candidate for single photon calorimetry, where an incident photon is absorbed by the graphene and sensed through the temperature rise of the electrons. In this talk, I will discuss our graphene-Josephson junction single-photon detector, which operates by measuring the heat deposited by a single photon. We are able to resolve single infrared (1550 nm) photons with a maximum quantum efficiency of ~90% and a dark count rate on the order of an hour. We can detect single-photons up to 1.2 K, paving the way to operate at liquid Helium-4 temperatures after further optimization. We further explore the heat propagation in our graphene. Using a milliKelvin optical scanner, we spatially resolve the heat propagation due to an incident single photon and find no substantial variation in switching rate over ~25 um, indicating that the thermal length scale is substantially larger. Further, by varying backgate, the efficiency of the detector wanes as the device is tuned away from the Dirac peak, confirming that we are measuring the electronic temperature rise due to a single photon. Host: Gleb Finkelstein
2022/11/03		Journal club ft. Yikang Zhang,   Unusual time: 12:20pm! "Introduction to renormalization group II: Wilson-Fisher fixed point and application to quantum systems" [1]  E. Fradkin, Field theories of condensed matter physics [2]  A. Altland and Ben D. Simons, Condensed matter field theory [3]  S. Sachdev, Quantum phase transitions
2022/10/27		Journal club ft. Yikang Zhang "Introduction to renormalization group I: dimensional analysis" [1]  E. Fradkin, Field theories of condensed matter physics [2]  J. Cardy, Scaling and renormalization in statistical physics [3]  A. Altland and Ben D. Simons, Condensed matter field theory [4]  S. Sachdev, Quantum phase transitions
2022/09/22		Alexander Khitun  (UC Riverside) "Quantum computing without quantum computers: Database search and data processing using classical wave superposition" Quantum computing is an emerging field of science which will lead us to new and powerful logic devices with capabilities far beyond the limits of current transistor-based technology. There are certain types of problems which quantum computers can solve fundamentally faster than the tradition digital computers. For example, Peter Shor developed a quantum algorithm which solves the factoring and discrete logarithm problems in time O(n^3), compared with the exponential time required for the best known classical algorithm. Quantum computers can search an "unsorted database" in time O(sqrt(N)), compared with the O(n) time that would be required classically. Superposition of states and entanglement are the key two ingredients which make quantum computing so powerful. Superposition of states allows us to speedup database search by checking a number of bits in parallel, while quantum entanglement is critically important for quantum cryptography. There are quantum algorithms which require both superposition and entanglement (e.g., Shor's algorithm). But neither the Grover algorithm nor the very first quantum algorithm due to Deutsch and Jozsa needs entanglement. Is it possible to utilize classical wave superposition to speedup database search? This interesting question was analyzed by S. Lloyd. It was concluded that classical devices that rely on wave interference may provide the same speedup over classical digital devices as quantum devices. There were several experimental works using optical beam superposition for emulating Grover's algorithm. It was concluded that the use of classical wave superposition comes with the cost of the exponential increase of the resources. Since then, it is widely believed that the use of classical wave superposition for quantum algorithms is inevitably leading to an exponential resources overhead (e.g., number of devices, precision, and/or power consumption). In this talk, we will describe a classical Oracle machine which utilizes classical wave superposition. We argue that the classical wave-based approach provides the same speedup in database search as quantum computers. We also present experimental data on database search through a magnetic database using spin wave superposition. [1]  M. Balynsky et al., J. Appl. Phys. 130, 164903 (2021) Host: Gleb Finkelstein
2022/08/25		Anatoly Dymarsky  (University of Kentucky) "Eigenstate thermalization hypothesis in 2d conformal field theories" I will discuss the eigenstate thermalization hypothesis (ETH) in conformal field theories, with an emphasis on the 2d case. In this case, infinite-dimensional Virasoro symmetry gives rise to integrable quantum Korteweg-de Vries (KdV) structure in the stress-energy sector. Taking this structure into account leads to a generalized ETH formulation matching an individual eigenstate to the KdV generalized Gibbs ensemble ensemble. Host: Thomas Barthel and Berndt Mueller
2022/05/05		Masud Haque  (TU Dresden) "Assigning temperatures to the eigenstates of isolated many-body systems" Understanding the statistical mechanics of isolated quantum systems is a topic of current interest. In this endeavor, an inescapable issue is the definition of temperature, which is not a priori defined within closed-system quantum mechanics. After motivating the general field and setup, I will examine and compare different possible ways of assigning temperatures to energies, or equivalently, to eigenstates. Host: Thomas Barthel
2021/09/30		Qiang Miao  (Duke University),   Unusual time: 12:01pm! "A quantum-classical eigensolver using multiscale entanglement renormalization" We propose a variational quantum eigensolver (VQE) for the simulation of strongly-correlated quantum matter based on a multi-scale entanglement renormalization ansatz (MERA) and gradient-based optimization. This MERA quantum eigensolver has substantially lower computation costs than corresponding classical algorithms. Due to its narrow causal cone, the algorithm can be implemented on noisy intermediate-scale (NISQ) devices and still describe very large systems. It is particularly attractive for ion-trap devices with ion-shuttling capabilities. While the total number of required qubits grows logarithmically in the size of the simulated system, the number of qubits needed in the interaction region is system-size independent. Translation invariance of the simulated systems can be used to make computation costs square-logarithmic in the system size and describe the thermodynamic limit. The approach is analyzed numerically for a MERA with Trotterized disentanglers and isometries. With a few Trotter steps, one recovers the accuracy of the full MERA. [1]  Q. Miao and T. Barthel, arXiv:2108.13401 (2021)
2021/05/06		Alannah Hallas  (University of British Columbia) "Chemically tuning the exotic ground states of pyrochlore magnets" Pyrochlore lattices, which are found in two important classes of materials -- the A2B2X7 pyrochlore family and the AB2X4 spinel family -- are the quintessential 3-dimensional frustrated lattice architecture. Pyrochlore magnets are renowned for their exotic magnetic ground states, ranging from classical spin ice to quantum spin liquid. While historically rare-earth titanium oxides (B = Ti, X = O) have played the starring role in this field, the past decade has seen material's synthesis breakthroughs that have lead to new families of oxide pyrochlores (B = Ge, Pt) as well as the emergence of fluoride (X = F) and chalcogenide (X = S, Se) pyrochlore lattice materials. In this talk I will describe how chemical substitutions can modify the single ion spin anisotropy due to crystal electric field effects, stabilize new magnetic atom combinations, or dramatically alter the exchange pathways and thereby lead to new magnetic ground states. Host: Sara Haravifard
2021/04/29		Jad C. Halimeh  (University of Trento) "Staircase prethermalization and constrained dynamics in lattice gauge theories" The dynamics of lattice gauge theories is characterized by an abundance of local symmetry constraints. Although errors that break gauge symmetry appear naturally in NISQ-era quantum simulators, their influence on the gauge-theory dynamics is insufficiently investigated. In this talk, we show that a small gauge breaking of strength g induces a staircase of long-lived prethermal plateaus. The number of prethermal plateaus increases with the number of matter fields L, with the last plateau being reached at a timescale g^(-L/2), showing an intimate relation of the concomitant slowing down of dynamics with the number of local gauge constraints. By means of a Magnus expansion, we demonstrate how exact resonances between different gauge-invariant supersectors are the main reason behind the emergence of staircase prethermalization. Our results bode well for NISQ quantum devices, as they indicate that the proliferation timescale of gauge-invariance violation is counterintuitively delayed exponentially in system size. From a phenomenological perspective, our work shows how prethermal behavior is significantly enriched in models with slight breaking of local gauge invariance relative to their counterparts where a global symmetry is broken. Host: Thomas Barthel
2021/02/11		Divine Kumah  (NC State University) "Atomic-scale control of emergent phases at complex oxide interfaces" Complex oxides exhibit a wide range of exciting physical properties including high temperature superconductivity, metal-insulating transitions, and tunable magnetic and electronic phases. The ability to reduce the dimension of these systems in thin films allows for the stabilization of novel electronic and magnetic ground states. The manipulation of these emergent properties is of great interest due to potential applications ranging from spintronics and orbitronics to photonics. We demonstrate the engineering of orbital and spin degrees of freedom at the interfaces between atomically thin layers of rare-earth manganite and chromate films synthesized by molecular beam epitaxy and report dynamic tuning of magnetism in multiferroic PbZrTiO3/LaSrCrO3/LaSrMnO3 heterostructures via external electric fields. Additionally, we report the gate and temperature modulation of a high spin-to-charge conversion efficiency in the high mobility two-dimensional electron gas formed at the interface between the polar antiferromagnetic LaCrO3 and insulating SrTiO3. These results demonstrate the control of interactions at interfaces in quantum oxide heterostructures and illustrate pathways for harnessing their unique functional properties in next-generation devices for energy, computing and information technologies. Host: Thomas Barthel, Gelb Finkelstein
2021/02/04		Vincenzo Alba  (Universiteit van Amsterdam) "Hydrodynamic framework for out-of-equilibrium entangled many-body systems" Entanglement and entropy are key concepts standing at the foundations of quantum and statistical mechanics, respectively. In the last decade the study of quantum quenches revealed that these two concepts are intricately intertwined. For integrable models, novel hydrodynamic approaches based on a quasiparticle picture emerged as a new platform allowing for a quantitative understanding of quantum information dynamics in quantum many-body systems. Remarkably, this gives fresh insights on how thermodynamics emerges in isolated out-of-equilibrium quantum systems. I will start by reviewing this new unifying framework. I will then discuss several applications to entanglement-related quantities, such as entanglement entropies, mutual information, logarithmic negativity. I will also show how the framework allows to study the interplay between quantum information dynamics and transport of local conserved quantities. Finally, I will derive some simple bounds on the quantum information scrambling in out-of-equilibrium systems. Host: Thomas Barthel
2021/01/14		Bella Lake  (Helmholtz Zentrum and TU Berlin) "First exploration of novel Bethe string states in the model magnetic material SrCo2V2O8" Complex bound states of magnetic excitations, known as Bethe strings, were predicted almost a century ago to exist in one-dimensional quantum magnets. The string states have so far remained the subject of intense theoretical studies but without experimental verification. Here, by performing neutron scattering experiments on the one-dimensional Heisenberg-Ising antiferromagnet SrCo2V2O8 in high longitudinal magnetic fields, we reveal and explore the string states and find excellent agreement with Bethe-ansatz calculations. Host: Thomas Barthel
2020/12/17		Igor Bondarev  (North Carolina Central University) "Strongly correlated collective excitations in planar transdimensional nanostructures" I will briefly review the latest experiments [1-7] and then enlarge on our theoretical efforts to unravel the properties of hybrid quasi-2D semiconductor and metallic nanostructures, efforts that have uncovered their intriguing optical attributes lending themselves to new device applications. For instance, we show that charged and neutral exciton complexes (trion, biexciton, quaternion) in highly excited van der Waals bound transition metal dichalcogenide (TMD) heterostructures can have substantial (up to a few tens of meV) binding energies [8,9], and can be controlled externally to form strongly-correlated long-range collective states representing the crystal phases of the excited system [9]. Exciton complexes in TMD systems are of interest for nonlinear optics and spinoptronics applications [8-10], and offer a new unconventional high-T superconductivity mechanism based on a charged Bose-Einstein condensate of quaternions [2]. We also develop a theory of exciton intermixing and polarization dynamics for quasi-2D crystalline semiconductors of organic molecules with two isolated low-lying Frenkel exciton states, such as transition metal phthalocyanines [6,11]. The third-order nonlinear polarization response function we derive exhibits the dynamical reorientation of molecular exciton polarization, pointing out the charge separation direction in organic molecular chains. Our results can be used for the interpretation of the optical properties of crystalline transition metal phthalocyanines - next generation organic semiconductors for advanced optoelectronics. Last but not least, we study theoretically the confinement related effects in the optical response of isotropic and anisotropic finite-thickness plasmonic films in the transdimensional regime (between 3D and 2D [12,3-5]). We show [13,14] that, while being constant for relatively thick films, the plasma frequency acquires spatial dispersion typical of 2D materials and gradually shifts to the red with the thickness reduction. This explains the experiments done on aligned carbon nanotube films [3] and on TiN films [4,5] of controlled variable thickness, offering ways to tune the spatial dispersion and so the magneto-optical properties of plasmonic films and metasurfaces - not only by changing their material composition but also by varying their thickness, substrate and superstrate materials [15-17]. [1]  L.A. Jauregui, et al., Science 366, 870 (2019) [2]  Z. Sun, et al. arXiv:2003.05850 (2020) [3]  J.A. Roberts, et al., Nano Lett. 19, 3131 (2019) [4]  L. Vertchenko, et al., Optical Mater. Express 9, 2117 (2019) [5]  D. Shah, et al., Adv. Optical Mater. 1700065 (2017) [6]  A. Popescu, et al., Nano Lett. 17, 6056 (2017) [7]  J.S. Ross, et al., Nano Lett. 17, 638 (2017); M. Baranowski, et al., ibid. 17, 6360 (2017) [8]  I.V. Bondarev and M.R. Vladimirova, Phys. Rev. B 97, 165419 (2018) [9]  I.V. Bondarev, et al., arXiv2002.09988 (2020) [10]  M.M. Fogler, L.V. Butov, and K.S. Novoselov, Nature Commun. 5, 4555 (2014) [11]  I.V. Bondarev, et al., Appl. Phys. Lett. 109, 213302 (2016) [12]  A.E. Boltasseva and V.M. Shalaev, ACS Photonics 6, 1 (2019) [13]  I.V. Bondarev and V.M. Shalaev, Optical Mater. Express 7, 3731 (2017) [14]  I.V. Bondarev, Optical Mater. Express 9, 285 (2019) [15]  I.V. Bondarev, H. Mousavi, and V.M. Shalaev, MRS Commun. 8, 1092 (2018) [16]  I.V. Bondarev, H. Mousavi, and V.M. Shalaev, Phys. Rev. Research 2, 013070 (2020) [17]  C.M. Adhikari and I.V. Bondarev, arXiv:2010.00139 (2020) Host: Thomas Barthel
2020/12/10		Lubos Mitas  (NC State University) "Electronic structure quantum Monte Carlo: recent progress on spins and fixed-phase/node methods" I will talk about electronic structure quantum Monte Carlo (QMC) based on sampling of particle coordinates. The method has a broad appeal since it is applicable to electronic structure of real materials, including strongly correlated systems, as well as to ultracold condensates, model systems, etc. I will talk about recent developments on sampling fermion spins on the same footing as spatial variables, expanding thus the method to spin-orbit and other spin-dependent Hamiltonians. I will also touch on current understanding of fermion signs, in particular, on fixed-phase/node techniques and properties of fermion nodes that enable to reach accuracy up to ~ 99% of correlation energy in systems with hundreds of fermions. Host: Thomas Barthel
2020/12/03		Pasquale Calabrese  (SISSA Trieste) "New results on the entanglement evolution in non-equilibrium quantum systems" Entanglement and entropy are key concepts standing at the foundations of quantum and statistical mechanics. In the last decade the study of the non-equilibrium dynamics of isolated quantum systems revealed that these two concepts are intricately intertwined leading to the emergence of thermodynamics in isolated quantum systems. In this seminar, I will present some old and new results to show the potential of this modern paradigm shift. Host: Thomas Barthel
2020/11/12		Marcos Rigol  (Penn State) "Entanglement entropy of highly excited eigenstates of many-body lattice Hamiltonians" The average entanglement entropy of subsystems of random pure states is (nearly) maximal. In this talk, we discuss the average entanglement entropy of subsystems of highly excited eigenstates of integrable and nonintegrable many-body lattice Hamiltonians. For translationally invariant quadratic models (or spin models mappable to them) we prove that, when the subsystem size is not a vanishing fraction of the entire system, the average eigenstate entanglement entropy exhibits a leading volume-law term that is different from that of random pure states. We argue that such a leading term is likely universal for translationally invariant (noninteracting and interacting) integrable models. For random pure states with a fixed particle number (random canonical states) away from half filling and normally distributed real coefficients, we prove that the deviation from the maximal value grows only with the square root of the system's volume (when the size of the subsystem is one half of that of the system). We then show numerical evidence that the average entanglement entropy of highly excited eigenstates of a particle number conserving quantum chaotic model is the same as that of random canonical states. Host: Thomas Barthel
2020/10/22		Chung Ting Ke  (TU Delft) "Ballistic superconductivity and tunable pi-junctions in InSb quantum wells" The superconductor-semiconductor hybrids provide a promising platform for developing topological superconductivity. Using a 2D electron gas (2DEG) could allow one to create complex networks required for Majorana qubit schemes. Recently, there has been a great interest in realizing topological superconductivity in planar Josephson junctions (JJs). The robust topological phase is predicted when the phase of the junction is tuned to pi. The exceptional electronic properties of InSb 2DEGs make it a perfect material to realize this pi-junction. However, so far, material challenges have prevented the study of hybrid superconducting devices in InSb 2DEGs. Here, we interface InSb 2DEGs with a superconductor (NbTiN) to create Josephson junctions, thus providing the first evidence of induced superconductivity in high-quality InSb 2DEGs. The JJs support supercurrent transport over several microns and display clear signatures of ballistic superconductivity. Furthermore, we exploit the large Lande g-factor and gate tunability of the junctions to control the current-phase relation and drive transitions between the 0 and pi states. The transition is determined by a simple resonance condition, where the Zeeman energy and Thouless energy become comparable. We demonstrate that both electric and magnetic fields control of 0-pi transitions. This control over the free energy landscape allows us to construct a phase diagram identifying the 0 and pi regions, in agreement with theory. Our results establish InSb 2DEGs as a promising new material platform to study the interplay between superconductivity, SOC, and magnetism. [1]  F. Pientka et al., PRX 7, 021032 (2017) [2]  M. Hell, M. Leijnse, M. Flensberg, PRL 118, 107701 (2017) [3]  C.T. Ke et al., Nat. Commun. 10, 3764 (2019) Host: Gleb Finkelstein
2020/09/24		Journal club ft. Lingfei Zhao "Graphene Fabry-Perot quantum Hall interferometer" [1]  C. Déprez et al., arXiv:2008.11222 (2008) [2]  Y. Ronen et al., arXiv:2008.12285 (2008)
2020/08/27		Informal seminar ft. Iman Marvian  (Duke University) "Locality and conservation laws: How, in the presence of symmetry, locality restricts realizable unitaries" According to an elementary result in quantum computing, any unitary transformation on a composite system can be generated using 2-local unitaries, i.e., those which act only on two subsystems. Beside its fundamental importance in quantum computing, this result can also be regarded as a statement about the dynamics of systems with local Hamiltonians: although locality puts various constraints on the short-term dynamics, it does not restrict the possible unitary evolutions that a composite system with a general local Hamiltonian can experience after a sufficiently long time. We ask if such universality remains valid in the presence of conservation laws and global symmetries. In particular, can k-local symmetric unitaries on a composite system generate all symmetric unitaries on that system? Interestingly, it turns out that the answer is negative in the case of continuous symmetries, such as U(1) and SO(3): unless there are interactions which act non-trivially on every subsystem in the system, some symmetric unitaries cannot be implemented using symmetric Hamiltonians. In fact, the difference between the dimensions of the Lie algebra of all symmetric Hamiltonians and its subalgebra generated by k-local symmetric Hamiltonians with a fixed k, constantly increases with the system size (i.e., the number of subsystems). On the other hand, in the case of group U(1), we find that this no-go theorem can be circumvented if one is allowed to use a pair of ancillary qubits. In particular, any unitary which is invariant under rotations around z, can be implemented using Hamiltonians XX+YY and local Z on qubits. We discuss some implications of these results in the context of quantum thermodynamics and quantum computing. [1]  I. Marvian, arXiv:2003.05524
2020/08/20		Journal club ft. Alexey Bondarev "Masses and Majorana fermions in a graphene-like system" [1]  C. Chamon, C. Y. Hou, C. Mudry, S. Ryu, L. Santos, Phys. Scr. T146, 014013 (2012) [2]  L. Santos, S. Ryu, C. Chamon, C. Mudry, PRB 82, 165101 (2010) [3]  P. Ghaemi, F. Wilczek, Phys. Scr. T 146, 014019 (2012)
2020/07/30		Journal club ft. Qiang Miao "Introduction to entanglement renormalization and MERA" [1]  G. Vidal, PRL 99, 220405 (2007) [2]  G. Evenbly and G. Vidal, PRB 79, 144108 (2009)
2020/07/09		Jianfeng Lu  (Duke University),   Unusual time: 10:30am! "Flattening the curve: Taming the dynamical sign problem in diagrammatic algorithms for open quantum systems" Numerical simulations for open quantum system dynamics is a profound challenge. In this talk, we will present some recent works on diagrammatic algorithms for open quantum systems. The focus will be an interplay between the dynamical sign problem and error amplification in numerical integration. In particular, our analysis demonstrates that the technique of partial resummation provides a tool to balance these two types of error, and the recently introduced inchworm Monte Carlo method is a successful case to suppress the numerical sign problem. [1]  Z. Cai, J. Lu, and S. Yang, arXiv:2006.07654 [2]  Z. Cai, J. Lu, and S. Yang, Comm. Pure Appl. Math. (2020)
2020/07/02		Journal club ft. Yikang Zhang "Quantum de Finetti theorem" [1]  Aram W. Harrow, arXiv:1308.6595 (2013) [2]  G. Chiribella, Proceedings of TQC, 9 (2010) [3]  R. F. Werner, PRA 58, 1827 (1998)
2020/04/23		Xin Zhang  (Duke University) "Driven-dissipative phase transition in a non-linear oscillator" I show that a second-order phase transition can exist in a surprisingly simple system, which is a single non-linear oscillator. This simple system can be seen as a paradigmatic example of open quantum many-body system, in which there has been great interest recently due to experimental advances such as quantum simulation. Using both analytical and numerical methods, I demonstrate the Z2 symmetry breaking and its corresponding critical phenomena.
2020/04/02		Xing He  (Duke University),   Unusual time & place: 12:00pm, Zoom! "Anharmonic effects on phonon eigenvectors and S(Q,E) in quantum paraelectric SrTiO3" The quantum paraelectric behavior and strongly anharmonic lattice dynamics of SrTiO3 have attracted interest for decades. Reflecting the incipient ferroelectric (FE) instability near the quantum critical point and couplings between transverse acoustic (TA) and optic (TO) phonons, anomalous phonon intensities were observed from inelastic neutron scattering (INS) experiments on SrTiO3 using the state-of-art instruments at Oak Ridge National Laboratory. The S(Q,E) data reveal a strongly anomalous evolution of TA intensity as TO softens. The experimental trends are confirmed and rationalized using DFT simulations including anharmonic renormalization. By analyzing the temperature-dependent force constants (FC) and eigenvectors, it is found that the structure factors of phonon modes change dramatically with temperature, as a direct consequence of the anharmonicity in this system. Moreover, we identify that the changes of Ti and O eigenvectors are responsible for these striking observations, originating from FC changes in the Ti-O bonds. These results establish how temperature-dependent phonon intensities from INS can provide direct insights into the behavior of phonon eigenvectors, and also show how first-principles simulations can rationalize such anharmonic effects. Host: Thomas Barthel
2020/04/02		Volker Blum  (Duke University),   Unusual place: Zoom! "Electronic structure theory for 2d hybrid organic-inorganic perovskites" Layered 2d hybrid organic-inorganic perovskites (HOIPs) can be created by combining a wide range of possible inorganic components with an even broader range of organic molecules, offering considerable flexibility to fine-tune their synthesizability and properties. This talk focuses on computational predictions of the electronic energy levels of new 2d HOIP materials. Such predictions pose a considerable challenge due to high required levels of theory and large unit cells (hundreds of atoms) associated with typical 2d HOIPs. We here use high-precision, all-electron hybrid density functional theory including spin-orbit coupling, showing that this combination provides descriptions of the quantum-well like energy level alignment in lead halide based oligothiophene perovskites in excellent agreement with experiments. We then employ the same approach to predictively address the electronic properties of a broad range of further 2d HOIPs, including lead-free (Ag-Bi) based ones. As a final point, we show that the details of the atomic structure used to predict electronic properties matter significantly, even in a qualitative sense, by determining energy level alignments and, therefore, which component (organic or inorganic) forms the band edges. A complete structural understanding of a given target 2d HOIP is thus essential for faithfully predicting the properties that can be leveraged within this promising new semiconductor materials space. This work is enabled by the very large community of developers and users of the FHI-aims code as well as by close collaborations with leading experimental colleagues, particularly the group of David B. Mitzi (Duke University). Host: Thomas Barthel
2020/02/13		Jedediah H. Pixley  (Rutgers University) "Twistronics in solid-state devices and beyond" The ability to control and manipulate the strength of correlations in quantum matter is one of the central questions in condensed matter physics today. While pressure, chemical doping, or magnetic field have served as conventional tuning knobs for a wide class of correlated systems, the ability to twist van der Waals materials has recently emerged as a novel scheme to engineer strong correlations and tune electronic properties. In the case of twisting two sheets of graphene, at a particular "magic-angle", the kinetic energy of electronic degrees of freedom is expected to vanish, and as a result, interaction effects should dominate. This has now been demonstrated experimentally following the recent discovery of superconductivity in close proximity to correlated insulating phases in magic-angle graphene. These results have now been reproduced by a number of experimental groups and extended to other two-dimensional van der Waals materials. In this talk, I will discuss our theory to describe the universal properties that give rise to strong correlations in twisted bilayer graphene and related systems. As a result, we will generalize this phenomenon to a wide class of simpler models and distinct physical settings, such as ultra-cold atomic gases, trapped-ions, and metamaterial experiments. We will show that the "magic-angle" in twisted bilayer graphene is, in fact, a single particle quantum phase transition that can be described by a delocalization transition in momentum space: the incommensurate potential, that is generated by the twist, eventually destabilizes the ballistic plane-wave eigenstates. Lastly, we will demonstrate the effects of correlations by constructing effective Hubbard models with dramatically enhanced interactions due to this novel quantum phase transition. Host: Gleb Finkelstein
2020/02/06		Israel Klich  (University of Virginia) "Highly entangled spin chains and exact holographic tensor networks" Tensor networks provide a useful class of variational wave functions suitable for numerical studies of ground states of quantum hamiltonian as well as certain classical problems. In spite of an extensive body of work on the subject, so far no examples have been found where tensor networks describe exactly ground states of gapless systems. Here, I will describe the first such example: a construction of the ground states of the deformed Motzkin and Fredkin models. These models are spin chains that pose a novel quantum phase transition between low and high entanglement, and, as I will show, their ground state can be written as a new type of tensor network, describing tiling models in which the physical degrees of freedom live on the edge. Host: Thomas Barthel
2020/01/16		Emanuel Gull  (University of Michigan) "Dynamics of Kondo voltage splitting after a quantum quench" We analyze the time-dependent formation of the spectral function of an Anderson impurity model in the Kondo regime within a numerically exact real-time quantum Monte Carlo framework. At steady state, splitting of the Kondo peak occurs with nontrivial dependence on voltage and temperature, and with little effect on the location or intensity of high-energy features. Examining the transient development of the Kondo peak after a quench from an initially uncorrelated state reveals a two-stage process where the initial formation of a single central Kondo peak is followed by splitting. We analyze the time dependence of splitting in detail and demonstrate a strong dependence of its characteristic timescale on the voltage. We expect both the steady state and the transient phenomenon to be experimentally observable. Host: Harold Baranger
2020/01/13		Arthur P. Ramirez  (UC Santa Cruz),   Unusual time: Monday, 2:15pm! "How to fall off a plateau in a Shastry-Sutherland lattice" Magnetization plateaus are related to topological quantization that entwines spin and spatial degrees of freedom leading to a gap in the energy spectrum. I will discuss an example of a plateau in a Shastry-Sutherland system TmB4, where we used high-precision specific heat and magnetization to characterize the stability of the so-called 1/8th phase. Related systems and prospects for topological quantum matter will be discussed.
2019/12/12		Sami Mitra  (APS, Physical Review Letters) "PRL at 60+: You have your physics results, now what?" In a talk that I am hoping will quickly morph into a free-flowing Q and A session, I will discuss the role that journals in general and PRL in particular play in disseminating your physics results. The process is a cascading sequence that entails interacting with journal editors, referees, conference chairs, journalists, department chairs, deans, funding agencies, and others. The tools, however, have changed in recent years; the arrival of social media, search engines, and electronic repositories have us in a state of flux. PRL published its first paper 60 (plus 1) years ago. Let's look back and forward. Host: Gleb Finkelstein
2019/10/22		Su Kong Chong  (University of Utah),   Unusual weekday: Tuesday! "Quantum transport in topological insulator based van der Waals heterostructures" The discovery of linear-dispersive topological surface states in three-dimensional (3D) topological insulators (TIs) has offered a platform to study the topological phases in Dirac materials. Different from conventional 2D electron gas and Dirac fermions, the topological surface states are a manifestation of their bulk band inversion and exhibit a unidirectional spin texture locked to their momenta. However, the research on 3D TI devices have been restricted by the abundance bulk carriers which interfere the access of the surface transport. In this talk, I will present my research progress on the transport properties of a recent discovered bulk insulating 3D TI, Bi_2-x Sb_x Te_3-y Se_y (BSTS). We introduce van der Waals (vdW) heterostructures in the form of TI/insulator/graphite to control chemical potentials of the topological surface states in a clean and efficient manner. The vdW heterostructures configured local gates also permit quantum capacitance measurements for quantitative evaluation of the surface states' Landau level (LL) energies. In the studies of variable thickness 3D TI devices towards 2D thin limit, we establish a tunable capacitive coupling between the top and bottom surfaces and study the effect of this coupling in the quantum Hall regime via dual-gate control. Lastly, I will discuss the possible topological phase transition of 3D TI in the inter-surface tunneling regime where a thermally-activated transport gap clearly resolved and manipulated by electric and magnetic fields. These works show the promise of the vdW platform in creating advanced high-quality and tunability 3D TI-based devices. Host: Gleb Finkelstein
2019/08/22		Alexey Gorshkov  (University of Maryland) "Information propagation and entanglement generation with long-range interactions" Atomic, molecular, and optical systems often exhibit long-range interactions, which decay with distance r as a power law 1/r^alpha. In this talk, we will derive bounds on how quickly information can propagate and entanglement can be generated in such quantum systems. We will also present protocols that attempt to saturate these bounds. Finally, we will discuss numerous applications of these bounds and protocols, ranging from bounding and enhancing the speed of quantum computers to illuminating the properties of quantum phases and phase transitions in and out of equilibrium. Host: Thomas Barthel
2019/08/21		Slava Rotkin  (Pennsylvania State University),   Unusual weekday & place: Wednesday, Physics 119! "Correlative near-field and Raman imaging for twisted 2D materials" Novel 2D materials (2DMs), a class of electronic single/a few atomic layer thick lattices, demonstrate a number of unique physical effects. Among those, their electronic properties were shown to depend on the stacking order of the individual layers, specifically on their angular registry aka twist. Intuitively clear picture bases on translation superlattice formed by twisted lattices, also known as Moire-lattice. Splash of interest to twisted 2DM (including but not limited to graphene multi-layers) is due to both their unusual - chiral - electronic/optical properties and potential for applications in biosensing, electronics and quantum information technology. This work focuses on graphene samples with 2+ layers, where plasmonic response varies with the twist angle. Since electronic/optical properties of twisted 2DMs can be modulated at the scale of a few nanometers, the ultimate methods of characterization are required, with both high spatial resolution and capability to detect optical modulation, not readily available with regular tools of a physics lab. Scattering-type scanning near-field optical microscope (sSNOM) will be shown to reveal such properties of twisted graphene, in particular phonon-plasmon polariton coupling. Furthermore, correlation of hyperspectral sSNOM data (that is taken at multiple excitation) to confocal Raman scattering microscopy data will be shown to lead to explicit chirality identification. Potential avenues for expanding this technique to big-data analysis will be discussed. Host: Gleb Finkelstein
2019/08/06		Ian MacCormack  (University of Chicago),   Unusual weekday: Tuesday! "Holographic duals of inhomogeneous systems: The rainbow chain and the sine-square deformation model" Starting with a system described by a conformal field theory (e.g. a critical spin chain or free fermions), one can find interesting violations to the typical logarithmic behavior of the bipartite entanglement entropy by introducing an inhomogeneous kinetic term in the Hamiltonian. Two examples of recent interest are the rainbow chain and the sine-squared deformed (SSD) model. Such systems can be equivalently described by placing the original CFT on a curved background manifold. Using the AdS/CFT correspondence, we develop a holographic dual description of inhomogeneous (1+1) dimensional systems by foliating the bulk spacetime with curved surfaces. Extending these foliations to the BTZ spacetime allows us to describe inhomogneous systems at finite temperatures. Using field-theoretic, holographic, and numerical techniques, we are able to compute the entanglement entropy at zero at finite temperatures, for the rainbow chain, the SSD, and other inhomogeneous systems. Host: Ethan Arnault
2019/06/17		Amit Ghosal  (Indian Institute of Science Education and Research Kolkata),   Unusual weekday: Monday! "Superconductivity in a disordered vortex lattice" Orbital magnetic field and strong disorder weaken superconducting correlations acting individually on a type-II s-wave superconductor. The Abrikosov vortex lattice, resulting from the applied magnetic field, melts with an increase of the strength of the field, turning the system into a metal. Similarly, the presence of disorder causes a superconductor to insulator transition beyond a critical strength of disorder. Here we show that the interplay of these two perturbations, when present simultaneously in a two-dimensional superconductor, leads to its intriguing evolution. In particular, we show that the local superconductivity can actually strengthen due to interesting spatial reorganization or order parameters in the presence of strong disorder. While at weak disorder strengths the critical magnetic field for the suppression of superconducting energy gap matches with the critical field at which superfluid density vanishes, the two 'critical' fields diverge from each other with the increase of the disorder strengths. Our results have important consequences for the strong magnetoresistance peak observed in disordered superconducting thin films. We illustrate this by calculating the dynamical conductivity and analyzing its low-frequency behavior. Our results, which emphasize the role of spatial fluctuations in the pairing amplitude, capture the non-monotonic evolution of the magnetoresistance, consistent with experiments. We will also demonstrate that the presence of even weak disorder causes the Caroli-deGennes-Matricon zero-bias peak in vortex-core density of states to disappear. The origin and consequences of such dramatic behaviors will be discussed along with their experimental relevance. Host: Harold Baranger
2019/06/13		Journal club ft. Qiang Miao "Entanglement in condensed matter systems: Scaling, topology, and spectrum" [1]  N. Laflorencie, Phys. Rep. 646, 1 (2016)
2019/06/06		Thomas Banks  (Rutgers University) "Instantons, colloids and convergence of the 1/N expansion for the homogeneous electron gas" I will discuss an approximate equation for the two-point charge density correlation of the homogeneous electron fluid based on the large N expansion of this model. Large N considerations also lead to a conjecture about a colloidal phase in between the Wigner-crystal and fluid phases as first discussed by Kivelson and Spivak. As a bonus we argue that the large N expansion is convergent in the fluid phase and the colloidal and crystal phases occur at density of order 1/N where the expansion breaks down. Host: Ronen Plesser
2019/05/07		Michel Gingras  (University of Waterloo),   Unusual weekday: Tuesday! "Has compelling experimental evidence for order-by-disorder at last been found in a frustrated magnetic material?" In some magnetic systems, known as frustrated magnets, the lattice geometry or the competition between different spin-spin interactions can lead to a sub-exponentially large number of accidentally degenerate classical ground states. Order-by-disorder (ObD) is a concept of central importance in the field of frustrated magnetism. Saddled with large accidental degeneracies, a subset of states, those that support the largest quantum and/or thermal fluctuations, may be selected to form true long-range order. ObD has been discussed extensively on the theoretical front for over 30 years and proposed to be at play in a number of experimental settings. Unfortunately, convincing demonstrations of OBD in real materials have remained scarce. In this talk, I will review the phenomena of thermal and quantum of order-by-disorder and discuss how recent studies suggest that there may exist compelling evidence for ObD in some frustrated XY pyrochlore antiferromagnetic materials. Host: Sara Haravifard
2019/04/18		Benjamin Hunt  (Carnegie Mellon University) "Novel superconductors in two-dimensional materials and heterostructures" The physics of superconductors in reduced dimensions - two in particular - is related to such varied phenomena as high-temperature superconductivity, topological superconductivity, and the paradigmatic quantum phase transition, the superconductor-insulator transition. The discovery of graphene has led to the ability to produce new, highly crystalline 2D superconductors from other van der Waals bonded materials, which has given us access to superconductivity in single atomic layers as well as new ways of tuning and understanding superconductivity in these systems. In this talk, I will present an overview of superconductivity in two dimensions and I will discuss our recent experiments on single- and few-atomic-layer devices constructed from the transition metal dichalcogenides TaS2 and NbSe2, which possess remarkable properties due to their crystal symmetry and strong spin-orbit coupling (known as "Ising superconductivity"). I will discuss their connection to exotic phenomena such as spin-triplet Cooper pairing and topological superconductivity. Additionally, I will discuss our measurement of the superconducting proximity effect in the helical metallic edge states of the two-dimensional topological insulator 1T'-WTe2. These experiments have implications for the realization of topological superconductivity in 1D and the creation of Majorana modes in a van der Waals material platform. Host: Gleb Finkelstein
2019/04/11		Vito Scarola  (Virginia Tech) "Topological Mott insulators in certain frustrated lattices" Our group models ways in which strong interactions can lead to interesting collective states of matter, e.g., fractional quantum Hall states. These and other states are examples of topological phases which typically encode topology in the single particle band structure and can, as a result, reveal quantum Hall effects. But a remarkable new set of studies of "topological Mott insulators" shows that quantum Hall effects can be driven exclusively by interactions, while the parent non-interacting band structure is topologically trivial. Here the topological Mott insulator forms even with no external magnetic field because it relies on interactions to spontaneously break time reversal symmetry to reveal a quantized Hall effect. Unfortunately, underlying models have so far remained somewhat academic. I will discuss our work which shows that realistic models of interactions between particles in certain frustrated lattices lead to robust topological Mott insulators. I will also discuss example two-dimensional systems where we might be able to find topological Mott insulators. Host: Gleb Finkelstein
2019/04/09		Matthew J. Gilbert  (Urbana-Champaign, Stanford),   Unusual weekday: Tuesday! "The non-Hermitian Chern insulator" During the past decade, the landscape of condensed matter physics has been dominated by a desire to understand the topological nature of materials and the observable consequences that are associated with the presence of topology. To this end, topological band theory has provided a foundation that has allowed for the definition of topological invariants, or conserved quantities that do not change based on adiabatic changes to the system parameters. Nonetheless, each of the systems that has been considered to this point, have relied on the fact that the system under consideration is both closed and Hermitian. Recent work has extended topological band theory to open, non-Hermitian Hamiltonians but little is understood about how non-Hermicity alters the topological quantization of associated observables. In this talk, we will begin to address these problems by examining the non-Hermitian Chern insulator where we focus on changes in observables and its relation to our current understanding about non-Hermitian topological band theory. Host: Gleb Finkelstein
2019/04/05		Martin Fraas  (Virginia Tech),   Unusual time: Friday, 1:00pm! "Integers in gapped quantum lattice systems" We propose an index for gapped quantum lattice systems that conserve a U(1)-charge. This index takes integer values and it is therefore stable under perturbations. Our formulation is general, but we show that the index reduces to (i) an index of projections in the non-interacting case, (ii) the filling factor for translational invariant systems, (iii) the quantum Hall conductance in the two-dimensional setting without any additional symmetry. Example (ii) recovers the Lieb-Schultz-Mattis theorem, (iii) provides a new and short proof of quantization of Hall conductance in interacting many-body systems. Additionally, we show Avron-Dana-Zak relations for both integer and fractional quantum Hall effect. Host: Jianfeng Lu
2019/02/07		Elizabeth L. Green  (Helmholtz-Zentrum Dresden-Rossendorf) "Fermi Surface Reconstruction in Nd-doped CeCoIn5" CeCoIn5, one of the most well-studied heavy fermion systems, exhibits a novel field-induced superconducting state above 10T for B||a known as the Q-phase. Recent neutron scattering measurements show a similar Q-vector for the 5% Nd-doped CeCoIn5 at zero applied magnetic field [1] which has initiated intense theoretical and experimental work on this doping series. In this talk I will present de Haas-van Alphen (dHvA) effect measurements which demonstrate a drastic Fermi surface reconstruction between 2 and 5% Nd-doping levels is responsible for the emergence of this unconventional superconducting state. The cylindrical Fermi surface develops a quasi-three-dimensional topology with increased doping levels thus reducing the likelihood of an enhanced nesting scenario, previously given as a possible explanation for the presence of the Q-phase. Effective masses remain unchanged up to 10% Nd, indicating the presence of a spin density wave type of quantum critical point. In addition, I will present evidence that by substituting Ce with Nd the electronic pairing potential is altered. These results highlight the need for additional experiments and further theoretical calculations to accurately model this unique system. [1]  S. Raymond et al., JPSJ 83, 013707 (2014) [2]  J. Klotz et al., PRB 98, 081105(R) (2018) Host: Sara Haravifard
2019/01/31		Michael J. Manfra  (Purdue University) "Aharonov-Bohm interference of fractional quantum Hall edge modes" The braiding statistics of certain fractional quantum Hall states may be probed via interferometry of their edge states. Practical difficulties including loss of phase coherence make this a challenging task. We demonstrate operation of a small Fabry-Perot interferometer in which highly coherent Aharonov-Bohm oscillations are observed in the integer and fractional quantum Hall regimes. Careful design of the heterostructure suppresses Coulomb effects and promotes strong phase coherence. We characterize the coherency of edge mode interference by the energy scale for thermal damping and determine the velocities of the inner and outer edge modes independently via selective backscattering of edge modes originating in the N=0,1,2 Landau levels. We also observe clear Aharonov-Bohm oscillations at fractional filling factors n=2/3 and n=1/3, which indicates that our device architecture provides a platform for measurement of anyonic braiding statistics. Host: Gleb Finkelstein
2019/01/17		Jason Petta  (Princeton University) "Microwave spectroscopy of valley states in silicon" The bulk conduction band of Si has six equivalent valleys. Strain in Si/SiGe heterostructures partially lifts the six-fold valley degeneracy by raising the energy of the four in-plane valleys. It is known that large electric fields can lift the degeneracy of the remaining two low-lying valleys. However, the measured valley splittings range from 10-300 micro-eV, suggesting that microscopic details such as interface roughness and disorder impact the valley splitting. In this lecture I will describe how microwave spectroscopy can be applied to probe valley states in silicon nanostructures [1]. In the first experiment, a cavity coupled Si double quantum dot is probed using microwave frequency photons. The transmission of the photons through the microwave cavity displays signatures that are consistent with the valley degree of freedom and the data can be modeled using cavity input-output theory [2]. We also use Landau-Zener interferometry to probe the low-lying energy level structure of a silicon double quantum dot. The observed Landau-Zener interference pattern persists down to low driving frequencies of 50 MHz, suggesting relatively long-lived charge coherence. Low-lying valley states result in a unique Landau-Zener interference pattern that is in contrast with measurements on conventional two-level charge qubits [3]. These new probes of valley states have high energy resolution and may be applied to other low energy degrees of freedom. [1]  G. Burkard and J. R. Petta, Phys. Rev. B 94, 195305 (2016) [2]  X. Mi, C. G. Péterfalvi, G. Burkard, and J. R. Petta, Phys. Rev. Lett. 119, 176803 (2017) [3]  X. Mi, S. Kohler, and J. R. Petta, Phys. Rev. B 98, 161404(R) (2018) Host: Harold Baranger
2018/12/13		Journal club ft. Xin Zhang "Phase transitions in driven-dissipative quantum optical systems" [1]  P.D. Drummond and D.F. Walls, J. Phys. A: Math. Gen. 13, 725 (1980) [2]  W. Casteels, F. Storme, A. Le Boité, and C. Ciuti, Phys. Rev. A 93, 033824 (2016) [3]  W. Casteels, R. Fazio, and C. Ciuti, Phys. Rev. A 95, 012128 (2017) [4]  T. Fink, A. Schade, S. Höfling, C. Schneider, A. Imamoglu, Nature Physics 14, 365 (2018)
2018/12/06		Luiz Henrique Bugatti Guessi  (Universidade de Sao Paulo, Duke) "Correlation effects in the emergence of bound states in the continuum" Bound States in the continuum (BICs) are states with localized wave-function even though lying in the continuum. Here, we explore theoretically the emergence of a spin-BIC in a system comprising two identical quantum dots side-coupled to a quantum wire. The dots are symmetrically coupled to the site at the center of the wire and to its nearest neighbors. Taking advantage of the dot symmetry, we work with the bonding and anti-bonding (AB) levels resulting from the symmetric and anti symmetric combinations of the dot levels. We consider a two-impurity Anderson model. In the non-interacting limit (U=0), the AB orbital is decoupled from the conduction band and is hence a BIC. For nonzero U, our numerical renormalization-group results show that the interaction couples the bonding and anti-bonding orbitals and hence broadens the latter. The RKKY interaction between the magnetic moments of the two dots can either be ferro- or antiferromagnetic, to form a singlet or a triplet, which affects the formation of the Kondo cloud. In the strongly particle-hole asymmetric coupling limit, the AB orbital is reduced to a singly occupied level that is decoupled from the continuum, i.e., a spin-BIC.
2018/11/29		Arthur P. Ramirez  (UC Santa Cruz) "Transport in undoped Weyl semimetals and a new type of spin glass" Two topics will be discussed. The first addresses transport in undoped Weyl semimetals (WSMs). Despite their name and the behavior of most WSMs, these materials should exhibit "semiconducting" temperature dependent resistivity, drho/dT<0, when undoped. We show that this form is obeyed in the rare-earth iridate pyrochlores, R2Ir2O7, in stoichiometric samples. We also show that for R = Yttrium, rho~1/T^4, consistent with a theory of charged impurity scattering, but extending into a temperature range where k_F l << 1, suggesting behavior of a "bad" WSM. The second topic will address an unusual spin glass, pseudobrookite Fe2TiO5 in which an Ising anisotropy develops due to interactions. This presents a second example of "quasi-spin" glass. Host: Harold Baranger
2018/11/15		Fabio Altomare  (D-Wave Systems) "D-Wave quantum annealer as a quantum simulator" D-Wave Systems builds superconducting quantum annealing processors that are primarily intended to be used for solving classical computation problems such as optimization and sampling. However, recent studies have shown how this computing platform can be used as a programmable quantum magnet that can be used to simulate physical systems relevant to the field of condensed matter physics. This lecture will provide a brief overview of D-Wave's technology, a summary of recently published quantum magnetism experiments, and a look ahead at next-generation quantum annealing processors. [1]  A. D. King et al., Nature 560, 456 (2018) [2]  R. Harris et al., Science 361, 162 (2018) Host: Albert Chang, Thomas Barthel
2018/11/08		Nicholas P. Butch  (NIST, UMD College Park) "Spin polarized half-gapped superconductivity" Nonunitary superconductivity is a rare and striking phenomenon in which spin up and spin down electrons segregate into two different quantum condensates. In this talk, I'll describe the discovery of spin-triplet superconductivity in paramagnetic UTe2, in which electrons with parallel spins pair, yet only half of the available electrons participate, yielding a spin-polarized condensate that coexists with a spin-polarized metal. The superconducting order parameter arises from strong ferromagnetic fluctuations, yielding a very high upper critical field with similar anisotropy to established ferromagnetic superconductors. This novel superconductor is a promising platform for exploration of topological in-gap states and their further uses. Host: Sara Haravifard
2018/10/11		Seyed M. Koohpayeh  (Johns Hopkins University) "Development, synthesis, and bulk crystal growth of novel/quantum materials" Almost every new technological innovation depends significantly upon the discovery of new materials with novel physical properties. The development, growth and structural analysis of materials, particularly in the form of single crystals, establishes how materials work at a fundamental level, helps to design better functional materials, and opens up pathways for deeper fundamental understanding of condensed matter physics to uncover novel phases of matter. Therefore, crystal growth of stoichiometric and high-quality solids of the increasingly complex materials of current interest is vital. Here, we show that achieving such perfection requires detailed understanding of the system under investigation, coupled with in-depth knowledge of crystal growth processes and the thermodynamics involved. In this talk, our recent development on some of the pyrochlore structure compounds will be presented. Strong dependence of their physical, compositional and structural properties on synthesis and float-zone growth conditions will also be shown. Host: Sara Haravifard
2018/05/10		Oleg L. Berman  (City University of New York) "Exciton Bose-Einstein condensation and superfluidity in two-dimensional nanomaterials" This talk reviews the theoretical studies of the Bose-Einstein condensation (BEC) and superfluidity of indirect excitons and microcavity polaritons in quasi-two-dimensional (quasi-2D) van der Waals nanomaterials such as graphene, phosphorene, and transition metal dichalcogenide (TMDC) heterostructures. Indirect excitons are the Coulomb-bound pairs of electrons and holes confined to different parallel monolayers of a layered planar nanomaterial structure. It has been shown that the indirect excitons in gapped bilayer graphene can form BEC and experience superfluidity [1]. The high-T superfluidity of the two-component weakly-interacting Bose gas of the A-type and B-type indirect excitons in the TMDC heterostructures is proposed [2,3]. The critical temperature and superfluid velocity of the indirect excitons in a bilayer phosphorene nanostructure is shown to be anisotropic, dependent strongly on the particular direction of the exciton propagation [4]. Also analyzed are the effects of high magnetic fields on the BEC and superfluidity of exciton polaritons formed by the direct excitons in gapped monolayer graphene embedded in an optical microcavity to show that the polariton BEC can be manipulated by an external magnetic field [5], whereas the superfluid behavior can be controlled by changing the graphene gap [6]. These results open up new avenues for the experimental realization of the exciton BEC and superfluidity phenomena as well as their practical applications in optoelectronics [7]. [1]  O. L. Berman, R. Ya. Kezerashvili, and K. Ziegler, PRB 85, 035418 (2012) [2]  O. L. Berman and R. Ya. Kezerashvili, PRB 93, 245410 (2016) [3]  O. L. Berman and R. Ya. Kezerashvili, PRB 96, 094502 (2017) [4]  O. L. Berman, G. Gumbs, and R. Ya. Kezerashvili, PRB 96, 014505 (2017) [5]  O. L. Berman, R. Ya. Kezerashvili, and Yu. E. Lozovik, PRB 80, 115302 (2009) [6]  O. L. Berman, R. Ya. Kezerashvili, and K. Ziegler, PRB 86, 235404 (2012) [7]  D. W. Snoke and J. Keeling, Physics Today 70, 54 (2017) Host: Harold Baranger
2018/05/07		Hannes Pichler  (Harvard University),   Unusual weekday: Monday! "Universal photonic quantum computation via time-delayed feedback" We propose and analyze a deterministic protocol to generate two-dimensionally entangled photonic states using a single quantum emitter coupled to a 1D waveguide. I will show that delayed quantum feedback dramatically expands the class of achievable quantum states in such settings and in particular allows to generate universal resources for measurement based quantum computation. As a physical implementation, we consider a nanophotonic setting where delayed feedback is introduced by terminating the waveguide on one side with a mirror. We identify the class of many-body quantum states that can be produced using this approach, characterize them in terms of 2D tensor network states and give an explicit protocol to generate the 2D cluster state. [1]  H. Pichler, S. Choi, P. Zoller, and M. D. Lukin, PNAS 114, 201711003 (2017) Host: Harold Baranger
2018/05/01		Chun Ning (Jeanie) Lau  (Ohio State University),   Unusual weekday: Tuesday! "Spin, charge and heat transport in low-dimensional materials" Low-dimensional materials constitute an exciting and unusually tunable platform for investigation of electronic interactions. Here I will present our results on integer and fractional quantum Hall states of high quality few-layer phosphorene devices, the unprecedented current carrying capacity of carbon nanotube "hot dogs", and our recent observation of robust long distance spin transport through the antiferromagnetic state in graphene. Host: Gleb Finkelstein
2018/04/26		Adolfo del Campo  (University of Massachusetts Boston) "Tailoring the dynamics of isolated and open quantum systems" Tailoring the far from equilibrium dynamics of quantum matter is an open problem at the frontiers of physics. Yet, it is also a necessity for the development of quantum science and technology. Conventional adiabatic protocols are ubiquitously exploited for the manipulation and control of quantum matter in a wide variety of fields. They require however long evolution times and are thus prone to noise and decoherence errors. Shortcuts to adiabaticity provide an alternative control paradigm, free from the requirement of slow driving. We shall review progress in the development of shortcuts to control complex systems, and choose a unitary Fermi gas as a paradigmatic example with strong interactions [1]. Moving to open systems, we shall discuss a scheme for the quantum simulation pf many-body decoherence using classical noise as a resource [2,3]. The resulting dynamics is of relevance to a variety of applications ranging from adiabatic quantum computation [4] to quantum metrology [5]. [1]  S. Deng, A. Chenu, P. Diao, F. Li, S. Yu, I. Coulamy, A. del Campo, H. Wu, arXiv:1711.00650 (2017) [2]  A. Chenu, M. Beau, J. Cao, A. del Campo, PRL 118, 140403 (2017) [3]  M. Beau, J. Kiukas, I. L. Egusquiza, A. del Campo, PRL 119, 130401 (2017) [4]  A. Dutta, A. Rahmani, A. del Campo , Phys. Rev. Lett. 117, 080402 (2016) [5]  M. Beau, A. del Campo, PRL 119, 010403 (2017) Host: Thomas Barthel
2018/04/19		Charles L. Kane  (University of Pennsylvania) "Topological superconductivity from Majorana to Fibonacci" Topological superconductivity is a topic of current interest because of its potential for providing a method to reliably store and manipulate quantum information. The most basic topological superconductor has an underlying Ising topological order, in which zero energy Majorana quasiparticle states are associated with topological defects. We will review recent experimental progress towards realizing those states in one and two dimensional superconducting devices. Ising topological order is too simple to allow universal quantum computation, but the richer Fibonacci topological order is in principle sufficient. We will formulate a theory of a Fibonacci phase of a topological superconductor based on a solvable model of interacting Majorana fermions. This theory provides new insight into the nature of the Fibonacci phase, and predicts a closely related "anti-Fibonacci" phase. We show that Majorana fermions can split into a pair of Fibonacci anyons, and propose an interferometer that directly probes Fibonacci non-Abelian statistics.
2018/04/17		Dan Shahar  (Weizmann Institute of Science),   Unusual weekday: Tuesday! "Hot superconductors and cold insulators" The study of the magnetic-field driven superconductor-insulator transition in thin superconducting films at low temperatures reveals an unusual insulator whose conductivity seems to approach zero at a finite temperature, while its current-voltage characteristics are bistable, indicating that the electrons decouple from the phonon system. In parallel, the superconducting state at lower magnetic fields exhibits a broad range where metallic behavior is seen down to the lowest temperatures. Host: Gleb Finkelstein
2018/04/10		Manuel Houzet  (Université Grenoble Alpes and CEA),   Unusual weekday: Tuesday! "Non-equilibrium quasi-particles in disordered superconductors" Experimentally, the concentration of quasi-particles in gapped superconductors always largely exceeds the equilibrium one at low temperatures. Since these quasiparticles are detrimental for many applications, it is important to understand the origin of the excess. We demonstrate that the dynamics of quasiparticles localized at spatial fluctuations of the gap edge becomes exponentially slow. This may give rise to the observed excess quasiparticles in the presence of a vanishingly weak non-equilibrium agent. Host: Gleb Finkelstein, Harold Baranger
2018/03/29		Journal club ft. Anne Draelos "Superconductivity and Mott insulating behavior in twisted bilayer graphene" [1]  Y. Cao, V. Fatemi, A. Demir, S. Fang, S. L. Tomarken, J. Y. Luo, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, E. Kaxiras, R. C. Ashoori, P. Jarillo-Herrero, "Correlated insulator behaviour at half-filling in magic-angle graphene superlattices", Nature (2018) [2]  Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, P. Jarillo-Herrero, "Unconventional superconductivity in magic-angle graphene superlattices", Nature (2018) [3]  E. J. Mele, "Novel electronic states seen in graphene", Nature (2018)
2018/03/22		Carlos A. R. Sa de Melo  (Georgia Tech) "Color superfluidity of neutral ultracold fermions in the presence of color-orbit and color-flip fields" I describe the emergence of color superfluid phases for systems of ultra-cold atoms with artificial color-orbit and color-flip fields for three-component (Red-Green-Blue) Fermi systems. For fermions interacting only in the s-wave channel, I describe the phase diagrams of color-flip fields versus interaction parameter for fixed color-orbit coupling. Various topological phases are encountered, where the quasiparticle excitation spectrum possesses nodal structures induced by the presence of color-orbit coupling. The order parameter tensor develops momentum dependent non-zero matrix elements in the singlet, triplet and quintet sectors. An unusual topological multicritical point arises, where several gapless phases converge. This point can be reached by tunning color-flip fields and interaction parameter. Finally, I discuss potential experiments to observe these phases in Fermi isotopes of Lithium, Potassium and Ytterbium.
2018/03/15		Journal club ft. Xin Zhang "Variational approach to non-equilibrium quantum impurity problems" [1]  Y. Ashida, T. Shi, M. Carmen Banuls, J. I. Cirac, E. Demler, arXiv:1801.05825 (2018) [2]  T. Shi, E. Demler, J. I. Cirac, arXiv:1707.05902
2018/02/22		James R. Williams  (University of Maryland, College Park) "Using Josephson junctions to probe quantum materials" Josephson junctions are quantum circuit elements commonly associated with use as fundamental units in quantum computation. These junctions may also play a fundamental role in probing complex materials that have eluded characterization by conventional means. In this talk I'll be presenting some of our group's recent results on using Josephson junctions (JJs) to look for exotic excitations in quantum materials. Specifically, we will present data on junctions made from Pb_xSn_(1-x)Te -- a topological crystalline insulator -- where deviations from conventional JJs becomes most evident under microwave radiation [1] and revel interesting signatures of an unconventional surface state. Finally, I will report on progress made in dynamically creating JJs in NbSe2 and the potential revelation of two independent superconducting order parameters present within the material. [1]  R. A. Snyder, C. J. Trimble, C. C. Rong, P. A. Folkes, P. J. Taylor, J. R. Williams, arXiv:1710.06077 (2017) Host: Gleb Finkelstein
2018/02/15		Journal club ft. Gu Zhang "Geometric phase at quantum phase transitions" [1]  A. C. M. Carollo and J. K. Pachos, PRL 95, 157203 (2005) [2]  A. Hamma, arXiv:quant-phy/0602091 (2006) [3]  L. Campos Venuti and P. Zanardi, PRL 99, 095701 (2007)
2018/01/11		Sho Yaida  (Duke University) "Novel phase transition within amorphous solids" Glassy materials are omnipresent in everyday life from windows to plastics to piles of sand. Yet our understanding of both their (equilibrium) liquid and (out-of-equilibrium) solid phases lags far behind that of crystalline counterparts. Recent advances are rapidly changing the ways in which we understand these common-yet-physically-enigmatic materials. This talk overviews one such advance -- the discovery of the Gardner phase transition from normal to marginally-stable glasses. Our work in particular indicates that such a transition, first found in abstract infinite-dimensional models, can survive down to the three-dimensional world. This transition reinforces the overriding role of rugged free-energy landscapes that dictate physics of glassy systems, with tangible consequences on jamming, yielding, and beyond.
2017/12/07		Thomas Barthel  (Duke University) "Fractal structures in the phase diagram of the honeycomb-lattice quantum dimer model" Quantum dimer models appear in different contexts when describing dynamics in constrained low-energy manifolds, such as for frustrated Ising models in weak transverse fields or Heisenberg magnets with quantum disordered ground states. In this talk, I address a particularly interesting case, where a quantum dimer model on the honeycomb lattice, in addition to the standard Rokhsar-Kivelson Hamiltonian, includes a competing potential term, counting dimer-free hexagons. It has a rich phase diagram that comprises a cascade of rapidly changing flux quantum numbers. Here, the flux corresponds to the average tilt in the height representation of the model. The cascade of phase transitions is partially of fractal nature ("devil's staircase") and the model provides, in particular, a microscopic realization for the Cantor deconfinement scenario. We have studied the system by means of quantum Monte Carlo simulations and the results can be explained using perturbation theory, RG, and variational arguments in terms of string configurations. [1]  E. Fradkin, D. A. Huse, R. Moessner, V. Oganesyan, S. L. Sondhi, PRB 69, 224415 (2004) [2]  T. M. Schlittler, T. Barthel, G. Misguich, J. Vidal, R. Mosseri, PRL 115, 217202 (2015) [3]  T. M. Schlittler, R. Mosseri, T. Barthel, PRB 96, 195142 (2017)
2017/11/16		Aashish Clerk  (University of Chicago) "Photonic analogues of topological superconductors" Interest continues to grow in photonic analogues of topological electronic phases. In most cases, these systems are non-interacting, and have the same band structure and edge state structure as their fermionic counterparts. In this talk, I'll discuss recent theory work in my group showing how parametric "two-photon" driving can be used to realize a new class of photonic topological systems that superficially resemble topological superconductors. Unlike standard particle-number conserving models of non-interacting topological phases, these new systems exhibit crucial differences between their bosonic and fermionic versions. Further, one can realize a situation where all bulk states are stable, but where edge states are guaranteed to be unstable. Such a system can form the basis of a useful device: a topologically-protected amplifier which operates close to the fundamental limits set by quantum mechanics. I'll discuss how these ideas could be realized in a variety of different experimental platforms, including superconducting quantum circuits and optomechanics. Host: Harold Baranger
2017/10/26		Dmitri Khveshchenko  (UNC Chapel Hill) "Thickening and sickening the Sachdev-Ye-Kitaev model" We discuss higher dimensional generalizations of the 0+1-dimensional Sachdev-Ye-Kitaev (SYK) model. Unlike the previous constructions where multiple SYK copies would be coupled via some spatially short-range one- and/or q-particle random hopping processes, this study focuses on the algebraically varying long-range (spatially and/or temporally) correlated random couplings in d+1 dimensions. Such pertinent topics as translationally-invariant strong-coupling solutions, emergent reparametrization symmetry, effective action for fluctuations, and chaotic behavior (or a lack thereof) are addressed. We find that the most important properties of the original SYK model that make it an appealing example of some potentially exact holographic correspondence do not survive the aforementioned generalizations, thus providing no immediate support for the hypothetical generalized holographic conjecture. Host: Thomas Barthel
2017/10/12		Giuseppe Calajo  (TU Wien) "Atom-light interactions in slow-light waveguide QED" Slow-light waveguide QED refers to a regime where the maximal photonic group velocity is significantly reduced compared to free space. Such conditions could be achieved in one-dimensional photonic periodic structures such as photonic crystals, coupled-resonator arrays, modulated fibres, etc. In this talk I will describe the main features of atom-light interactions in this slow-light regime. In the first part I will discuss the bound states formed by an atom and a localized photonic excitation that represent the continuum analog of the familiar dressed states in single-mode cavity QED. The extension to the multi-photons and multi-atoms case is also considered. In the second part I will describe the coupling of moving atoms to such slow-light waveguide in the regime where the atomic velocities are comparable to the effective speed of light. In this case, the interplay between a velocity-induced directionality and the emergence of new divergences in the photonic density of states gives rise to a range of novel phenomena and nonperturbative effects in the emission of photons and the resulting photon-mediated interactions between moving atoms. Host: Harold Baranger
2017/09/21		Kirill Shtengel  (University of California Riverside) "Designer non-Abelian anyons and exotic quantum circuitry" Non-Abelian anyons are widely sought for the exotic fundamental physics they harbour as well as for their possible applications for quantum information processing. Currently, there are numerous blueprints for stabilizing the simplest type of non-Abelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a so-called "Majorana wire" can be made by judiciously interfacing readily available materials; the experimental evidence for the viability of this approach is presently emerging. Following this idea, we introduce a device fabricated from conventional fractional quantum Hall states and s-wave superconductors. Similarly to a Majorana wire, the ends of our "quantum wire" would bind "parafermions", exotic non-Abelian anyons which can be viewed as fractionalized Majorana zero modes. In this talk, I will briefly discuss their properties and describe how such parafermions can be used to construct new and potentially useful circuit elements which include current and voltage mirrors, transistors for fractional charge currents and "flux capacitors". Host: Gleb Finkelstein
2017/09/07		Elbio R. A. Dagotto  (University of Tennessee and ORNL) "Computational studies of iron-based high critical temperature superconductors" Fermi surface nesting guided the initial theoretical studies of iron-based high critical temperature superconductors but evidence is accumulating that these materials are more complex than previously anticipated. For example, competition between antiferromagnetic and ferromagnetic tendencies leads to frustration and unusual magnetic states. In this framework, the "iron ladders" is an area of research that is receiving considerable attention and will be the primary focus of my presentation. The two-leg ladder compound BaFe2S3 is the only member of the iron-based family that becomes superconducting (at high pressure) without having iron layers in its crystal structure [1,2]. Recent theoretical results using the density matrix renormalization group for a two-orbital Hubbard model applied to both ladders and chains [3,4] will be discussed. They correctly reproduce the dominant magnetic order and have revealed intriguing indications of pairing tendencies at intermediate/strong Hubbard couplings upon doping. Results for the dynamical spin structure factor of ladders will also be presented [5]. Time allowing, other exotic states of iron superconductors will be briefly reviewed such as the orbital selective Mott phase [6]. Also the much discussed spin nematic regime will be briefly addressed from the perspective of Spin-Fermion model Monte Carlo simulations including lattice orthorhombic and monoclinic distortions [7,8,9]. [1]  H. Takahashi et al., Nat. Mater. 14, 1008 (2015) [2]  T. Yamauchi et al., PRL 115, 246402 (2015) [3]  N. D. Patel et al., PRB 94, 075119 (2016) [4]  N. D. Patel et al., PRB 96, 024520 (2017) [5]  A. Nocera et al., arXiv:1707.02626 (2017) [6]  J. Rincon et al., PRL 112, 106405 (2014) [7]  S. Liang, A. Moreo and E. Dagotto, PRL 111, 047004 (2013) [8]  S. Liang et al., PRB 92, 104512 (2015) [9]  C. B. Bishop et al., PRL 117, 117201 (2016) Host: Harold Baranger
2017/09/05		Václav Janiš  (Charles University in Prague, Academy of Sciences of the Czech Republic),   Unusual weekday: Tuesday! "Quantum dot attached to superconducting leads: Andreev bound states and 0-pi transition" Carbon nanotubes with pronounced electron repulsion and well separated energy levels attached to superconducting leads display an impurity quantum phase transition at low temperatures from a spin singlet to a spin doublet state (0-pi transition). The transition is accompanied by a change of the sign of the Josephson current through the nanotube and by a crossing of the Andreev bound (Yu-Shiba-Rousinov gap) states. The quantum critical behavior is strongly influenced by electron correlations on the dot and reflects interplay of superconductivity and Kondo physics. We discuss a microscopic description of experimentally realized nanoscopic Josephson junctions via a single impurity Anderson model in a superconducting environment. We review the present status of the many-body perturbation theory and its ability to describe reliably the 0-pi transition. We compare various approximate solution resulting from the diagrammatic expansion with numerically exact methods, numerical renormalization group and continuous-time quantum Monte Carlo. We analyze the crossing of the Andreev bound states by splitting the spin symmetry and lifting the degeneracy of the pi phase by applying an external magnetic field. Host: Thomas Barthel
2017/05/25		Journal club ft. Gu Zhang "Transport through many-terminal Majorana islands" [1]  K. Michaeli, L. A. Landau, E. Sela, L. Fu, arXiv:1608.00581 [2]  L. Herviou, K. Le Hur, C. Mora, Phys. Rev. B 94, 235102 (2016)
2017/05/11		Journal club ft. Xin Zhang "Topology of density matrices" [1]  J. C. Budich, S. Diehl, Phys. Rev. B 91, 165140 (2015) [2]  A. Collinucci, A. Wijns, arXiv:hep-th/0611201
2017/04/20		Zhenzhong Shi  (National High Magnetic Field Lab) "Hidden order of Cooper pairs in a striped cuprate at high magnetic fields" The electronic phase diagrams of many highly correlated systems, and in particular the cuprate high temperature superconductors, are complex, with various types of charge and spin order competing each other. Recent progress in the field has revealed a ubiquitous presence of periodic modulations of charge density in all underdoped cuprate superconductors. The interplay of charge order with superconductivity in these materials under extreme conditions of high magnetic fields, however, remains an open question. We have investigated the nature of the magnetic-field-driven superconducting-normal phase transitions in La1.7Eu0.2Sr0.1CuO4, a superconductor with a "striped" charge order present already in zero field, under extreme conditions (T down to 0.016 K and H up to 45 T). To this end, a variety of linear and non-linear transport measurements has been performed. The results reveal a full sequence of states in the zero-temperature limit as a function of magnetic field: a superconductor, a wide regime of superconducting phase fluctuations or a vortex liquid, which is a superconductor only at T = 0, and a high-field normal state. Within the vortex liquid, an unanticipated, domelike (T, H) region of insulating, hysteretic behavior emerges below about 60 mK. The data are consistent with a novel type of ordering of localized Cooper pairs within the charge stripes in CuO2 planes. Despite not being predicted, the ordering of Cooper pairs in the domelike (T, H) region is not inconsistent with theoretical proposals, such as the putative pair-density-wave or Wigner crystal of Cooper pairs. Host: Sara Haravifard
2017/04/06		Subir Sachdev  (Harvard University) "Confinement transitions of Ising gauge theories" Wegner showed that the Ising lattice gauge theory in 2+1 dimensions has a confinement transition between confining and deconfined states. He also argued that this transition is in the universality class of the 3-dimensional Ising ferromagnet. I will begin with a review and update of these results, using the modern perspective of topological order and deconfined criticality. The confinement transition can be defined precisely also in the presence of matter fields, and I will discuss the relevance of such models to the physics of the hole-doped cuprates near optimal doping. Host: Shailesh Chandrasekharan
2017/03/30		Leo Fang  (Duke University) "Non-Markovian dynamics of a qubit due to photon scattering in a waveguide" Recently there is a revival of interest in non-Markovian (NM) dynamics as memory effect is not negligible in many quantum systems. Cavity quantum electrodynamics (QED), for example, has been used as a testbed for NM studies in both theory and experiment. In this talk I propose and discuss in detail another NM model system in the context of waveguide QED that is solvable and readily implementable using superconducting circuits. Specifically, I consider a qubit coupled to a 1D semi-infinite waveguide that has a perfect mirror at the end, which creates a feedback loop and thus a finite time-delay. In order to understand and to quantify the non-Markovianity of this system, a time-dependent calculation is inevitable. I present the exact solution for single-photon scattering, which complements the well-known result of spontaneous emission. In the two-excitation sector, a 1+1D delayed PDE emerges which I solved by adapting the finite-difference time-domain (FDTD) method. Combining all time-dependent wavefunctions in both 1- and 2-excitation sectors, the dynamical map for the qubit subject to a single-photon wavepacket can be constructed, from which NM measures can be calculated. It turns out that there are two kinds of NM sources: wavepacket and time-delay. The effect of the former is most transparent in the limit of small qubit-mirror separation L and finite wavepacket width w, while the latter is more important for finite L and small w. Finally, I will briefly address the open question on how non-Markovianity is affected by by a structured environment.
2017/03/23		Francesco Ciccarello  (University of Palermo, NEST, Duke) "Quantum 'non-Markovianity': new definitions and some recent developments" While in classical physics the notion of what is Markovian or not is well defined, this is not the case when it comes to open quantum systems. What makes a quantum dynamics Markovian or non-Markovian (NM)? Traditional answers to this question involve the well-known Lindblad master equation (ME) and/or the ability of an open dynamics to be governed by a time-local ME. The last few years, yet, witnessed a thorough revision of such concepts in the attempt to establish in a rigorous way exact criteria for assessing whether or not, and even quantifying, a dynamics is NM. This resulted in a number of NM "measures" that have been put forward. Based on these and a number of further theoretical progresses, the lack of a Lindblad ME or time-local ME turns out not to be a reliable criterion. For instance: there are Markovian dynamics that are not described by a Lindblad ME. On the other hand, open dynamics that are strongly NM can often be shown to fulfill a time-local ME. After reviewing all these new concepts in depth, I will discuss a specific instance of application of non-Markovianity measures from my latest research work: an atom emitting into a disordered medium [1]. In such a case, the occurrence of localized field modes due to Anderson localization makes the atom dynamics non-Markovian, as confirmed by the behavior of non-Markovianity measures against the amount of disorder. The functional shape of this relationship is well reproduced by an effective phenomenological model. [1]  S. Lorenzo, F. Lombardo, F. Ciccarello, and G. M. Palma, Sci. Rep.7, 42729 (2017) Host: Harold Baranger
2017/02/16		Po-Hao Huang  (Boston University) "Coupled spin-1/2 ladders as microscopic models for non-Abelian chiral spin liquids" Topologically ordered states of matter have attracted much interest in the past decade. The search for emergent point-like non-Abelian anyons in two-dimensional electronic system is one of the most exciting problems in condensed matter physics, since it opens many possibilities in quantum computing. In this talk, I will construct a two-dimensional lattice model that is argued to realize a chiral spin liquid with Ising topological order. The logic of the construction is to prepare an array of one-dimensional spin ladders with Ising criticality, then couple them into the two-dimensional chiral spin liquid with spin-spin interactions that breaks time-reversal symmetry. This coupled wire construction takes advantage of describing the one-dimensional spin system by conformal field theory. I will discuss mainly the theoretical proposal while provide some numerical evidence to support it. Host: Harold Baranger, Thomas Barthel
2017/01/12		Dong Liu  (Microsoft Station Q) "Majorana Search: Challenges and opportunities" Topological materials provide a protection from decoherence at the hardware level by using emergent non-Abelian anyons. The simplest non-Abelian anyon involves a defect that binds a Majorana zero-energy mode (MZM), predicted to appear quite naturally in certain superconducting systems. I will discuss the challenges and difficulties in the detection of Majorana zero modes, and then introduce a simple measurement scheme to overcome the problem, and show robust, clear, and universal experimental signatures of MZMs. I will also discuss a serious type of errors in topological quantum computation and Majorana qubit: Diabatic corrections only vanish as a power-law function with the length of time for the braid. This power-law behavior can wash out the advantages of topological quantum computation. We found that such diabatic errors can be detected and corrected by applying a sequence of parity measurements. Finally, I will discuss an interesting Majorana-impurity model, which provides an opportunity to study the competition between different correlations. Specifically, we consider a spinful quantum dot couples to a Majorana Kramer's pair, and investigate the effect of local repulsive interactions leading to an interplay between Kondo and Majorana correlations. We discovered and characterized several novel phases. Host: Harold Baranger
2016/12/01		Alexander Kemper  (NC State University) "Understanding complex materials using nonequilibrium spectroscopy: What can theory tell?" A powerful method to study the interactions between electrons and bosons in high-Tc superconductors is the measurement of the single-particle spectral function. The recent development of time-resolved ARPES (tr-ARPES) has allowed this measurement of be performed out of equilibrium, where the material is driven by an ultrafast laser pump pulse. We have developed a theoretical framework to complement to these experiments, and here we report on several aspects of electron-boson coupling out of equilibrium. First, we will illustrate how time-resolved spectroscopy can be used to study the coupling between electrons and phonons observing the decay rate of the transient signals as a function of energy, momentum, and time. A sufficiently strongly coupled phonon will exhibit a signature in the tr-ARPES spectra as both a kink in the dispersion as well as a sharp change of the decay rates, and we will discuss how these effects appear out of equilibrium. [1,2] Second, we will focus on the return to equilibrium in systems with multiple interaction types, and show that there are two distinct types of scattering processes: those types of interactions that conserve the energy within a subsystem, and those that do not. While in equilibrium these two contribute equally to the linewidth, we will show that out of equilibrium they behave differently -- the first type are mainly responsible for thermalization within the electronic subsystem, whereas the second type drain the energy out. As a result, the scattering rates out of equilibrium can be vastly different from the linewidth, and the features of the second type of interactions can be clearly observed. [3,4] In addition, I will present some aspects of non-equilibrium physics in BCS superconductors. We solve the Nambu-Gorkov equations for superconductivity within the Migdal-Eliashberg approximation, obtaining a full dynamic description of non-equilibrium BCS superconductivity. The temporal behavior after a pump exhibits characteristic 2D oscillations, which we attribute to the Higgs, or amplitude mode [5]. Finally, motivated by recent experiments [6], I will illustrate how superconductivity can be enhanced or suppressed through non-linear phononics. By modifying the physical parameters, we can model the driving of a lattice distortion, leading to an enhanced Tc [7]. [1]  M. Sentef et al., Phys. Rev. X 3, 041033 (2013) [2]  A.F. Kemper et al., Phys. Rev. B 90, 075126 (2014) [3]  S.L. Yang et al., Phys. Rev. Lett. 114, 247001 (2015) [4]  J. Rameau et al., submitted to Nature Communications [5]  A.F. Kemper et al., Phys. Rev. B 92, 224517 (2015) [6]  M. Först et al., Nature Physics 7, 854 (2011) [7]  M. Sentef et al., Phys. Rev. B 93, 144506 (2016) Host: Thomas Barthel
2016/11/22		Informal seminar ft. Ben Lawson  (University of Michigan),   Unusual time: Tuesday, 1:30pm! "Torque magnetometry on topological superconductor candidate doped Bi2Se3" Bi2Se3 is a known topological insulator. When doped with Cu or Nb, this compound becomes superconducting. Doping Bi2Se3 has been one of the leading avenues to search for topological superconductivity. Here I will present toque magnetometry studies on topological superconductor candidates, Cu-doped and Nb-doped Bi2Se3. Quantum oscillations were observed Cu-doped and Nb-doped Bi2Se3. In both compounds, the angular dependence of the oscillation period follows the prediction for an ellipsoidal Fermi surface. In the Cu-doped compound, the Fermi velocity is unchanged from Bi2Se3 implying it has a Dirac electronic band. In Nb-doped Bi2Se3, two quantum oscillations frequencies are observed indicating a multi-orbit electronic state distinct from its parent compound. In addition, the magnetic response in the superconducting state of the Nb-doped compound is enhanced along one direction confirming the presence a nematic superconducting order, which has been predicted to be strong evidence of odd-parity topological superconductivity. Host: Sara Haravifard
2016/11/22		Informal seminar ft. Elbio R. A. Dagotto  (University of Tennessee and ORNL),   Unusual time: Tuesday, 9:30am! Current research topics from condensed matter theory and materials science. Host: Thomas Barthel, Shailesh Chandrasekharan
2016/11/15		Kin Chung Fong  (Raytheon BBN Technologies),   Unusual weekday: Tuesday! "Listening to the noise of Dirac fluid in graphene" Interactions between the Dirac fermions in graphene can lead to new collective behavior described by hydrodynamics. At high temperature near the neutrality point, using high frequency, wide bandwidth Johnson noise thermometry, we find a strong enhancement of the thermal conductivity and breakdown of Wiedemann-Franz law in graphene. This is attributed to the non-degenerate electrons and holes forming a strongly coupled Dirac fluid, as predicted in hydrodynamic theory. At low temperature, the Dirac fermions are in extreme thermal isolation with minute specific heat that can be exploited for ultra-sensitive photon detection. We will present our latest experimental result towards observing single microwave photons and explore its role in scaling up the superconducting qubit systems. Our model suggests the graphene-based Josephson junction single photon detector can have a high-speed, negligible dark count, and high intrinsic efficiency for applications in quantum information science and technologies. [1]  J. Crossno et al., Science 351, 1058 (2016) Host: Jungsang Kim
2016/11/10		Shiwei Zhang  (College of William and Mary) "Computing electron correlation effects: an auxiliary-field perspective" Understanding the properties of strongly interacting quantum matter remains a grand challenge in physical sciences. Computation has an integral role to play in tackling this challenge. I will give an introduction to the fundamental issues facing accurate and predictive computations of quantum systems, and then describe recent progress in combining field-theory and Monte Carlo simulations for computations in many-fermion systems. This framework can be used for ab initio materials simulations as well as lattice model studies. As an example of the former, results will be presented on the binding and magnetic properties of Cobalt adatom on graphene, a setup that has drawn interest for possible spintronics applications. As an example of lattice model calculations, we determine ground-state properties of the Hubbard model, which is important in the context of high-Tc superconductivity and whose laboratory emulation is one of the major goals for optical lattice experiments. Host: Jianfeng Lu
2016/10/17		Denis Ullmo  (LPTMS, Université Paris Sud),   Unusual weekday: Monday! "Schrödinger approach to Mean Field Games" Mean field games theory is a recent research area, at the frontier between applied mathematics, social sciences, and physics. It was initiated a decade ago by Pierre-Louis Lions and Jean-Michel Lasry as a tool to model certain social phenomena involving a significant number of actors - through a game theory approach - while maintaining a reasonable level of simplicity thanks to the concept of mean-field imported from physics. After a general introduction to mean field games, I will show that there is a formal, deep link between an important class of these models and the nonlinear Schrödinger (or Gross-Pitaevskii) equation encountered in many circumstances in physics, and which describes in particular the evolution of a set of interacting bosons. This link makes it possible to develop highly effective approximation schemes to solve the mean field game equations. I will show in particular how to obtain in this way an intuitive and detailed understanding of a population dynamics model wherein the agents are under a strong incentive to coordinate themselves. [1]  I. Swiecicki, T. Gobron, D. Ullmo, Phys. Rev. Lett. 116, 128701 (2016) Host: Harold Baranger
2016/10/11		Yoshitomo Kamiya  (CMT Laboratory, RIKEN),   Unusual weekday: Tuesday! "Multiferroics by design with frustrated molecular magnets" Geometric frustration in Mott insulators permits perturbative electron fluctuations controlled by local spin configurations [1]. An equilateral triangle ("trimer") of spins with S=1/2 is the simplest example, in which low-energy degrees of freedom consist of built-in magnetic and electric dipoles arising from the frustrated exchange interaction. Such trimers can be weakly coupled to make multiferroics by design [2]. An organic molecular magnet known as TNN [3], with three S=1/2 nitronyl nitroxide radicals in a perfect C3 symmetric arrangement, is an ideal building block as demonstrated by recent experiments on a single crystal comprising TNN and CH3CN. The fascinating thermodynamic phase diagram of this molecular crystal, TNN-CH3CN, is in excellent agreement with our theory, which predicts multiferroic behavior and strong magnetoelectric effects arising from an interplay between magnetic and orbital degrees of freedom [4]. Our study thus opens up new avenues for designing multiferroic materials using frustrated molecular magnets. [1]  L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008) [2]  Y. Kamiya and C. D. Batista, Phys. Rev. Lett. 108, 097202 (2012) [3]  Y. Nakano, T. Yagyu, T. Hirayama, A. Ito, K. Tanaka, Polyhedron 24, 2147 (2005) [4]  Y. Kamiya et al., in preparation. Host: Sara Haravifard
2016/09/29		Carlos J. Bolech  (University of Cincinnati) "Consistency between bosonization and nonequilibrium transport setups" In this talk I will cover two different scenarios in which the technique of bosonization, as well as the reversed process of de-bosonization, are applied to problems that involve transport calculations across junctions. The case of a junction between two Fermi liquids will be used to illustrate how the use of bosonization in the conventional ways can give rise to results which are not fully consistent with the exact direct solution of the problem. The differences are not only quantitative, but for certain aspects the two solutions are also qualitatively different. We recently proposed a consistent way of proceeding [1] that allows to recover the same results as with the direct solution but after the re-fermionization of the problem. This takes the form of additional considerations related to the regularization of fermionic bilinears that supplement the conventional bosonization procedure and can be carried over to the study of strongly correlated systems like, for instance, the Kondo problem. In the second part of this presentation I will discuss a Kondo junction [2], which is a simplified low-energy model for transport applicable to certain types of quantum dots, molecular junctions, and other specialized transport setups. The so called Toulouse limit of this problem is mappable (using a re-fermionization based on bosonic field transformations combined with a previous bosonization and a subsequent de-bosonization) to the problem of a non-interacting resonant level model. The new considerations to seek consistency are seen to preserve the existence of the Toulouse limit but render it more complex, and I will argue they significantly modify the results for physical observables like the differential conductance of the junction. [1]  N. Shah and C. J. Bolech, Phys. Rev. B 93, 085440 (2016) [2]  C. J. Bolech and N. Shah, Phys. Rev. B 93, 085441 (2016) Host: Harold Baranger
2016/09/22		Siyuan Dai  (UC San Diego) "Hyperbolic phonon polaritons in hexagonal boron nitride" Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [1]. Additionally, we carried out the modification of hyperbolic response in meta-structures comprised of a mononlayer graphene deposited on hBN [2]. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the "hyperlens" for subdiffractional focusing and imaging using a slab of hBN [3]. [1]  S. Dai et al., Science 343, 1125 (2014) [2]  S. Dai et al., Nature Nanotechnology 10, 682 (2015) [3]  S. Dai et al., Nature Communications 6, 6963 (2015) Host: Maiken Mikkelsen
2016/07/18		Huaixiu Zheng  (Yale University),   Unusual weekday: Monday! "Quantum state smoothing for single microwave photon detection" We propose a circuit-QED-based single-microwave-photon detector for dark matter axion searches. The strong coupling between a superconducting qubit and microwave photons makes it possible to detect the extremely weak axion signal with a high sensitivity. With realistic experimental parameters, the single-photon detector has a detection efficiency 99.5% and a very low dark count of 0.5% of the thermal noise. At low temperatures, the single-photon detector can outperform linear amplifiers by several orders of magnitude in the signal-to-noise ratio (SNR). This improved SNR could significantly speed up the search for axion particles. Beyond axion searches, the proposed single-photon detector can become an important component of circuit-QED toolbox and find direct applications in experiments such as remote entanglement. Host: Harold Baranger
2016/04/07		Waseem S. Bakr  (Princeton University) "Strongly-interacting fermionic superfluids: spin-imbalance, lower dimensionality and topological defects" Ultracold atomic gases are a pristine platform for studying the physics of strongly-correlated quantum systems. Motivated by the search for exotic forms of superfluidity, we have studied a two-component, strongly-interacting two-dimensional Fermi gas with spin imbalance. The low-temperature phase diagram of such a gas may include several interesting phases, including conventional superfluids, Sarma or FFLO states as well as Fermi liquid phases. We have observed phase separation in the trapped gas between a spin-balanced condensed phase, a partially polarized phase and fully polarized normal gas. We have mapped out the behavior of the gas at different polarizations and interaction strengths across the BEC-BCS crossover. The lower dimensionality enhances the stability of exotic superfluid phases like the FFLO phase, a superfluid with a spatially modulated gap, whose direct signatures we are well-equipped to search for using a microscope capable of detecting single atoms in the gas. In the BEC limit, the FFLO phase can be thought of as a soliton train in the superfluid. Connecting to this idea, I will describe experiments where we have imprinted solitons onto a fermionic superfluid and observed their motion in the trap and their decay into other topological excitations. Host: Thomas Barthel
2016/03/24		Konstantin Matveev  (Argonne National Lab) "Electrical and thermal transport in inhomogeneous Luttinger liquids" We study the transport properties of long quantum wires by generalizing the Luttinger liquid approach to allow for the finite lifetime of the bosonic excitations. Our theory accounts for long-range disorder and strong electron interactions, both of which are common features of experiments with quantum wires. We obtain the electrical and thermal resistances and thermoelectric properties of such quantum wires and find a strong deviation from perfect conductance quantization. We cast our results in terms of the thermal conductivity and bulk viscosity of the electron liquid and give the temperature scale above which the transport can be described by classical hydrodynamics. Host: Stephen Teitsworth
2016/03/21		Hao Zhang  (TU Delft),   Unusual weekday: Monday! "Majorana zero modes in InSb nanowires" Majorana modes in hybrid superconductor-semiconductor nanowire devices can be probed via tunneling spectroscopy which shows a zero bias peak (ZBP) in differential conductance [1]. However, alternative mechanisms such as disorder or formation of quantum dots can also give rise to ZBPs, and obscure experimental studies of Majoranas. Further, a soft induced superconducting gap commonly observed in experiments presents an outstanding challenge for the demonstration of their topological protection. In this talk we show that with device improvements, we reach low-disorder transport regime with clear quantized conductance plateaus and Andreev enhancement, approaching the theoretical limit. Tunneling spectroscopy shows a hard induced superconducting gap without any formation of quantum dots. Together with extremely stable ZBPs observed in large gate voltage and magnetic field ranges, we exclude various alternative theories besides the formation of localized Majorana modes for our observations. Majoranas are formed when the Zeeman energy E_Z and the chemical potential mu satisfy the condition E_Z>(Delta^2+mu^2)^(1/2), with Delta the superconducting gap. This Majorana condition outlines the topologically non-trivial phase and predicts a particular dependence of ZBPs on the gate voltage (chemical potential) and the external magnetic field (Zeeman). Our gate voltage and magnetic field dependence of ZBPs map out a ZBP phase diagram which is consistent with the Majorana topological phase diagram. [1]  V. Mourik, K. Zuo, S.M. Frolov, S.R. Plissard, E.P.A.M. Bakkers, L.P. Kouwenhoven, Science 336, 1003 (2012) Host: Albert Chang
2016/03/10		Eduardo Novais  (Universidade Federal do ABC, Brazil) "Fidelity of a quantum state protected by the surface code at finite temperatures" A fundamental challenge to quantum information processing is to protect information from the detrimental effects of the environment. A milestone in addressing this problem was the development of the theory of quantum error correction (QEC). In this work, we build upon previous studies in order to evaluate the fidelity of a quantum state protected by one of the best QEC codes: the surface code. We discuss the protection of a quantum state for different spectral functions, bath temperatures and QEC cycles times. Analytical results are supported by finite size scaling analyses of Monte Carlo and exact diagonalization of finite lattices. Our results demonstrate a finite threshold that explicitly depends on the bath mediated qubit-qubit interaction range and temperature of the bath. Host: Harold Baranger
2016/02/29		Journal club ft. Moritz Binder,   Unusual weekday & place: Monday, Physics 205! "Identification of Hamiltonians and Liouvillians" [1]  J. Zhang and M. Sarovar, Phys. Rev. Lett. 113, 080401 (2014) [2]  J. Zhang and M. Sarovar, Phys. Rev. A 91, 052121 (2015)
2016/02/25		Adriana Moreo  (University of Tennessee, ORNL) "Robust nematic state in electron-doped pnictides induced by isotropic quenched disorder" The phase diagram of electron-doped pnictides as a function of temperature, electronic density, and isotropic quenched disorder strength obtained by means of computational techniques applied to a three-orbital (xz, yz, xy) spin-fermion model with lattice degrees of freedom will be presented. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. The use of computational techniques allows to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, Monte Carlo simulations reveal that the fast reduction with doping of the Néel T_N and the structural T_S transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range considered, changes in the Fermi Surface produced by electron doping affect only slightly both critical temperatures. Comparisons with STM and neutron scattering experiments will be presented. [1]  S. Liang, C. Bishop, A. Moreo, and E. Dagotto, Phys. Rev. B 92, 104512 (2015) Host: Harold Baranger
2016/02/11		Arnab Banerjee  (Oak Ridge National Laboratory) "In search of exotic fermions in spin-liquids - the case of a-RuCl3" In 2006, Alexei Kitaev proposed that certain 2D honeycomb magnets with a strong spin-orbit coupling in the low-spin (S=1/2) ground state and a high degree of bond anisotropy will form a quantum-spin-liquid (QSL). This QSL is special since its excitations can be solved exactly to predict the presence of 2D Majorana Fermions. If realized in a solid-state material, these hold the promise for the technology of lossless topological qubits that survive to high temperatures. In this talk, I will present the search of these elusive quasi-particles in a two-dimensional (2D) honeycomb magnet a-RuCl3. Using a combination of thermodynamic, diffraction and neutron-scattering measurements in powder and single-crystal samples, we delve in detail the ground-state in the material, and show that this material has the ingredients necessary for satisfying the Heisenberg-Kitaev Hamiltonian. A spin-wave analysis yields that anisotropic Kitaev interactions are indeed the leading-order coupling in this material. Although the ground state is antiferromagnetic at lowest temperatures, this material shows a broad excitation spectrum independent of the long-range ordering, - which could not be explained with standard classical theories. Excitingly, when compared with the exact quantum calculations of the pure Kitaev QSL ground state, the data matches the broad fractionalized excitations expected from itinerant Majorana Fermions. Host: Sara Haravifard
2016/01/21		Gu Zhang  (Duke University) "Rescuing a quantum phase transition with quantum noise" We show that placing a quantum system in contact with an environment can enhance non-Fermi-liquid correlations, rather than destroying quantum effects as is typical. The system consists of two quantum dots in series with two leads; the highly resistive leads couple charge flow through the dots to the electromagnetic environment (noise). The similarity to the two impurity Kondo model suggests that there will be a quantum phase transition between a Kondo phase and a local singlet phase. However, this transition is destabilized by charge tunneling between the two leads. Our main result is that sufficiently strong quantum noise suppresses this charge transfer and leads to stabilization of the quantum phase transition. We present the phase diagram, the ground state degeneracy at the four fixed points, and the leading temperature dependence of the conductance near these points.
2015/12/07		Journal club ft. Moritz Binder,   Unusual weekday: Monday! "Photonic quantum circuits with time delays" [1]  H. Pichler, P. Zoller, arXiv:1510.04646 (2015)
2015/11/19		Daniel Silevitch  (University of Chicago, Caltech) "Tunable ground states in a model quantum magnet" Changing the ground state of a system via a non-thermal parameter such as doping or pressure has been a topic of great interest in condensed matter physics for decades, most notably in the study of quantum phase transitions and the associated quantum critical regime. The rare-earth fluoride LiHoF_4 and the related dilution series LiHo_{1-x}Y_xF4, long known to be a realization of the S=1/2 Transverse Ising Model, can be used to study state tuning as a function of several tuning parameters. In addition to the well-understood field-induced ferromagnet to paramagnet quantum phase transition in pure LiHoF_4, there are a number of subtler transitions which are seen in the diluted materials. For x~=0.5, application of a magnetic field transverse to the Ising axis transitions the system into a realization of the Random-Field Ising Model, an important generic model for studying the effects of disorder. In the more highly dilution limit, at x~0.05, ordered ferromagnetic states are no longer stable due to frustration effects. Instead, the system shows behavior consistent with either a spin glass or a non-freezing spin liquid, and can be tuned between these two regimes by varying the degree of thermal connectivity with an external heat bath or by tuning the rate of quantum tunneling via an external magnetic field. Host: Sara Haravifard
2015/11/12		Catherine Marcoux  (Duke University) "Limit-periodic structures: self-assembly and sparse vibrational modes" Recent advances in techniques for synthesizing multivalent micron-sized particles have generated interest in the types of structures that can be formed through self-assembly. Simulations have shown that relatively simple building blocks can create complex structures, including crystals and quasicrystals. We are interested in the creation of a limit-periodic structure, which is ordered but nonperiodic and consists of a union of periodic lattices with ever-increasing lattice constants. We consider the possibility of forming a limit-periodic structure out of a collection of particles based on the Taylor-Socolar tile. We find through Monte Carlo simulations that identical hard disks with magnetic dipoles embedded around the perimeter spontaneously form the limit-periodic structure through a hierarchy of phase transitions even when the ground state is periodic. The initial phase transitions drive the system into a deep free energy well, preventing the formation of competing periodic phases. The possibility of the realization of the limit-periodic structure raises the question about the nature of its phonon modes. We use standard techniques to calculate the phonon modes of ball and spring models of periodic structures that contain elements of the limit-periodic pattern. We identify interesting sets of extended low-participation-ratio modes of the limit-periodic structure. We show that each set of modes forms a hierarchy and present a heuristic argument for the existence of the set with the simplest geometry.
2015/10/22		Adrian E. Feiguin  (Northeastern University, Boston) "Building and understanding magnetic nano-structures, one atom at a time" Understanding magnetism is a complex undertaking: it relies on our knowledge of the exact position of magnetic ions in a crystal and their interactions. More important, at its core, this is fundamentally a quantum problem. In general, knowledge of the magnetic properties of a single atom will not tell much about the magnetic properties of a material, and requires understanding the cooperative effects of many degrees of freedom, particularly the spin. Starting from the chemistry, "cooking" a single crystal with enough purity and without defects is already an enormous challenge. In addition, being able to "design" a material with the desired geometry and interactions is only possible in a few cases, usually using organic molecules as a fundamental building block. In the past decade we have witnessed enormous progress in experiments that consist of placing magnetic atoms at predetermined positions on substrates, and building magnetic nanostructures, one atom at a time. The electrons in the substrate mediate the interactions between the spins, and scanning tunneling microscopy allows one to study their properties. In order to understand these interactions, we rely on a theory developed decades ago by Ruderman, Kittel, Kasuya, and Yosida, dubbed "RKKY theory", which applies when the spins are classical. The quantum nature of the electronic spin introduces another degree of complexity, and competition with other quantum phenomena: the Kondo effect. This competition is quite subtle and non-trivial, and can only be studied by numerical means. We have found that there is a critical distance at which the Kondo effect dominates, translating into a finite range for the RKKY interaction. We study this mechanism on different lattice geometries in 2 and 3 dimensions by introducing an exact mapping onto an effective one-dimensional problem that we can solve with the density matrix renormalization group method (DMRG). We show a clear and departure from the conventional RKKY theory, and important differences that can be attributed to the dimensionality and geometry. In particular, for dimension d>1, Kondo physics dominates even at short distances, while the ferromagnetic RKKY state is energetically unfavorable, which can have important implications in our understanding of heavy fermion materials and magnetic semiconductors. Host: Thomas Barthel
2015/10/08		Christopher Wilson  (University of Waterloo) "Quantum electrodynamics in 1D using a superconducting artificial atom" We have studied the strong coupling of a single artificial atom, formed from a superconducting qubit, to an open transmission line. This produces an almost ideal 1D quantum electrodynamic system. In a series of experiments, we have already demonstrated a number of interesting physical effects. For instance, we demonstrated that the coherent scattering of microwaves from the atom leads to the strong extinction (>99%) of the transmitted light, an effect long predicted but never observed in conventional quantum optics. In the same work, we demonstrated a rudimentary single-photon router in the microwave regime. In the next experiment, we proved that the microwaves scattered from the artificial atom are definitively nonclassical. We did this by demonstrating photon antibunching in the reflected signal, using a microwave photon statistics analyzer developed in our lab. In a third experiment, we demonstrated the giant cross-Kerr effect, an effective interaction between two photons mediated by the artificial atom. There has been great interest in the cross-Kerr effect for a number of potential applications such as photonic quantum gates and quantum nondemolition measurements (QND) of photons. In more recent experiments, we have studied the interaction of the artificial atom with its own image in a superconducting "mirror." In this talk, I will review these experiments and discuss prospects for future work. Host: Harold Baranger
2015/10/01		François Amet  (Appalachian State University) "Superconductivity in the quantum Hall regime" Combining superconductivity and the quantum Hall (QH) effect is a promising route for creating new types of topological excitations. Despite this potential, signatures of superconductivity in the quantum Hall regime remain scarce, and a superconducting current through a quantum Hall weak link has so far eluded experimental observation. Here we demonstrate the existence of a novel type of Josephson coupling through a QH region at magnetic fields as high as 2 Tesla. The supercurrent is mediated by states encompassing QH edge channels, which are flowing on the opposite sides of the sample. The edges are coupled together by the hybrid electron-hole modes at the interfaces between the QH region and the superconducting contacts. These chiral modes, which share some features with Majorana modes, are formed when electron and hole edge states are mixed by the superconductor. Host: Gleb Finkelstein
2015/09/24		Journal club ft. Gu Zhang "Fate of the Kondo effect and impurity quantum phase transitions through the lens of fidelity susceptibility" [1]  L. Wang, H. Shinaoka, M. Troyer, arXiv:1507.06991 (2015) [2]  A. Bayat, H. Johannesson, S. Bose, P. Sodano, Nature Comm. 5, 3784 (2014) [3]  B. Alkurtass, A. Bayat, I. Affleck, S. Bose, H. Johannesson, P. Sodano, E. S. Sørensen, and K. Le Hur, arXiv:1509.02949 (2015)
2015/09/10		Journal club ft. Leo Fang "A chemical potential for light" [1]  M. Hafezi, P. Adhikari, J. M. Taylor, arXiv:1405.5821 (2014) [2]  P. Wurfel, J. Phys. C 15, 3967 (1982)
2015/09/03		Journal club ft. Anne Watson "Hot-carrier Seebeck effect: Diffusion and remote detection of hot carriers in graphene" [1]  J. F. Sierra, I. Neumann, M. V. Costache, S. O. Valenzuela, Nano Lett. 15, 4000 (2015) [2]  Y. M. Zuev, W. Chang, P. Kim, Phys. Rev. Lett. 102, 096807 (2009)