## Condensed Matter Seminar,
Regular time and place: Thursdays at 11:30am in Physics 298. |

2018/11/08 |
Nicholas P. Butch (NIST, UMD College Park)title and abstract t.b.a.Host: Sara Haravifard |

2018/11/15 |
Fabio Altomare (D-Wave Systems)title and abstract t.b.a.Host: Albert Chang |

2018/11/29 |
Arthur P. Ramirez (UC Santa Cruz)title and abstract t.b.a.Host: Harold Baranger |

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, K. Le Hur | |

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) |

have a nice day!