Harold U. Baranger
Gonzalo Usaj (post-doc)..................quantum dots and spintronics
Hong Jiang (student)........................DFT of real quantum dots
Serguei Vorojtsov (student).............effects of dot/lead coupling
Anand Priyadarshee (student)........correlations investigated using QMC
The interplay of electron-electron interactions and quantum interference has been a central theme in condensed matter physics over the last 40 years. This interplay plays a role in, for instance, dirty superconductors, localization of electrons, density-functional theory, the quantum Hall effect, multi-impurity many-body effects, heavy-fermion superconductivity, and various metal-insulator transitions. In mesoscopic physics--by which I mean the study of quantum coherent phenomena in solids on length scales much larger than single atoms--interactions have been largely neglected over the last 15 years of intensive study. This has been justified because of, first, the rich physics of non-interacting particles that was uncovered, and, second, the use of open structures made from relatively simple materials in which interaction effects were not expected to be crucial. However, as the field of mesoscopic physics turns to ever smaller structures of a wider variety of materials--thus becoming an important part of the larger area of Nanoscience--the interplay of interactions and interference has come to the fore in this area of condensed matter physics as well.
Conceptually, the main interest in this topic is the experimental control over and variety of many-body physics that nanosystems provide. The coupling between the nanosystem and the outside world can be tuned, the density of electrons can be changed, the form of the interaction can be somewhat modified, and by changing materials the relative strength of the different terms can be changed. Perhaps the most spectacular examples of this control is the recently observed Kondo effect in quantum dots and the as yet unexplained metal-insulator transition in two dimensions. Another particularly important type of control, is the ability to change size, from a few electrons to thousands, and thus the ability to study not only atomic-like effects but also the properties of large numbers of interacting particles.
On the other hand from a practical viewpoint, there are several aspects of this type of nanophysics which are interesting for potential avant-garde applications. First, the Coulomb blockade is the basis for the single electron transistor (SET) which has been proposed as a possible post-silicon technology. There are severe problems with reproducibility and control, however, which require smaller more uniform systems and a detailed understanding of the physics. My goal, then, is to understand how one might make SET's in molecular materials containing chemically defined ``grains''. Second, the combination of magnetic and electronic properties is a very active area with regard to both storage and functional devices--a topic dubbed ``spintronics''. While bulk and two-dimensional systems have been intensively investigated, nanostructures needed for high-density applications are only beginning to be studied. I would like to understand what controls the magnitude and character of spintronic effects at the nanoscale with an eye towards possible device innovation.
My research interests at this time (8/01) are concentrated on four topics in this general area of interactions and interference in nanosystems.
Several aspects of experiments on the Coulomb blockade in quantum dots remain
unexplained. The most intriguing is the observed distribution of spacings
between the peaks in the conductance as a function of gate voltage. This
is basically a measure of the compressibility of the dot--the change in the
ground-state energy as electrons are added. The experimental distribution
is broader than theoretical expectations and lacks spin structure. We are
approaching this problem using both analytical and numerical methods. Our
analytical approach is to combine a random matrix decription of the single
particle properties with a method developed in nuclear physics for adding
residual interaction effects to mean-field potential known as the Strutinsky
method. These analytical results will be compared to density functional theory
--The spin properties of magnetic
nanoparticles are a fascinating context in which to study interaction and
interference effects--exchange, correlation, and spin-orbit interactions
all play an important role. We are calculating the tunneling magnetoresistance
for nanoparticles of various types using simple models. The possible use
of density functional theory techniques for these issues is being evaluated.
Correlations: From Single
Particle to Many--We propose
to investigate interaction effects at several different scales. First, at
the smallest scale, several correlation effects seem more important for experiments
than expected--the formation of a Wigner crystal at the edge of an electron
gas, and a metal-insulator transition in two-dimensions, for instance. We
plan to address these issues as far as possible with the recently developed
cluster quantum monte carlo (QMC) techniques. Second, at the scale of a few
nano-systems, the interaction among particles leads to, for instance, entangled
states crucial for quantum computing. These can be studied with full DFT
techniques. Finally, it is critical to see how these effects scale into the
many nanoparticle limit. Using a simplified DFT technique developed recently
by W. Yang, we plan to address this issue in carbon nanotube systems.
New Types of Nanostructures:
is a field in transition. A set of systems and problems has been extensively
studied and the field is greatly expanding its view as it turns to other issues.
In particular, the rapid development of new synthesis techniques for creating
nanostructures, such as quantum dot growth by self-assembly or DNA-assisted
assembly, is opening up fundamentally new types of structures. I am interested
in studying the electronic and transport properties of these new structures
as they become available.
Last modified: 30-Sep-01