Condensed Matter Seminars
2004 – 2005

Detailed studies of carbon nanotube physics and devices require independent control of nanotube parameters. It is particularly important to control electron confinement (quantum dot) and constriction (point contact) effects for applications such as quantum computation. In this talk, I will show how we achieve independent control of nanotube parameters via multiple electrostatic gates. Local gates can be used to fabricate and measure nanotube-based quantum dots and quantum point contacts. I will discuss how this control of a nanotube-based double quantum dot allowed us to manipulate and study single-electron charging effects as well as charge interactions within the nanotube. I will also discuss the appearance of quantized conductance steps when local gates are used to create point-contact-like constrictions in nanotubes.

Doping the Kondo insulator FeSi with Co at the Fe site leads to a ferromagnetic metal that is fully spin polarized below the Curie temperature. I will present transport, magnetic, and optical data that reveal that the transport in this material is highly senstive to external magnetic fields. All of these quantities reveal significant effects of quantum interference effects of the diffusively conducting carriers. As such the behavior is not like more standard magnetic semiconductors or half metallic materials.

We outline a general approach to the computation of transport properties of interacting systems at low temperetures and frequencies. We show that if the fixed point and the irrelevant operators around it are known, then by studying the structure of the softly violated conserved currents chracterizing the fixed point one may set up an effective calculation in terms of a memory matrix formalism. We apply this approach to the computation of thermal conductivity of spin chains and ladders embedded in a matter matrix and interacting with its phonons.  The results are found to be in very good agreement with experiment.

A density matrix renormalization group (DMRG) approach, has recently been developed by Shibata and Yoshioka to treat quantum Hall systems.  This computational method has the ability to treat larger system sizes then other accurate ³unbiased² numerical methods (i.e. the ³gold standard² in the field, direct diagonalization).  This opens up the possibility of accurate numerical treatment of compressible systems, for example, stripes, Wigner crystals and bubble states. In the first part of the talk, a review of the dmrg method suitable for a general audience will given and an introduction to the quantum Hall effect (in the context of semiconductor physics, no atomic gases!) will be provided.  The second part of the talk will present our preliminary numerical results for the DMRG applied to high Landau levels.

As the size scale of electronic devices continues to decrease, the dissipation of heat generated in device operation becomes a critical problem, and understanding the mechanism of heat transport in nanostructures becomes of increasing importance. This aspect of the physics of nanostructures is only beginning to be explored. In this talk, I will outline the techniques we have developed to perform thermal measurements on nanostructures, and the results of our experiments on thermal transport in devices incorporating normal-metals and superconductors, where a number of new and interesting effects are observed.

The experimental realization of Bose-Einstein condensation in a dilute atomic gas has inspired a lot of theoretical and experimental work on these systems. One of the many interesting properties of these systems is their suitably to engineer hamiltonians, well-known from condensed matter physics, in a very controllable way. Perhaps the best-known example along these lines is the use of an optical lattice to engineer the so-called Bose-Hubbard hamiltonian, and study its Mott-insulator to superfluid phase transition, as has been achieved recently. In this talk, I'll discuss other examples of ideas from condensed matter physics applied to the field of atomic gases. First, I'll show that a Bose-Einstein condensate in a co-rotating optical lattice provides an excellent system to study the pinning of vortices and structural transitions between different types of vortex lattices. After this, I will consider fermions and briefly discuss the possibility of experimentally realizing and detecting the Bardeen-Cooper-Schrieffer state, known from the theory of superconductivity. The latter state occurs for attractive interatomic interactions, and I'll also briefly discuss the possibility of realizing a ferromagnetic state for the case of repulsive interatomic interactions.

Recent advances have allowed the exploration of true one-dimensional electron transport in two new systems: carbon nanotubes and conjugated organic molecules.  In each case electrons are conducted through the extended -orbital network of carbon.  I will discuss recent experiments in my lab to determine the fundamental conduction properties of semiconducting carbon nanotubes, and recent results from a collaborative effort to synthesize, measure, and model transport through individual organometallic molecules.  Growth of very long (up to 1 millimeter), very clean semiconducting carbon nanotubes has allowed determination of the charge carrier mobility in this material.  The mobility may exceed 105 cm2/Vs at room temperature, higher than any other known semiconductor.  Schottky-barrier electrodes allow simultaneous injection of electrons and holes at high bias, with recombination in the nanotube.  A simple model allows determination of the saturation velocity of carriers in the nanotube of 2 x 107 cm/s, twice that in silicon.  We have studied the conduction through a ferrocene-based organometallic molecule, and, in excellent agreement with theoretical results, observe a Lorentzian resonance in the bias-dependent conduction with a peak differential conductance of up to 70% of Go, the theoretical maximum.  The results are in sharp contrast to those of our group and other groups on conjugated all-organic oligomers, where a high conductance resonance is also expected, but not observed experimentally.  We suggest some solutions to this dilemma.

We summarize experimental observations relating to novel photo-excited vanishing resistance states in the ultra high mobility 2DES at low temperatures, in a large filling factor limit. GaAs/AlGaAs Under microwave photoexcitation, GaAs/AlGaAs 2DES specimens exhibit vanishing diagonal resistance, without Hall resistance quantization, about B = 4/5 Bf and B = 4/9 Bf, where Bf = 2π f m*/e, m* is the electron mass, e is electron charge, and f is the EM-wave frequency, as the resistance-minima follow B = [4/(4j+1)] Bf with j=1,2,3… In this report, we illustrate the basic characteristics and highlight supplementary features, in light of recent theory for this remarkable effect.

This talk will give a review of STM-induced light emission from a variety of systems: metal surfaces, semiconductors, molecules etc. I will then focus on discussing light emission from metallic quantum well states, an effect displaying an interesting interplay between enhanced spontaneous light emission due to plasmon resonances and the electronic structure of the quantum well system, an Na overlayer on a Cu(111) surface.

Although the intriguing non-Fermi liquid signatures of the two-channel Kondo (2CK) effect, have been observed in several nanoscopic transport experiments, A clear microscopic model for its realization is still lacking even after a decade of research.
After a brief review of the experimental situation and the theoretical difficulties for the stabilization of the 2CK fixed point, a physical realization of the 2CK effect will be proposed in this talk, where a dynamical defect in a metal has a partially broken SU(3) symmetry with a unique ground state and twofold degenerate excited states. The defect can be comprised of an interstitial atom moving in a modulated Mexican hat potential, which is formed by the lattice. A perturbative renormalization group analysis shows that the coupling to the conduction electrons renormalizes the excited defect levels below the non-interacting ground state, thus stabilizing the 2CK fixed point. For a wide range of parameters the level crossing occurs in the weak coupling region. The model may explain, on the same footing, the 2CK zero-bias anomaly and the conductance spikes observed in ultrasmall metallic point contacts.