Condensed Matter Seminars
2003 – 2004

Deconfined quantum critical points
Prof. Senthil Todadri, MIT

The theory of second order phase transitions is one of the foundations of modern statistical mechanics and condensed matter theory. A central concept is the observable “order parameter,” whose non-zero average value characterizes one or more phases and usually breaks a symmetry of the Hamiltonian. At large distances and long times, fluctuations of the order parameter(s) are described by a continuum field theory, and these dominate the physics near such phase transitions. This talk will show that near second order quantum phase transitions, subtle quantum interference effects can invalidate this paradigm. A theory of quantum critical points in a variety of experimentally relevant two-dimensional antiferromagnets will be presented. The critical points separate phases characterized by conventional “confining” order parameters. Nevertheless, the critical theory is naturally expressed in terms of new emergent, “deconfined” degrees of freedom associated with fractionalization of the order parameters, along with an emergent gauge field. This new paradigm for quantum criticality may be the key to resolving a number of experimental puzzles in correlated electron systems.

Electron and Electron Spin Transport in Multi Wall Carbon Nanotubes
Prof. Bruce Alphenaar, University of Louisville

In this talk, I will describe three recent experiments we have performed on electron and electron spin transport in carbon nanotubes (MWNT):

First, we have carried out a careful experimental study of the suppression of the tunneling conductance of a MWNT in the vicinity of the Fermi energy. This so called “zero-bias anomaly” has recently been attributed to the formation of a MWNT Luttinger liquid. In our experiments we observe several previously unreported features in the bias, magnetic field, and temperature dependence of the tunneling conductance. These features, as well as those described previously, can be explained in terms of the inter-shell interaction in the MWNT within the one-electron framework, without the need for incorporating electron-electron interactions.

Second, we have developed a technique to produce liquid metal contacts to nanotubes in a planar geometry. This allows us to directly measure the change in contact resistance due to the immersion of the nanotube into the liquid metal. The liquid metal contacts are extremely transmissive with a two-terminal MWNT conductance approaching 2e²/h. The total conductance is independent of the length of the nanotube, in agreement with previous experiments performed used a scanning probe technique.

Third, I will describe temperature dependence measurements of the resistance switching in ferromagnetically contacted nanotubes. We observe that the sign of the resistance switch can change from positive to negative as the temperature increases. This suggests that the sign of the injected spin polarization depends on the thickness of a non-ferromagnetic “dead-layer” between the contact and the nanotube. The change in sign also provides additional confirmation that spin injection and not stray field effects are the main influence on the resistance switching.

Molecular Kondo Resonance in Atomic Fermi Gases
Prof. Henk Stoof, Utrecht University

Recent experiments performed in the group of the Nobel laureate Carl Wieman (JILA) have shown coherent oscillations between an atomic and a molecular Bose-Einstein condensate. In the first part of the seminar we show how to arrive at a microscopic theory that is capable of accurately describing the Josephson oscillations observed in these experiments. In the second part of the seminar we then generalize the theory to be also able to consider ultracold atomic Fermi gases. In particular, we show that in this case a coherent coupling between atoms and molecules leads to a new physical phenomena, which we have called the bosonic Kondo effect.

Molecular Scale Conduction: Few-Atom Junctions and Single-Molecule Transistors
Prof. Doug Natelson, Rice University

Recently developed techniques allow the fabrication of metal electrodes connected by a few atoms or single molecules. Such electrodes are tools for examining conduction at the nanometer scale. In disordered atomic scale gold junctions made electrochemically, we find conductance effects that may be understood qualitatively as the nonperturbative result of electron-electron interactions in a granular metal. Using an electromigration technique, we have also successfully made transistors out of individual C₆₀ molecules. In some of these devices, we see the signature of Kondo physics, a many-body correlated state. I will describe our recent results, and explain the great potential of these novel devices.

Tracking the Elusive d-Wave Quasiparticle
Nuh Gedik, UC Berkeley

I will report measurements of the dynamics of nonequilibrium quasiparticles in cuprate superconductors. In these experiments, quasiparticles are injected into single crystal samples by short laser pulses with photon energy 1.5 eV. At low laser intensities, the time evolution of the quasiparticle density yields the rate at which Cooper pairs reform and enter the superfluid condensate. At higher laser intensities, the nonequilibrium quasiparticle population induces a superconductor to normal phase transition, from which we determine the difference in kinetic energy between the two states. By exciting sample with two coherent laser pulses we create a spatially periodic quasiparticle distribution. Probing the evolution of this distribution through space and time allows precise measurement of the quasiparticle diffusion coefficient. We argue that these parameters shed light on the character of d-wave antinodal quasiparticles in these materials.

Electron Gas Redux: From Laudau to Luttinger And Back Again
Dr. Emiliano Papa, University of Texas at Austin

Most properties of condensed matter systems originate from the quantum mechanics of many interacting electrons. For most electronic systems, interactions do not alter properties qualitatively even though they are strong, a fact first understood by Landau. Electron systems in one space dimension, commonly known as Luttinger liquids, are an exception. I will explain why Luttinger liquids occur as edge states in quantum Hall systems, and review the fundamental experimental studies of these systems that have been completed over the past few years. A key aspect of the folklore surrounding quantum Hall Luttinger liquids has been the belief that their properties depend only on interger or fractional quantum numbers. I will explain how interactions can alter the properties of quantum Hall Luttinger liquids and use these considerations to explain recent experiments.

Dots for Dummies
Prof. Ramamurti Shankar, Yale University

I will give a pedagogical introduction to the problem of interacting electrons in a ballistic quantum dot with chaotic boundary conditions. In a particular limit, the problem becomes exactly solvable. I will describe the resulting phases and quantum phase transitions, as well as some predictions for the Coulomb Blockage peak spacing distributions and Fock-space delocalization. The latter is reflected in the quasiparticle width and ground state wavefunction.

Localization-Delocalization Transition in a Quantum Dot
Prof. Raymond Ashoori, MIT

In a two-dimensional quantum dot with inherent disorder, we investigate the sensitivity of single electron wavefunctions to changes of the confining potential. Using a capacitive technique, we detect electron additions to the dot starting from the first electron up to many hundreds. This capability permits us to observe the evolution of the electronic system the dot from an arrangement of electrons localized by disorder at low densities to a high-density electronic “liquid” whose wavefunctions extend over the entire dot. In the low-density system, localized electrons inhabit distinct spatial sites each with different sensitivity to the change of the confining potential at the edge of the dot. As density increases, the electrons gradually evolve into a metallic-like state where all wavefunctions are delocalized and similarly spread fully over the dot’s area. We find that as one approaches the delocalization transition, the last remaining localized states exist at the perimeter of the dot. The data indicate that this is a very unusual type of localization that includes an unexpected binding of electrons. Finally, strong perpendicular magnetic field squeezing electron wavefunctions destroys the metallic-like state restoring localization.

Phenomena in Cold Exciton Gases: Condensation, Macroscopic Ordering And Beyond
Prof. Leonid V. Butov, University of California at San Diego

Bound electron-hole pairs — excitons — are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin, the temperature many orders of magnitude higher than for atoms. Experimentally, the exciton temperature well below 1 Kelvin is achieved in coupled quantum well semiconductor nanostructures. We overview the problem of exciton condensation and report on new experiments, revealing bosonic stimulation of exciton scattering. Novel experiments with pattern formation in exciton system and macroscopically ordered exciton state will also be presented.

Ultrafast Spectroscopy of Carbon Nanotubes
Prof. Tobias Hertel, Vanderbilt University

Single-wall carbon nanotubes hold promise for application in a variety of nano-electronic and optical devices. This presentation will introduce the topic by a brief introduction to their fundamental electronic and mechanical properties that have sparked the interest and imagination of researchers worldwide. The remainder of the talk will focus on optical studies of single- and double-wall carbon nanotubes, which are investigated in quasi-crystalline form — as so called nanotube ropes — or as individual entities in aqueous solution. Results from CW absorption, time-resolved photoemission, and time-resolved photoluminescence spectroscopy provide us with a wealth of information on free-carrier and exciton dynamics. Combined with recent advances in nanotube processing, purification and separation capabilities, the current spectroscopic work is expected to lead to a better understanding of dynamical processes in these unique one-dimensional materials.

Suppression of Spin-Orbit Scattering in Strong-Disordered Gold Nanojunctions
Prof. Dragomir Davidovic, Georgia Tech

We discovered that spin-orbit scattering in strong-disordered gold nanojunctions is greatly suppressed relative to that in weak-disordered gold thin films. This property is unusual because in weak-disordered films, spin-orbit scattering increases with disorder. Granularity and freezing of spin-orbit scattering inside the grains explains the suppression of spin-orbit scattering. We propose a generalized Elliot-Yafet relation that applies to strong-disordered granular regime.

Thermal Conductivity and Thermoelectric Power of Carbon Nanotubes
Marc Llaguno, University of Pennsylvania

We have investigated phonon quantization in single wall carbon nanotubes (SWNTs). Phonons in carbon nanotubes are expected to be quantized due to the periodic boundary conditions for the wavevector around their circumference. The zone-folding model for phonons predicts that the energy splitting of the phonon subbands is inversely proportional to the diameter. To verify this prediction, we have investigated the diameter dependence of the thermal conductivity of SWNTs.

We have also developed techniques for measuring the thermo-electric power (TEP) of individual SWNTs. The TEP of a semiconducting nanotube has been measured in the Coulomb blockade regime, and TEP oscillations have been observed. As the tube is depleted of carriers, the oscillations grow larger and are centered around an offset value of the TEP. We attribute the offset to defects on the tube and the Schottky barriers at the contacts that contribute a negative thermopower. From the TEP offset, we are also able to estimate the size of the depleted regions in the nanotube.

Electronic Properties of InSb Quantum Wells and Mesoscopic Structures
Prof. Michael B. Santos, University of Oklahoma

In narrow-gap semiconductors, electrons have properties that are much different than in free space. For example, the effective mass in InSb is nearly two orders of magnitude smaller than the mass in free space. This property can be exploited in applications, such as magnetoresistive sensors or ballistic transport devices, where a high mobility or a long mean free path is required. The strength of the interaction between an electron?s spin and a magnetic field is also enhanced in InSb. The effects of a small effect mass and large spin-orbit coupling are seen through far infrared magneto-spectroscopy of quantum wells and transport measurements of mesoscopic structures.

Transport in a Luttinger Liquid
Prof. Leonid Glazman, FTPI, University of Minnesota

Conduction of quantum wires is strongly affected by the interaction between electrons. Many of the interaction-induced modifications are adequately described within the Luttinger liquid model, while understanding of some phenomena requires going beyond it. The unusual properties of one-dimensional electron transport include the suppression of tunneling conductance, electron spin-charge separation, and an enhanced Coulomb drag effect. The talk reviews theory and some experiments in this growing research field, and describes its relation to the physics of other low-dimensional systems.