October 6, 2003
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 nonzero 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 twodimensional 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.

November 3, 2003
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 “zerobias
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 intershell interaction in the MWNT
within the oneelectron framework, without the need for
incorporating electronelectron 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 twoterminal
MWNT conductance approaching 2e^{2}/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 nonferromagnetic
“deadlayer” 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.

November 4, 2003 (Tuesday)
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 BoseEinstein 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.

November 17, 2003
Molecular Scale Conduction:
FewAtom Junctions and SingleMolecule 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
electronelectron interactions in a granular metal. Using an
electromigration technique, we have also successfully made
transistors out of individual C_{60} molecules. In some
of these devices, we see the signature of Kondo physics, a
manybody correlated state. I will describe our recent results,
and explain the great potential of these novel devices.

November 24, 2003
Tracking the Elusive dWave
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 dwave
antinodal quasiparticles in these materials.

December 1, 2003
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.

December 8, 2003
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 Fockspace
delocalization. The latter is reflected in the quasiparticle
width and ground state wavefunction.

January 26, 2004
LocalizationDelocalization
Transition in a Quantum Dot
Prof. Raymond Ashoori, MIT
In a twodimensional 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 highdensity electronic “liquid”
whose wavefunctions extend over the entire dot. In the lowdensity 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
metalliclike 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 metalliclike state restoring
localization.

February 9, 2004
Phenomena in Cold Exciton
Gases: Condensation, Macroscopic Ordering And Beyond
Prof. Leonid V. Butov, University of California at San Diego
Bound electronhole pairs — excitons — are Bose
particles with small mass. Exciton BoseEinstein 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.

February 23, 2004
Ultrafast Spectroscopy of
Carbon Nanotubes
Prof. Tobias Hertel, Vanderbilt University
Singlewall carbon nanotubes hold promise for application in
a variety of nanoelectronic 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 doublewall carbon nanotubes, which are investigated
in quasicrystalline form — as so called nanotube ropes
— or as individual entities in aqueous solution. Results
from CW absorption, timeresolved photoemission, and timeresolved
photoluminescence spectroscopy provide us with a wealth of
information on freecarrier 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 onedimensional materials.

March 8, 2004
Suppression of SpinOrbit Scattering in
StrongDisordered Gold Nanojunctions
Prof. Dragomir Davidovic, Georgia Tech
We discovered that spinorbit scattering in strongdisordered gold
nanojunctions is greatly suppressed relative to that in weakdisordered
gold thin films. This property is unusual because in weakdisordered
films, spinorbit scattering increases with disorder. Granularity and
freezing of spinorbit scattering inside the grains explains the
suppression of spinorbit scattering. We propose a generalized ElliotYafet
relation that applies to strongdisordered granular regime.

March 15, 2004
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 zonefolding 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
thermoelectric 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.

April 12, 2004
Electronic Properties of InSb Quantum Wells
and Mesoscopic Structures
Prof. Michael B. Santos, University of Oklahoma
In narrowgap 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 spinorbit
coupling are seen through far infrared magnetospectroscopy of quantum
wells and transport measurements of mesoscopic structures.

April 26, 2004
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 interactioninduced modifications are
adequately described within the Luttinger liquid model, while understanding
of some phenomena requires going beyond it. The unusual properties of
onedimensional electron transport include the suppression of tunneling
conductance, electron spincharge 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
lowdimensional systems.
