September 07, 2006
Probing Hybridization through Optics
Dr. Ken Burch
Center for Integrated Nanotechnologies
Los Alamos National Laboratory
In this talk I detail recent optical spectroscopic
studies of as grown and annealed thin films of Ga1xMnxAs
across its phase diagram. We
find that the spectra significantly differ from what is expected and has
been observed in semiconductors doped with nonmagnetic impurities. These
studies indicate that the Insulator to Metal transition in this magnetic
semiconductor is significantly different from most other doped
semiconductors, indicating an important role for magnetism. I will then
switch to discussing magnetic impurties in the lattice via our work on Yb14MnSb11.
We found that this system is the first underscreened delectron Kondo lattice.
Our optical studies were the first to demonstrate the heavy electron nature
of the carriers and the interestingly interplay between the Kondo and JahnTeller
interactions in this system. If time allows, I will also discuss recent
optical pumpprobe measurements on this material that indicate the unconventional
nature of the Ferromagnetic state.

September 25, 2006
Anisotropy in 2D electronic quantum Hall systems
Professor Orion Ciftja
Prairie View A&M University
Strongly correlated electronic systems in twodimensions
(2D) have constantly been a source of new discoveries. Novel phenomena such
as the integer and fractional quantum Hall effect have emerged when such
systems have been subjected to very strong perpendicular magnetic fields.
A recent experimental discovery in the quantum Hall regime has been the observation
of very strong magnetotransport anisotropies at certain low values of magnetic
field below a critical temperature of about 100 mK. While the origin of
such anisotropy is yet unknown, we explain the emergence of such anisotropies
in terms of electronic liquid crystalline states with broken rotational symmetry.
We investigate the stability of liquid crystalline phases with nematic order
at fractional fillings of the valence Landau level. Monte Carlo results
indicate that while isotropic states are stable in the lowest Landau level,
there are regions of instability towards liquid crystalline states in higher
Landau levels. A possible connection of the recently discovered magnetotransport
anisotropy in low magnetic fields and these liquid crystalline states is
discussed.

October 02, 2006
FieldInduced Superconductivity in an Organic
Insulator
Professor Kevin Storr
Prairie View A&M University
Organic conductors are excellent candidates for
high magnetic field studies because of their anisotropy, instabilities and
interactions at low temperatures. l(BETS)2FeCl4
a quasi twodimensional organic conductor undergoes an antiferromagneticinsulator
and metallic phase before displaying strong evidence for Field Induced Superconductivity
(FISC) which persists in fields up to 40 tesla after which it is reentrant.
By gradual replacement of the Fe in the FeCl4 anion,
the superconducting phase moves towards lower fields with a zero field superconductor
resulting with total substitution in the form of l(BETS)2GaCl4.
We explain the continuance of the FISC in terms of the JaccarinoPeter
effect which requires that the applied magnetic field be compensated by
the exchange field of the aligned Fe3+ ions.

October 30, 2006
Magnet on Magnet: the Exchange Bias Effect in
Co/CoO Nanoparticles
Professor Meigan Aronson
Univeristy of Michigan
When a ferromagnet is in close proximity to an
antiferromagnet, the direct exchange interaction between the moments
in each can create a unidirectional anisotropy which impedes the reversal
of the ferromagnetic moment. It is thought that this is a promising way
to stabilize the moment for the smallest nanoparticles, but so far understanding
the fundamental mechanism has been obscured by lack of information and control
of the antiferromagneticferromagnetic interfaces. We present here
a neutron scattering study of the internal magnetic structure and dynamics
of Co/CoO coreshell nanoparticles. Transmission electron microscopy measurements
show that the Co/CoO interface is both highly directional and virtually
epitaxial. Neutron diffraction measurements show that antiferromagnetic
order occurs below a Neel temperature much suppressed from the bulk CoO
value. Further, the magnetic structure involves the spatial segregation
of oxygen interstitials near the interface, inducing a large uncompensated
moment. Normal CoO antiferromagnetism is found in the outer regions of the
CoO shell. Our results suggest that the interface layer enhances the ferromagneticantiferromagnetic
coupling, and is responsible for the
large exchange bias observed in our samples. We have also performed inelastic
neutron scattering measurements which show that the interface has a dual
role, providing an unusual braking mechanism which slows the reorientations
of the ferromagnetic core at the onset of the exchange bias.

November 06, 2006
Anisotropic thermodynamics of unconventional
superconductors
Professor Ilya Vekhter
Louisiana State Univeristy
Determination of the shape of the energy gap in
unconventional superconductors is of significant importance since it is believed
to be related to the pairing mechanism. A candidate technique for finding
the positions of zeroes (nodes) of the gap is via measurements of the anisotropy
of thermal and transport properties under a rotated magnetic field. This
anisotropy was predicted and found experimentally in the specific heat and
thermal conductivity in a number of materials. However, limitations of the
original semiclassical theory made unambigious interpretation of experiments
not easy or impossible.
After giving the background to the problem, I will present the results
of a microscopic fully selfconsistent calculation of the anisotropy of the
specific heat and thermal conductivity in anisotropic superconductors across
the TH phase diagram. I will explain the physical origin for the unexpected
inversion of the anisotropy. For heavy fermion CeCoIn5 I
will show that the results are in a semiquantitative agreement with experiment
and resolve the controversy between the existing specific heat and thermal
conductivity measurements.

November 13, 2006
Quantum ElectroMechanics
Professor Keith Schwab
Cornell University
The technology is at hand to produce and measure
mechanical structures at quantum mechanical limits. We have demonstrated
the closest approach to the Heisenberg Uncertainty Principle and the quantum
ground state for a radiofrequency nanomechanical structure, achieved by
coupling to a single electron transistor. Recently, we have measured
the quantum noise of the transistor and discovered that cooling of the mechanical
mode is possible by an electronic process analogous to laser cooling. Future
experiments will pursue more sophisticated situations by integrating quantum
coherent electronic elements (qubits) and will probe the production and
detection of the true quantum states of the resonator, including squeezed,
superposition, and entangled states.

December 04, 2006
Magnetic field asymmetry and interactions in
singlewalled carbon nanotubes
Professor Dave Cobden
University of Washington
The length scales and scattering processes in the
onedimensional electron system in singlewalled carbon nanotubes remain only
partially understood. Measuring the magnetoresistance, in both linear and
nonlinear response, is away to investigate these processes. In disordered
nanotubes with ballistic paths much shorter than the length, we observe magnetoresistance
in the metallic regime which at low temperatures resembles the universal
fluctuations and weak localization seen in higher dimensional metals. A
parabolic magnetoresistance persists at room temperature, indicating a significant
role for phase coherence and/or interactions at high temperatures. While
the linear resistance of a twoterminal sample must be an even function of
magnetic field B by Onsager's principle, the nonlinear resistance need not
be. Importantly, the Basymmetric nonlinear terms can in principle
be used to infer the strength of electronelectron interactions in the sample.
We have therefore also measured in detail the lowest order Basymmetric
current contributions, with a focus on the Blinear term. This has
apparently not been done before in any system. Consistent with general theory,
at high temperatures the term is small and has a constant sign independent
of Fermi energy. At low temperatures it grows and develops mesoscopic
fluctuations. Although these result imply that interactions are involved
in the transport, calculations specific to nanotubes will be needed in order
to extract interaction parameters.

December 11, 2006
From noisy quantum dots
to quantumcritical heavy fermions:
recent developments of the numerical renormalization group method
Dr. Matthew Glossop
University of Florida
I will discuss recent progress in describing
BoseFermi quantum impurity models using the numerical renormalization group
approach. In particular I will focus on results obtained for the BoseFermi
Kondo model (BFKM), describing a local magnetic moment coupled to a conduction
band and a dissipative bosonic bath. The model is of current interest in
connection with noisy quantum dot systems and nonFermiliquid behavior
observed in quantum critical heavyfermion materials. The latter systems
are described by the Kondo lattice model, which maps onto a selfconsistent
BFKM within the extended dynamical meanfield theory (EDMFT) framework. I
will discuss our recent NRGEDMFT solution of the anisotropic Kondo lattice
model, addressing in particular the topical issue of local quantum criticality.

February 05, 2007
Universal phase diagram
of interacting quantum liquids near the unitarity
limit
Dr. Predrag Nikolic
Harvard University
We consider several models
of particles with shortrange attractive interactions
whose universal properties are controlled by an unstable renormalizationgroup fixed point at zero density and temperature.
The fixed point corresponds to the Feshbach
resonance, and relevant perturbations are the
detuning of the resonance, and parameters that control the particle densities. Some critical exponents
are determined exactly, and scaling functions
are expressed as expansions about two or four
spatial dimensions. The existence of a renormalizationgroup fixed point implies a universal phase diagram as a function of
density, temperature, population imbalance,
and detuning. We study this phase diagram in
the context of BECBCS crossover of swave paired fermions. We develop a 1/N expansion, based upon models with
Sp(2N) symmetry, and use it to systematically
analyze the universal properties of interacting fermions near the unitarity limit. This approach
overcomes several limitations of the expansions
about two and four dimensions, and allows a
well controlled exploration of the full phase diagram of imbalanced fermion populations in the experimentally relevant threedimensional
space.

February 09, 2007
Density Wave Formation in Graphene Facilitated
by inplane Magnetic Field
Dr. Sebastian Reyes
Brookhaven National Lab
We suggest that by applying a magnetic field lying
in the plane of graphene layer one may facilitate an excitonic condensation
of electronhole pairs with opposite spins. The provided calculations yield
a conservative estimate for the transition temperature Tc~0.1B. Below Tc
the system is in insulating state with critical spin fluctuations.

February 19, 2007
POWERLAWS IN ONEDIMENSIONAL TRANSPORT: Luttinger Liquid or Disorder?
Prof. Michael Fogler
University of California
San Diego
When the conductance of a 1D wire shows a powerlaw
dependence on temperature T and voltage V, it is often attributed to Luttinger
liquid or related interaction effects. For this explanation to hold the
powerlaw exponents in T and V must be equal. This condition is systematically
violated in long wires (typically, longer than 10 micron), where the thermal
exponent is much larger than the voltage one. To shed light on the physics
involved we study a theoretical model of a long 1D wire with a finite density
of strong random impurities that convert it into a chain of weaklycoupled
quantum dots. Electron transport in such a system is shown to exhibit a
rich dependence on V and T due to the interplay of sequential and cotunneling.
Remarkably, we indeed find a broad parameter range where the conductance
exhibits an algebraic dependence on T and V with unequal exponents, in agreement
with the experiments. At much lower temperatures the conductance eventually
crosses over to the stretched exponential laws typical of the variablerange
hopping. Reference: M. M. Fogler, S. V. Malinin, T. Nattermann, "Coulomb
blockade and transport in a chain of onedimensional quantum dots," Phys.
Rev. Lett. 97, 096601 (2006).

February 27, 2007
Transport in Nanofabricated
Electronic Devices
Dr. Gavin Scott
Two separate nanoscale electronic
device projects will be discussed, addressing issues of fabrication and
transport in the fewelectron regime. The first topic focuses on the construction
and characterization of single molecule transistors (SMTs) using the recently
synthesized Borromean Ring complex. While stability and reliability
have been pervasive issues in the study of SMTs, a comprehensive understanding
of the primary physical influences enables us to ascertain new means
of exerting further control in these systems. For example, I will
introduce a simple means of enhancing rectification properties in SMT devices.
The second topic concerns our effort to fabricate and characterize electrostatically
defined lateral quantum dots on modulation doped Si/SiGe heterostructures
for future quantum computation applications. Towards attaining the
goal of developing Si/SiGe based nanostructures with a level of control near
what has been achieved with GaAs/AlGaAs, I will detail our progress overcoming
fabrication obstacles related to the use of surface gates. Proposed,
nearfuture experiments for both of these projects are directed along strikingly
similar paths, the success of which may underscore their respective viabilities.

March 19, 2007
Orbitalfree Density Functionals for Materials
Simulations
Prof. Samuel B. Trickey
University of Florida
Prediction of the structure and properties of
a single complicated material is a demanding task. The task is more difficult
in the presence of solvents, and especially so in combination with imposed
mechanical stress. Such "chemomechanical" systems require quantum mechanical
treatment of critical reactive zones, but cannot be facilitated by either
localization (as in molecular calculations at high accuracy) or translational
periodicity (as in perfect crystals).
In view of the successes of density functional theory, especially continued
improvement of approximate exchangecorrelation functionals, a reasonable
goal is to move the nuclei in the material (or a subsystem) via molecular
dynamics (MD) on the ground state potential surface (BornOppenheimer approximation).
Despite advances (pseudopotentials, order$N$ methods), solution
of the KohnSham equations is too slow computationally to be a fully viable
approach. An alternative,CarParrinello dynamics, does not guarantee motion
on the BO surface.
Another approach, orbitalfree DFT, therefore is appealing. But
that approach has a long history of difficulties in constructing reliable
approximations to the KohnSham kinetic energy functional. To be useful
for MD, such functionals must be local ({\it i.e.} onepoint), which makes
the task harder. However, we have made progress by requiring only
that the approximate functional deliver highquality forces, by exploiting
the ``conjointness'' hypothesis of Lee, Lee, and Parr, and by parameterizing
to simple molecules (e.g., SiO). This talk will survey the fundamentals,
discuss our approach, report a range of test results, and discuss a gradedsequenceofapproximations
procedure for making relatively inaccurate functionals useful.

April 16, 2007
TBA
Prof. Ron Naaman
Department of Chemical Physics
Weizmann Institute

April 26, 2007
ULTRA SLOW MOTION OF AN INTERFACE PINNED BY IMPURITIES
Prof. Alberto Rosso
Massachusetts Institute of Technology
Many experimental systems can be modeled as interfaces
pulled by a force on a disordered landscape. Prominent examples are the magnetic
domain walls continuously pushed by magnetic fields while our computer hard
disks work, or similar walls in ferroelectric systems and crack propagation
in solids. In all these systems the physical properties are drastically modified
by the existence of impurities in the material on which the interface pins.
Understanding these properties is thus a considerable challenge, both from
a fundamental and a practical point of view. Although the problem is simple
to model, it is extremely complicated to solve. Indeed, while for large forces
the interface simply slides, for small forces it is pinned by the impurities.
Motion is still possible thanks to thermal kicks, but becomes exceedingly
slow. The relevance for applications of this regime has motivated a continuous
research activity during the last two decades. In spite of these efforts,
the slow motion of the interface has remained numerically inaccessible. We
have developed a new technique that allows to overcome this problem and to
get the important physical quantities characterizing this ultraslow motion.
Our work thus opens the way for a better understanding of the motion of a
wide class of interfaces.

May 21, 2007
Theory of dwave Fermi surface symmetry breaking
Dr. Hiroyuki Yamase
Max Planck Institute for Solid State Physics
The Fermi surface usually respects the pointgroup
symmetry of the underlying lattice. However, recently it was shown [1,2]
that such symmetry of the Fermi surface can be broken spontaneously by electron
correlation effects (Pomeranchuk instability). In this talk, after reviewing
a possible relevance to actual materials, we analyze a pure forward scattering
model, which exhibits spontaneous Fermi surface symmetry breaking. We clarify
the phase diagram including a possible quantum critical point, and universal
quantities charactering the phase diagram.
[1] H. Yamase and H. Kohno, JPSJ 69 , 332 (2000); 69, 2151 (2000).
[2] C.J. Halboth and W. Metzner, PRL 85, 5162 (2000).
