September 19,
2005
Periodic Systems: New Insight into their
Properties
Dr. Emil Prodan
Princeton University
Quantum systems with periodic potentials
play an important role when it comes about our understanding of the
condensed matter.
In a seminal paper, Bloch showed that the solutions of the Schrodinger
equation
for electrons in periodic potentials are still waves and, ever since,
these
solutions were labeled Bloch waves. In 1959, Walter Kohn published a 1D
study
on these functions, in which he revealed the analytic structure of
Bloch
waves as a function of the k vector. Understanding this structure, for
real
as well as for complex k vectors, turned out to be the key for
understanding, besides many other things, the exponential
localization of different correlation functions in periodic insulators.
In this talk I will discuss recent and first extensions of these
results to higher dimensions. I
will present the analytic structure of the Bloch functions for linear
molecular chains and cubic crystals and discuss several applications
which include: estimating the changes of the particle density and local
density of states in periodic crystals due to impurities, surfaces and
interfaces

September 27, 2005
Generalized BoseEinstein Condensation in
ManyFermion Systems
Prof. Manuel de Llano
University of Houston
By recognizing the vital importance of
twohole Cooper pairs in addition to the standard twofermion ones in a
manyfermion system such as a superconductor or neutralfermion
superfluid, the concept of pairing has been reexamined with striking
conclusions. Based on this, BoseEinstein condensation (BEC) theory is
generalized to include not bosonboson interactions (also neglected in
BCS theory) but rather bosonfermion interactions. The new formalism
reduces to all the old known statistical theories as special
casesincluding the socalled BCSBose crossover picture which in
turn
generalizes BCS theory. With no adjustable parameters, it yields
substantially
higher superconducting transition temperatures without invoking
nonphonon
dynamics. The implications of all this in neutralfermion superfluids
like
trapped ultracold Fermi atomic clouds, is still to be explored.

October 3, 2005
Quantum Criticality in Ferromagnetic
SingleElectron
Transistors
Stefan Kirchner
Rice University
Quantum Dots and SingleElectron Transistors
have
been used over the past decade to model Fermi liquid or Kondo states in
and
near
equilibrium. The universal features of the Kondo effect and its
significance
in describing strongly correlated electron materials have led to a
strong
interest both from a technological as well as scientific point of view.
Here, we introduce what we believe to be the first realistic system  a
quantum dot attached to ferromagnetic leads  that models nonFermi
liquid
states near a quantum phase transition. We theoretically demonstrate a
gatevoltage
induced quantum phase transition. At the transition the Kondo effect
becomes
quantum critical, leading to distinct, universal properties. We find a
fractionalpowerlaw
dependence of the conductance on temperature (T). The AC conductance
and
thermal noise spectrum have related powerlaw dependences on frequency
(omega)
and, in addition, show an (omega/T) scaling. Our results imply that the
ferromagnetic
nanostructure constitutes a robust and realistic model system to
elucidate
magnetic quantum criticality that is central to the heavy fermions and
other
strongly correlated electron systems with nonFermi liquid behavior.

October
17, 2005
NonFermiLiquid
Physics in a
Magnetically Frustrated Nanostructure
Prof. Kevin Ingersent
University of Florida
Interest in the Kondo effect
has been revived by experimental manifestation of this manybody
phenomenon in tunneling through a quantum dot and through a magnetic
adatom on a metallic surface.
Studies have begun of multiple"impurity" configurations in which Kondo screening of local spins competes
with magnetic ordering of the
same spins. In this context, a cluster of three antiferromagnetically coupled spins is of fundamental
importance as the simplest example of frustration, a feature of many magnetic
systems. Scanning tunneling
microscopy points to the existence of a novel Kondo state in Cr trimers on a gold surface, but the
nature of this state is somewhat controversial. Interest is also growing
in the possible interplay
between Kondo physics and interdot quantum entanglement in triangular quantumdot devices.
After reviewing this
background, the talk will describe a nonFermi liquid phase of the Kondo model for
three halfinteger spins with
frustrating antiferromagnetic interactions. The phase, which is stabilized by the triangular symmetry of
the model, arises without
finetuning of couplings, and (unlike the nonFermi liquid regimes found in other impurity models) is
stable against magnetic fields
and breaking of particlehole symmetry. Various exact, universal lowenergy properties will be presented.
Signatures predicted in
electrical transport may be testable in scanning tunneling microscopy on adatom trimers or in
transport through quantum dots.

November
07, 2005
Microwave
and rf spectroscopy of
twodimensional electron solids in high magnetic field
Prof. Lloyd Engel
Florida State University
Twodimensional electron
systems (2DES), hosted in GaAs, are best known for exhibiting the
integer and fractional quantum Hall effects. The electrons in such
systems are predicted to form electronic crystals in a number of
different ranges of magnetic field, B. The archetypical electron solid,
expected to be the ground state of disorderfree 2DES in the high B or
low density limit, is the Wigner crystal, a triangular lattice of
electrons stabilized by their mutual repulsion.
We have found that a striking rf or microwave resonance in the spectrum
is a generic feature of the electron solids. The observed resonance
frequencies range from 70 MHz to 10 GHz. The electron solids are
insulators due to pinning by disorder, and the resonances are
interpreted as "pinning modes" of the solids, in which crystalline
domains oscillate within the disorder potential. After introducing the
phenomenology of the resonances, I will present results for three
distinct types of electron solid: 1) the phases terminating the
fractional quantum Hall series at high magnetic field, 2) electron
crystals concomitant with integer quantum Hall effects, and 3) bubble
phases, which are crystals with clusters of electron guiding centers at
each site, and which are observed when multiple Landau levels are
occupied.

November
14, 2005
GRAPHENE
NANOSTRUCTURES: Physics
and Device
Prof. Li Lu
Chinese Academy of Sciences
Considerable efforts have been
made in recent years to explore using carbon nanotubes as the building
blocks for future nanoelectronic circuits. In this approach, however,
there are unsolved difficulties such as how to grow carbon nanotubes
with the desired diameter and chirality and in particular with
intramolecular junctions, and how to select nanotubes of a desired
type, then to assemble them in the right positions in circuits. In this
talk I will report our proposal and investigation on a completely new
approach, in which a topdown technique is incorporated to tailor
graphene sheets directly to nanodevices and nanoelectronic circuits.
We demonstrate the feasibility of this idea by fabricating specially
designed multiterminal graphene patterns down to a minimum strip width
of 50 nm. Electron tunneling measurement confirms the formation of
quasionedimensional subbands due to the quantum size confinement of
the electrons in the fabricated strips. This new approach would in the
future provide an efficient way of producing numerous layers of
identical graphene nanoelectronic circuits.

November
28, 2005
Magnetism,
Superconductivity and
Quantum Criticality in the HeavyFermion Compound CeRhIn5
Prof. Joe D. Thompson
LANL
Though
a longstanding problem, the relationship between magnetism and
superconductivity has become a particularly lively topic, especially in
the context of strongly correlated heavyfermion materials, such
as CeRhIn5. In these systems, unconventional superconductivity
emerges as a magnetic phase transition is tuned by applied
pressure toward zerotemperature. Longrange magnetic order,
however, terminates abruptly when the magnetic and
superconducting transition temperatures become equal, and there
is no evidence that the magnetic transition actually reaches T=0.
Recent measurements reveal the emergence of fieldinduced
antiferromagnetism in the superconducting state of CeRhIn5. This
magnetism, hidden by superconductivity in zero field, reappears
in an applied field and terminates at the expected quantumcritical
point where the effective mass of quasiparticles diverges and the
Fermisurface volume increases without a change in topology. The
relationship between magnetism, superconductivity and quantum
criticality found in CeRhIn5 may be applicable to other strongly
correlated systems, such as those based on copperoxide and
plutonium.

January
30, 2006
The Anomalous Hall Effect
in a Paramagnetic 2DEG
Prof. John Cumings
University of Maryland
In
ferromagnetic conductors, the Hall Effect generally acquires an
additional contribution, even in the absence of a magneticfield, due
to
spinorbit coupling of the carriers and their inherent spin
polarization. This is known as the Anomalous Hall Effect (AHE).
Despite being discovered more than a century ago, the basic physics
behind the AHE is still a subject of intense debate. In recent
measurements, we have shown that this phenomenon can also be
exhibited in a paramagnetic semiconductor. The twodimensional
nature of the system allows us to independently characterize the
material through quantum transport and also to tune the effect with
an applied electric field. Our results provide clues to the origin
of the AHE and even track the effect into the littleexplored realm
of carrier localization.

March
06, 2006
ATOMIC
FERMI GASES WITH UNEQUAL SPIN POPULATIONS
Dr. Meera Parish
Cambridge University
I will investigate
the properties
of a gas of fermionic atoms where the two spin populations are
unequal. By considering how the ground state evolves as a function of
interatomic interaction and population imbalance, I will determine
what the measurable differences are between the various theoretical
models of the BCSBEC crossover in atomic gases. In addition, I will
present the phase diagram at finite temperature.

March 20,
2006
POSITRON
BINDING ON MOLECULES IN
POSITRON EMISSION TOMOGRAPHY
Prof. Lukas Pichl
International Christian University, Tokyo
Positron
binding on molecules have been long believed to be rather of
theoretical than practical interest, not only because of the ultimate
annihilation (i.e., quasibound states of positron), but especially
because of the positronium formation
channel.
Recent experiments, however, show direct signs of positron
annihilation in bound states on molecules. The talk will discuss the
role of molecular vibrations in
the bound states of positron on alkali metal hydrides supported by
the large dipole moment of these molecules, and also demonstrate the
importance of positron binding on biomolecules for medical PET imaging.

April 03,
2006
FIND
YOUR PARTNER OR EXPEL YOUR
COMPETITOR:
States in Exotic
PairingFermionic Superfluids with Unbalanced Pairing Species
Prof. Kung Yang
National High Magnetic Field Lab, Florida
Superfluidity
in fermionic systems originates from pairing of fermions,and Bose
condensation of these Cooper pairs. The Cooper pairs are usually made
of fermions of different species; for example in superconductors they
are pairs of electrons with opposite spins. Thus the most favorable
situation for pairing and superfluidity is when the two species of
fermions that form pairs have the same density, a situation
successfully described by the BardeenCooperSchrieffer theory. It
has become clear recently that pairing and superfluidity can also
occur when the fermion species have different densities, in systems
ranging from superconductors in a strong magnetic field to trapped
cold atom systems, and quark matter in the core of neutron stars.
Such a situation will necessarily lead to unpaired fermions in the
ground state, and possibly nontrivial spatial structure in the
superfluid order parameter. In this talk I will discuss the physics
of such exotic pairing states, and their possible realization and
detection in superconductors and cold atom systems.

April 10,
2006
Observation
of the TwoChannel Kondo Effect in a Semiconductor Nanostructure
Dr. Ilena Rau
Stanford University
The
twochannel Kondo Hamiltonian is a prototype for studying nonFermi
liquid behavior in strongly correlated electron systems. Quantum dots
have proved to be excellent systems for studying the singlechannel
Kondo effect, that is, the manybody ground state resulting from an
excess spin _ electron in the quantum dot interacting with a
reservoir of conduction electrons. The twochannel Kondo effect is
achieved by coupling the dot with equal strength to two independent
reservoirs. In this configuration the screening of the excess spin is
not successful anymore as the reservoirs compete in an attempt to
form the Kondo state with the dot.
Creating
two distinct screening reservoirs for a quantum dot is a nontrivial
problem. In a new semiconductor double quantum dot geometry, proposed
by Y. Oreg and D. GoldhaberGordon (1), we observe spin twochannel
Kondo physics. Low temperature transport measurements through the
smaller of the two dots reveals that the cmbined system of double dot
and leads undergoes a quantum phase transition from one
competing singlechannel Kondo state to another. By tuning between
these two Fermi liquid regimes, we observe the nonFermi liquid
signature of the twochannel Kondo state. I will explore recent
experimental progress and open questions.
(1)
Y. Oreg, D. GoldhaberGordon, PRL 90 136602 (2003).
