Septmeber
17, 2007
Exploring unusual
charge ordering in high mobility twodimensional hole system
Dr. Mike Manfra
Bell Laboratories
Recent developments in the
epitaxial growth of carbondoped, twodimensional hole systems (2DHS)
in GaAs have resulted in samples of unprecedented quality. These
new samples allow for the exploration of correlations in two dimensions
in a regime where strong spinorbit coupling plays an important
role. In this talk I will describe the growth of such structures
and discuss some of our recent observations of anisotropic charge
transport in a perpendicular magnetic field at filling factors v=7/2,
11/2 and 13/2 at low temperatures. In start contrast with 2d
electron systems, the transport at v=9/2 is isotropic for all
temperatures. Isotropic hole transport at v=7/2 is restored for
sufficiently low 2D densities or an asymmetric confining
potential. The density and symmetry dependences of the observed
anisotropies suggest that strong spinorbit coupling in the hole system
contributes to the unusual transport behavior.

September
20, 2007
Light
induced dynamics of magnetization in ferromagnetic IIIV semiconductors
Professor Munekata
Tokyo Institute of Technology
Holemediated ferromagnetism in
IIIV ferromagnetic semiconductors gives us the opportunity of studying
direct manipulation of local spins through carrier spins. Studying the
influence of optical excitation on magnetic moments in the time regime
of nanosecond or less is especially interesting, since it gives
insight about the interaction among electron, lattice, and spin
systems. The interaction between electron and spin systems via the
lattice system has been studied extensively, and can be understood in
terms of thermalization of spins. On the other hand, understanding the
direct pathway of energy and momentum transfer between the electron and
spin systems has just begun with novel materials and advanced
experimental/theoretical approaches [13].
In this presentation, we are concerned with the effect of optical
excitation on ferromagnetism in IIIV ferromagnetic semiconductors
(IIIV FMS). I will first describe materials aspect of IIIV FMS and
physics of carriermediated ferromagnetism. This is followed by the
discussion of three different lightinduced phenomena: lightinduced
precession of ferromagneticallycoupled Mn spins (magnetization) [4],
dynamic perpendicular magnetization [5], and ultrafast demagnetization
[3]. The first two phenomena have been studied by collecting
timeresolved magnetooptical (MO) signals from ferromagnetic (Ga,Mn)As
through onecolor, pumpandprobe technique, whereas the third
phenomenon by timeresolved polar Kerr signals from ferromagnetic,
narrowgap (In,Mn)As through twocolor, pumpandprobe technique. As to
the lightinduced precession (5 GHz), experimental results indicate
that optical excitation can result in the variation of magnetic
anisotropy driven at least in part by some nonthermal process. As for
the dynamic perpendicular magnetization, experimental data suggest that
excitation with circular polarization causes ultrafast inducement of
magnetization along optical orientation followed by a single
exponential decay (50 psec). The experimental data showing the
ultrafast demagnetization under intense light pulses (100fsec) suggest
the direct heat exchange between electron and spinsubsystems,
presumably through dynamic polarization of hot holes followed by
ultrafast spin relaxation. These experimental data would lead us to
establish fundamental concepts of ultrafast manipulation of collective
spins and magnetization.
References
[1] E. Beaurepaire, J.C. Merle, A. Daunois, J.Y. Bigot, Phys. Rev.
Lett., 76, 4250 (1996).
[2] A.V. Kimel, A. Kirilyuk, A. Tsvetkov, R.V. Pisarev, and Th Rasing,
Nature 429, 850 (2004).
[3] J. Wang, et al., Phys. Rev. Lett. 95, 167401 (2005).
[4] A. Oiwa, et al., J. Supercond. Novel Mag. 18, 9 (2005); Y.
Hashimoto and H. Munekata, arXiv: 0707.4055v1.
[5] Y. Mitsumori, et al., Phys. Rev. B 69, 033203 (2004); A. Oiwa, et
al., Phys. Rev. Lett. 88, 137202 (2002).

September 21, 2007
Metallic Spin Liquid, Anomalous Hall
Transport, and Spin Chirality on a Pyrochlore Lattice
Professor Satoru Nakatsuji
University of Tokyo
Metallic magnets on geometrically frustrated
lattices have attracted interest because of their novel low temperature
phenomena such as the delectron heavy fermion behavior of LiV2O4,
YMn2, and the anomalous Hall effect (AHE) of Nd2Mo2O7.
Among them, the pyrochlore magnet Pr2Ir2O7
is quite unique for its metallic spin liquid behavior, and for novel
transport phenomena due to the strong geometrical frustration. We
discuss the origin of pronounced nonFermiliquid behavior and
unconventional AHE in terms of spinchirality of Isinglike Pr moments..

October
01, 2007
Suspended
CarbonNanotube Based Structures: Mechanical Resonator, Field Effect
Transistor and Tunneling Gap
Professor
Haibing Peng
University of Houston
Carbonnanotube provides a
unique platform for fabricating
nanostructures beyond the reach of standard lithography techniques.
Such nanostructures can be explored in broadscope mechanical or
electronic applications. I will give a few examples including
electromechanical resonator, fieldeffect transistor and tunneling gap.
Apart from their potential applications in sensors or actuators, these
nanostructures reveal interesting physical phenomena such as nonlinear
mixing effect and random telegraph signals. In the long run, they may
provide promising systems for probing the imprint of quantum
interactions in the macroscopic level.

October
29, 2007
Diagrammatic Monte Carlo
method for quantum impurity models
Dr. Philipp Werner
Columbia University
I will present a recently
developed continuoustime QMC method for impurity models [1,2] which is
based on a diagrammatic expansion of the partition function in the
impuritybath hybridization. This algorithm enables the efficient
DMFTsimulation of strongly correlated systems and I will illustrate
its power and flexibility with applications to Hubbard,
HolsteinHubbard, Kondolattice and multiorbital models.
[1] PRL 97, 076405 (2006)
[2] PRB 74, 155107 (2006)

November
12, 2007
Fibonacci
Anyons, Topological Quantum Computation, and Fractional
Quantum Hall Effect
Professor Woowong Kang
University of Chicago
Topological quantum computation
is a new, emerging paradigm for faulttolerant quantum computation. The
proposed topological quantum computer relies on the existence of
nonAbelian anyons, which are thought to occur in certain fractional
quantum Hall states as elementary excitations. In this talk, I will
talk about (a) the motivation for studying topological quantum
computation, (b) how Fibonacci anyons may be used for topological
quantum computation, and (c) our recent effort on interferometry
experiments to study the postulated nonAbelian behavior in the
fractional quantum Hall effect.

November
19, 2007
Inaugural
lecture in the series of "Distinguished Lectures of Quantum Magnetism"
Superconductivity at
heavyfermion quantum critical points
Professor Frank Steglich
MaxPlanckInstitute for Chemical Physics of Solids
Dresden
The present focus on quantum
critical phenomena in correlated matter is driven by the puzzling
enigma concerning the origin of the superconductivity (SC) in these
materials. The prototypical heavyfermion superconductor CeCu2Si2
yields a possible clue to this question: For this material, itinerant
antiferromagnetic (AF) or spindensitywave (SDW) order was found to be
smoothly suppressed by applying a critical pressure, pc, of a few kbar
only. Here, pc denotes a SDW quantum critical point (QCP) which
involves threedimensional (3D) critical fluctuations. Since the
integrity of the ("composite") heavy charge carriers is guaranteed at a
3DSDW QCP, it is often called a conventional QCP.
In the first part of this lecture, recent results of inelastic
neutronscattering experiments are presented which suggest that in
CeCu2Si2 quantum critical SDW fluctuations may mediate the formation of
Cooper pairs. In the second part of the lecture, the isostructural
compound YbRh2Si2 which shows weak AF order below a N�el temperature
TN 70 mK will beintroduced. TN was found to smoothly vanish upon
applying a very low magnetic field ( 60 mT). As demonstrated by a
number of different experimental probes, the heavy quasiparticles seem
to disintegrate on the approach of this fieldinduced QCP.
In conclusion, while the conventional QCP in CeCu2Si2 appears to favor
unconventional SC, in the vicinity of the unconventional QCP in
YbRh2Si2 no SC was observed, at least down to T 10 mK.
This work was done in collaboration with J. Custers, P. Gegenwart, C.
Geibel, H. S. Jeevan, R. K�chler, S. Paschen, J. Sichelschmidt, O.
Stockert and S. Wirth (MPI CPfS) as well as E. Abrahams and P. Coleman
(Rutgers University), C. P�pin (CEA Saclay) and Q. Si (Rice University).

November
26, 2007
Classical
and Quantum Frustrated Magnets
Professor YongBaek Kim
University of Toronto
We will discuss the properties
of classical and quantum Heisenberg models on the lattices with corner
sharing triangles; namely two dimensional Kagome and threedimensional
HyperKagome lattices. The roles of thermal and quantum fluctuations,
and the resulting ground states will be the focus of the discussion.
Connection to recent experiments on Volborthite, Na4Ir3O8, and
Znparatacamite will also be discussed.

December
03, 2007
The
classical limit of quantum transport
Professor Piet Brouwer
Cornell University
The interference of multiply
scattered quantum mechanical matter waves causes small but noticeable
corrections to the conductance of a metal at low temperatures.
Historically, one separates the interference correction to the
electrical conductance into "weak localization", a small negative
correction to the conductance averaged over an ensemble of conductors
with different impurity configurations, and the "conductance
fluctuations", the sampletosample fluctuations measured with respect
to the ensemble average.
What is the fate of quantum interference corrections in the limit that
the wavelength of the electrons becomes small in comparison to all
other relevant length scales? This limit is a "classical limit" similar
to the transition from wave optics to ray optics that occurs when the
typical size of optical elements becomes much larger than the
wavelength of light. I'll show that, whereas the interference
correction to the ensembleaveraged conductance (weak localization)
disappears in this classical limit, the quantum interference
contribution to the samplespecific conductance fluctuations remains
surprisingly unaffected.

January
14, 2008
Higher Angular Momentum Kondo Liquids
Dr. Pouyan Ghaemi
MIT
The variation in the
quasiparticle weight Z on moving around the fermi surface in correlated
metals is studied theoretically. Our primary example is a heavy Fermi
liquid treated within the standard hybridization mean field theory. The
most dramatic variation in the quasiparticle weight happens in
situations where the hybridization vanishes along certain directions in
momentum space. Such a "hybridization node" is demonstrated for a
simplified model of a Ceriumbased cubic heavy electron metal. We show
that the quasiparticle weight varies from almost unity in some
directions, to values approaching zero in others. This is accompanied
by a similar variation in the quasiparticle effective mass. Some
consequences of such hybridization nodes and the associated angle
dependence are explored. Comparisons to somewhat similar phenomena in
the normal metallic state of the cuprate materials are discussed. A
phenomenological picture of the pseudogap state in the cuprates with a
large Fermi surface with a severely anisotropic spectral weight is
explored. In certain cases, the quasiparticle residue goes to zero at
isolated points (in two dimensions) on the Fermi surface. We also point
out the possibility of quantum Hall phenomena in two dimensional Kondo
insulators, if the Kondo singlet has complex internal angular momentum.

January
25, 2008
First Order Quantum Phase Transition into a
Quantum Critical Point
Dr. Pallab Goswami
University of California (UCLA)
We provide a heuristic argument
for disorder rounding of a first order quantum phase transition into a
continuous quantum phase transition. This is illustrated by
renormalization group analysis of the Ncolor AshkinTeller chain, the
q state Potts chain, the biquadratic spin one antiferromagnetic chain
and the O(N) vector model with cubic anisotropy. In most of the cases,
the results of renormalization group analysis are in excellent
agreement with our heuristic argument. In a certain range of parameter
space of the AshkinTeller model renormalization group calculations
break down and indicate lack of universality. This may imply the
persistence of the first order quantum phase transition and a possible
modification of AizenmanWehr theorem for the quantum phase transitions.

January
28, 2008
Exciton
Superfluid in 2D Hole Bilayers
Professor
Emanuel Tutuc
Microelectronics Research Center
University of Texas at Austin
When
two layers of twodimensional carriers are brought into close
proximity, new phenomena resulting form the carriercarrier
interaction in opposite layers occur. Here, we explore a few
of these phenomena observed in interacting GaAs hole bilayers, at low
temperatures and in high magnetic fields. One key experimental
observation in these systems is a charge
neutral superfluid at total Landau level filling (nu)
factor one when equal and opposite currents are passed in the two
layers (counterflow). At the lowest temperatures (T) both
Hall and longitudinal counterflow resistivities (rho_{xx}
and rho_{xy})
vanish at nu=1, a
finding which demonstrates
the existence of a counterflow superfluid in the limit of T=0 at this
filling factor. This phenomenon can be
explained by the formation of electronhole pairs (excitons) in the
two, halffilled layers, and the resulting excitonic condensation at
the lowest temperatures. The counterflow rho_{xx}
and rho_{xy}
dependence on
temperature and bilayer density suggests that the counterflow
dissipation is caused by existence of mobile vortices which move
across the superfluid current.

February
01, 2008
Nonequilibrium
quantum criticality in an open itinerant electron magnet
Dr. So Takei
University of Toronoto
The study of critical phenomena
in equilibrium manybody systems is by now a wellestablished
subfield of condensed matter physics. The central issue in equilibrium
critical phenomena is the emergence of universal behaviour among
various systems with different microscopic properties when they are in
the vicinity of a critical point. Whether or not this
universality persists in systems driven far from equilibrium is
an interesting but largely unexplored problem. In this talk, I
will address this issue by considering a thin itinerant electron
magnet driven into a nonequilibrium steady state by current flow
across the system. The theory is formulated using the Keldysh
functional integral formalism, and the renormalization group
scheme is generalized to the nonequilibrium case to distinguish
different universality classes of nonequilibrium perturbations.
We show the relevance of departures from equilibrium at an
equilibrium critical point and present the consequent decoupling
of statics and dynamics by the nonequilibrium drive. By mapping
our problem onto an effective classical theory we find that the
leading effect of the nonequilibrium drive is to generate an
effective temperature.

February
04, 2008
The
Role of Disorder in Supersolidity
Dr. Sophie Rittner
Cornell University
A supersolid state of matter,
i.e. a solid with crystalline structure that exhibits frictionless
superflow, was theoretically proposed to occur in solid 4He almost 40
years ago. The first experimental indication for the existence of
supersolidity in helium was discovered in 2003 when Kim and Chan
employed the torsional oscillator technique to observe nonclassical
rotational inertia (NCRI). In order to determine if supersolidity is an
intrinsic property of helium we have studied the effect of disorder on
the NCRI. We find that the supersolid signal can be substantially
reduced or sometimes eliminated by annealing of the
helium crystal. Also, we have been able to increase the supersolid
signals by three orders of magnitude by geometrically confining the
sample space. We believe that the supersolid signal is strongly
modified by varying the amount of crystalline disorder.

February
18, 2008
The Suprafroth and Topological
Hysteresis
Professor Ruslan Prozorov
Ames National Laboratory, Iowa State University
Usually magnetic hysteresis is
caused by the imperfections and defects in the crystal structure
resulting in pinning of magnetic flux in superconductors or domain
walls in ferromagnets. The hysteresis may also be caused by shape and
surface  related energy barriers (e.g., geometric and BeanLivingston)
that result in a spatially nonuniform free energy and corresponding
metastable states of the system.
I will describe a different type of magnetic hysteresis that is only
observed in clean, pinningfree samples where structure of the mixed
phases patterns becomes a variable in finding the ground state. This
hysteresis cannot be annealed or removed by any sample improvement. We
call this phenomenon a “topological hysteresis“ to indicate that the
difference in the topologies of the intermediate state in typeI
superconductors or ferromagnetic domains in soft ferromagnets can lead
to a measurable hysteretic response of magnetization. Furthermore,
structure of the intermediate state in typeI superconductors maps onto
a 2D froth. However, unlike usual soap froth where time – driven
coarsening is irreversible (drainage and drying), coarsening of the
suprafroth is driven by easily manageable parameters – temperature and
magnetic field. Similarities and differences between suprafroth and
conventional froths will be discussed.
1. R. Prozorov, R. W. Giannetta, A. A. Polyanskii and G. K. Perkins,
"Topological hysteresis in the intermediate state of typeI
superconductors", Phys. Rev.B 72, 212508 (2005).
2. R. Prozorov, "Equilibrium topology of the intermediate state in
typeI superconductors of different shapes", Phys. Rev. Lett. 98,
257001 (2007).

February
28, 2008
Photon
localization and Dicke
superradiance : a crossover to small world networks
Professor Eric Akkermans
Technicon, Israel
We
study photon localization in a gas of cold atoms, using a Dicke
Hamiltonian that accounts for photon mediated atomic dipolar
interactions. The photon escape rates are obtained from a new class
of random matrices. A scaling behavior is observed for photons
escape rates as a function of disorder and system size. Photon
localization is described using statistical properties of random
networks which display a "small world" crossover. Those results
are compared to the Anderson photon localization transition.

March
03, 2008
Modulation spectroscopy
with ultracold atoms in an optical lattice
Dr. Aníbal Iucci
University of Geneva
The
remarkable control on the parameters reached
in cold atoms loaded in optical lattices setups has led to the employ
of out of equilibrium processes to increase the ability to probe the
properties of these systems. We study the response of one dimensional
1D ultracold atoms in optical lattice to external timedependent
perturbations like the periodic modulation of the lattice height. We
find that for bosons, sharp energy absorption peaks are not unique to
the Mott insulating phase at commensurate filling but also exist for
superfluids at incommensurate filling. For strong interactions, the
peak structure provides an experimental measure of the interaction
strength. For fermions, the spectrum of the induced double occupancy
allows to determine the pairing gap in a superfluid state and the
interaction energy in a Mottinsulating state.

March
24, 2008
Raman
spectroscopic studies of Cu_xTiSe_2
Professor Lance
Cooper
University of Illinois at UrbanaChampaign
One of the most important
current goals of condensed matter physics research involves elucidating
the competition between diverse quantum phases  such as between
antiferromagnetic or chargedensity wave (CDW) and superconductor (SC)
phases  in strongly correlated materials. By studying the low
temperature inelastic light scattering spectra of correlated materials
while pressure and/or magneticfieldtuning their phase behavior, we
have been able to investigate the evolution of the low frequency
excitation spectra through various quantum phase transitions in
correlated materials. In this talk, I'll focus on the remarkable
competition among novel phases observed as a function of both pressure
and Cuintercalation in 1TTiSe2, which is a semimetal or smallgap
semiconductor that develops a commensurate CDW with a
2a×2a×2c superlattice structure at temperatures below a
secondorder phase transition at TCDW ~ 200K. Our low temperature
pressuredependent studies of 1TTiSe2 revealed that lattice
compression leads to quantummechanical (T ~ 0) melting of the CDW
phase through a novel incommensurate phase that may have hexatic order.
More recently, Morosan et al. [1] discovered that lattice expansion of
1TTiSe2 via copper intercalation (i.e., CuxTiSe2) leads to the
suppression of the CDW transition temperature, and the emergence near
x=0.04 of a SC phase having a maximum Tc of 4.15 K at x=0.08. Among
other interesting results, we find evidence for xdependent ‘quantum’
(T~0) softmode behavior in CuxTiSe2 that is consistent with a quantum
critical point hidden in the superconductor phase of CuxTiSe2,
suggesting a possible connection between quantum criticality and
superconductivity.
[1]. E. Morosan et al., Nature Phys. 2, 544 (2006).

April 14, 2008
Spin Resonance in the
dwave Superconductor CeCoIn_{5}
Professor Collin Broholm
John Hopkins University
A range of
strongly
correlated metals feature an intricate mix of magnetism and
superconductivity. While copper oxides famously display such behavior
on the 100 Kelvin temperature scale, closely related phenomena are
encountered in rare earth and actinide intermetallics on the 1 Kelvin
temperature scale. Applications and two orders of magnitude in energy
not withstanding, much can be learned about both classes of materials
by comparing and contrasting their properties. Following a brief
introduction to the now quaint subject of heavy fermion
superconductivity, I shall discuss our recent discovery of a spin
resonance in the superconducting state of CeCoIn_{5} [1]. A
surprising result that follows from the neutron data is that the
reduction in 4f intersite spin exchange energy is substantially
greater than the reduction in thermal energy associated with
superconductivity. This suggests that the energy gain from
strengthening
near
neighbor spin exchange may actually be able to drive an
antiferromagnetically correlated metal to dwave superconductivity.
[1]
C. Stock, C.
Broholm, J. Hudis, H. J. Kang, and C. Petrovic, Phys. Rev. Lett. 100,
087001 (2008)

April
21, 2008
Is
there a quantum phase transition in trapped polarized Fermi gases?
Professor H.T.C. Stoof
Instituut voor Theoretische Fysica
Universiteit Utrecht
We review the contradictory
experimental results obtained by the MIT and Rice groups, lead
respectively by Wolfgang Ketterle and Randy Hulet, on
resonantlyinteracting polarized Fermi gases. In particular, the
former group appears to see a quantum phase transition between a paired
superfluid state and a polarized normal state, whereas the latter group
does not see such a transition. We discuss the progress that has been
made in the last year to resolve this discrepancy and present the open
problems in the field.

April
28, 2008
Challenges and chemical trends in achieving
a room temperature dilute magnetic semiconductor: a spintronics tango
between theory and experiment
Professor Jairo Sinova
Texas A&M University
In the world of spintronics the
revolution of the strongly correlated systems dubbed as diluted
magnetic semiconductors has seen a resurgence over the past five years
in attempting to achieve ever higher Curie temperatures. This has been
fueled in part from our clearer understanding of the paradigm of these
materials, (Ga,Mn)As, which is at present one of the best understood
ferromagnetic systems in its most metallic form. Through a diversity of
theoretical tools and in constant comparison with experiments, the
understanding of what it takes to achieve a room temperature DMS has
gained a strong foothold and new attempts to find the ultimate
combination which optimizes the diverse parameters which determines the
Curie temperature is underway. I will review part of the current
understanding of (Ga,Mn)As gained through phenomenological effective
Hamiltonian models and other more microscopic calculations and what it
makes us believe its understanding is quite good, and what new
materials are being explored to reach a room temperature ferromagnetic
semiconductor.

July 17,
2008
Quantum
Phase Transitions in a DoubleQuantumDot Device
Professor Kevin Ingersent
University of Florida
Gainesville
Quantum dots offer unique
possibilities for the controlled study of manybody correlations. This
talk presents theoretical predictions for a doublequantumdot device
consisting of one dot in its Kondo regime and a larger, effectively
noninteracting dot, both coupled in parallel to a pair of external
leads. This system maps onto a singleimpurity Anderson model with a
structured (nonconstant) density of states. By tuning a plunger gate
voltage on the noninteracting dot, different features of this effective
density of states can be brought to the Fermi energy. In one regime,
band filtering through the noninteracting dot splits the Kondo
resonance, even though the singlet ground state remains robust. The
system can also be continuously tuned to create a powerlaw vanishing
of the effective density of states and to access a pair of quantum
phase transitions separating Kondoscreened manybody ground states
from a nonKondo phase. These zerotemperature transitions have
characteristic signatures in the zerobias conductance at temperatures
T > 0. The prospects for experimental detection of the transitions
are enhanced by the fact that the conductance features increase in
prominence with increasing T up to quite high temperatures. If time
permits, the talk will end with a discussion of the effect on the
aforementioned results of having weakbutnonvanishing Coulomb
interactions in the larger dot.
