September
14, 2009
Nonlinear WavePacket Dynamics in a Disordered Medium
Prof. Alexander Finkelstein &
Dr. Georg Schwiete
Texas A&M University
Recent experiments on pulse propagation in a nonlinear and
disordered medium, realized in (i) photonic crystals and (ii) cold atomic
gases, opened a new theoretical problem that has neither been discussed in
the context of nonlinear physics, nor in condensed matter physics. In this
talk I will explain why despite the apparent success of the experiments that
aimed at the visualization of Anderson localization the essential physics of
nonlinear pulse propagation in disordered media still remains to be
observed. We have formulated an effective theory of pulse propagation in a
nonlinear and disordered medium in terms of a nonlinear diffusion equation.
Despite its simplicity this equation describes novel phenomena which we
refer to as ?locked explosion? and ?diffusive? collapse.

September
28, 2009
Electron transport
through a singlemolecule magnet from first principles
Prof. Kyungwha Park
Virginia Tech University
Recently, there have been a great amount of experimental efforts
to build and characterize nanoscale singlemolecule magnets
deposited on surfaces or bridged between electrodes aiming at
applications for information storage devices or quantum computing
materials. Singlemolecule magnets showed magnetic quantum
tunneling and quantum interference, and may exhibit an intriguing
effect of the coupling between spin and charge degrees of freedom
on transport. Reported transport measurements through the prototype
singlemolecule magnet Mn12 demand theoretical inputs on the roles
of the interfaces and molecular geometries on the transport, and
on whether electronic and magnetic properties are maintained in
lowdimensional structures. To provide such theoretical inputs,
largescale simulations at the atomistic level are required. We
simulate semiinfinite electrodes and different molecular geometries
and interfaces which would mimic experimental setups. Then we
compute transport properties through the singlemolecule magnet
Mn12, using the nonequilibrium Green's function method
in conjunction with densityfunctional theory. We discuss the
coupling between the Mn12 and the electrodes, as well as charge
distribution of conduction electrons over the Mn12 depending on
molecular geometries and interfaces. We also present a possibility
of using the Mn12 as a spin filter at low bias voltages.

October
5, 2009
Direct observation of spincharge separation and interaction effects in
GaAs quantum wires by momentumconserved tunneling
Prof. Chris Ford
University of Cambridge and University of Birmingham
Coulomb interactions have been predicted to have a profound effect on the
behaviour of electrons in one dimension. We have fabricated a 1D system in
which we observe spincharge separation in momentumconserved tunneling
from an array of 1D wires into a 2D electron gas, and also a powerlaw
suppression of tunneling into the wires. These are as predicted for a
TomonagaLuttinger Liquid (TLL), the simplest analytic model of an
interacting 1D system. The use of an array of wires averages out impurity
effects and allows the lowest 1D subband to be probed with precise control
of electron density. We observe spincharge separation in the dispersion
relation of the 1D wires, mapped by varying the inplane magnetic field and
the dcbias. We find that the separation persists beyond the regime of the
TLL approximation. Furthermore, the measured 1D2D tunneling current is
suppressed at zero dc bias in the presence of a magnetic field, confirming
that interactions are important in the 1D wires. This suppression has been
measured as a function of temperature and sourcedrain voltage. These both
have similar powerlaw dependences, as predicted by the TLL model.

October
21, 2009
Lattice distortion and magnetic quantum phase transition in CeFeAs1xPxO
Dr. Clarina R. dela Cruz
University of Tennessee, Knoxville
With the advent of Febased superconductivity initially discovered in the prototypical electron doped Fepnictide LaFeAsOxF1x, came a surge of renewed interest in high temperature superconductivity. Neutron scattering was used to show the antiferromagnetic (AFM) order in the parent compounds of the iron arsenide superconductors. The discovery of the ubiquitous magnetic order, and its competition with superconductivity, across the various families brings attention to understanding the interplay between magnetism and hightransition temperature (highTc) superconductivity in these materials. A feature of the parent compounds is the structural distortion that occurs in the vicinity of the onset of long range magnetic order of the Fespins. In the RFeAsO(R=rare earth) family, the magnetostructural transition is suppressed in favor of superconductivity upon doping charge carriers into the system, which alters the system electronically and crystallographically as well. To understand the lattice effect on the suppression of the AFM ground state itself, it is important to isoelectronically tune the crystal lattice structure without the influence on charge carrier doping and superconductivity. Here we use neutron powder diffraction to show that replacing the larger arsenic with smaller phosphorus in CeFeAs1xPxO simultaneously suppresses the AFM order and orthorhombic distortion near x = 0.4, providing evidence for a magnetic quantum phase transition. Furthermore, we find that the pnictogen height in these iron arsenides is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity.

October
22, 2009
From Black Holes to Strange Metals: ManyBody Physics Through A Gravitational Lens
Prof. Hong Liu
Massachusetts Institute of Technology
In the last ten years string theory has revealed a surprising and deep connection between gravity and manybody physics. Puzzling issues in quantum gravity can now be formulated as questions in a manybody system without gravity, where conventional quantum mechanics applies. Alternatively, difficult questions in some strongly coupled manybody systems can be answered by simple calculations in classical gravity. The physical intuition behind this connection will be briefly described, as well as new insights into the quantum nature of black holes that have been obtained. I will then focus on some recent work where the connection has been used to find universality classes of nonFermi liquids, with a possible application to the strange metal phase of the cuprate high temperature superconductors.

November
3, 2009
Interplay between Antiferromagnetic Quantum Critical Point and Selective Mott Transition in Pure and Doped YbRh2Si2
Prof. Frank Steglich
MaxPlanck Institute for Chemical Physics of Solids, Dresden, Germany
In the heavyfermion metal YbRh2Si2 a quantum critical point (QCP) has been established by driving a continuous antiferromagnetic (AF) phase transition from TN ~70 mK at B = 0 to TN = 0 via application of a tiny magnetic field Bc (?c) ~ 60 mT. New results on the Hall coefficient, magnetic Gr¨¹neisen ratio and thermoelectric power support the conclusion drawn from earlier studies that this AF QCP coincides with a Kondobreakdown QCP or Mott transition, selective to the Yb3+  4f states. In a recent investigation, (positive and negative) chemical pressure was applied to YbRh2Si2 to explore the evolution of its BT phase diagram under changes of the unitcell volume: Clear signatures of the selective Mott transition were observed within the magnetically ordered phase under volume compression (i.e., Co substitution for Rh). Here, the AF QCP appears to be of the conventional (3D SDW) type. Under slight volume expansion (doping with 2.5 at % Ir) the AF instability and the selective Mott transition were found to still coincide at Bc (?c) ~ 40 mT. For 6 at% Ir doping, however, AF order appears to be largely suppressed (TN < 20 mK), while the Kondobreakdown QCP remains virtually unchanged. For this composition, a new type of lowT spinliquid phase shows up in a finite range of magnetic fields. Further ongoing studies concerning the interplay between the AF QCP and the selective Mott transition in this material will also be briefly mentioned. In collaboration with: M. Brando, S. Friedemann, P. Gegenwart, C. Geibel, S. Hartmann, S. Kirchner, C. Krellner, M. Nicklas, N. Oeschler, Q. Si, O. Stockert, Y. Tokiwa, T. Westerkamp and S. Wirth.

November
9, 2009
Emergent Supersymmetry and String in Condensed Matter Systems
Prof. SungSik Lee
McMaster University, Canada
Quantum field theories arise as low energy effective descriptions for
gapless states in condensed matter systems. Although strongly coupled
quantum field theories are rather common, currently there is no
systematic way of understanding those theories. In this talk, I will
discuss about two condensed matter systems where nonperturbative tools
may shed some light on the strongly coupled low energy physics. In the
first part, I will talk about a 2+1 dimensional lattice model where
emergent superconformal symmetry enables one to understand a strongly
interacting critical point nonperturbatively. In the second part, a 2+1
dimensional nonFermi liquid state will be discussed where a
matrix/string theory emerges in the low energy limit.

November
19, 2009
From Frustration to Correlation via Fluctuation
Prof. Yong Baek Kim
University of Toronto, Canada
Competing interactions between electrons or spins on geometrically frustrated lattices may not be satisfied simultaneously. The resulting frustration often leads to macroscopically degenerate classical ground states. We take an example of the frustrated magnets, namely interacting local moments on geometrically frustrated lattices, and discuss how thermal/quantum fluctuations on the degenerate classical ground states lift the degeneracy. Relieving the frustration via such fluctuations leads to various conventional and exotic quantum ground states with characteristic correlations. Applications to the experiments on newly discovered frustrated quantum magnets are discussed.

January
12, 2010
1D Bose Gas as the NonRelativistic Limit of the sinhGordon Model
Dr. Marton Kormos
SISSA, Italy
Due to recent experimental achievements with trapped ultracold atoms,
the properties of the 1D nonrelativistic Bose gas are of great
interest. In many experimental setups the behavior of the particles is
very well described by the LiebLiniger model which in many aspects
can be regarded as a theoretical benchmark in the research of
integrable models. In spite of its integrability, calculating
correlation functions in the model is notoriously difficult. In the
talk I will propose a novel approach to compute expectation values and
other physical quantities in the LiebLiniger model. The method is
based on the fact that in the repulsive case the Smatrix, the
Lagrangian and the operators can be obtained from a certain
nonrelativistic limit of the sinhGordon model. This observation
allows us to compute expectation values in the LiebLiniger system
both at zero and finite temperature.

January
25, 2010
What Do We Learn from the Thermoelectric Transport in Graphene?
Prof. Jing Shi
University of California, Riverside
Graphene has attracted much attention in the condensed matter physics and materials
science communities due to its exotic electron excitation spectrum and unique physical
properties. Electrical transport has already revealed a great deal of interesting
physical properties such as the vanishing cyclotron mass and quantum anomaly at the
Dirac point. In this talk, I will present our recent thermoelectric transport property
study on graphene in both classical and the quantum transport regimes. The
thermoelectric coefficients such as the Seebeck and Nernst coefficients probe the energy
derivatives of the electrical conductivities (both longitudinal and the Hall);
therefore, they show more pronounced anomalous behaviors near the Dirac point. In
addition to the unique band structures, the nature of the impurities may also be
inferred from the temperature dependence of these thermoelectric transport coefficients.

January
26, 2010
NonEquilibrium Steady State Charge Transport in an Interacting Open Quantum System
Dr. Dibyendu Roy
University of California, San Diego
I will discuss how LippmannSchwinger scattering theory can be
employed to calculate nonequilibrium steady state current through an open
quantum dot with local electronelectron interaction. The twoparticle
current is evaluated exactly while we use perturbation theory to
calculate the current when the leads are Fermi liquids at different
chemical potentials. Finally I
will tell some applications of this technique to demonstrate different
interesting phenomena like twoparticle resonance, current asymmetry, and
spinfiltering in different dot models.

February
5, 2010
The Phase Diagrams of Strongly Interacting Ultracold Atoms
Dr. Charles Mathy
Princeton University
The experimental realization of model condensed matter
systems in ultracold atoms allows for a complete exploration of a
variety of phase diagrams, as one is able to tune such parameters as
mass, polarization and interaction strength, with a flexibility
unmatched in any other experiments. In this talk I will discuss two
ultracold atomic systems : twocomponent fermions in an optical
lattice, and imbalanced fermi gases. The interest in these systems
stems from their relation to important problems in condensed matter
physics, and their experimental accessibility. I will discuss what has
already been achieved in these systems, what still remains to be done,
and what the main experimental and theoretical challenges are to that
end.

February
8, 2010
SpinTriplet Superconductivity in CoBased Josephson Junctions
Prof. Norman Birge
Michigan State University
Superconducting/Ferromagnetic (S/F) hybrid systems exhibit a
number of interesting properties due to the interplay between the competing
symmetries of their order parameters. With conventional spinsinglet
superconductors, the proximity effect in S/F systems decays over an
extremely short length scale in the ferromagnet due to the large exchange
splitting between the spinup and spindown electron bands. In S/F/S
Josephson junctions, the critical current oscillates and decays rapidly as a
function of the ferromagnetic layer thickness. If there were spintriplet
superconducting correlations present, however, then both the proximity and
Josephson effects would persist over much longer distances. Such
correlations have been predicted to occur in S/F systems with certain forms
of magnetic inhomogeneity near the S/F interface. Moreover, these
correlations exhibit a strange symmetry never before observed: they are odd
in frequency or time. In this talk I will discuss our efforts to produce and
measure these elusive spintriplet correlations in S/F/S Josephson
junctions, culminating in our very recent success.

February
15, 2010
SpinTriplet Superconductivity in CoBased Josephson Junctions
Dr. Junliang Song
University of British Columbia, Canada
Spin correlations and coherent dynamics in a spinor BEC are usually driven
by mean field energies, either due to scattering between atoms or due to coupling to
external fields; coherent quantum dynamics have been observed in various cold atom
experiments. In this talk, I will report our studies on some beyondmeanfield phenomena
that are driven by quantum fluctuations in hyperfine spintwo ultracold atoms. It is
shown that zero point quantum fluctuations of collective spin coordinates can completely
lift the accidental continuous degeneracy that is found in mean field analysis of quantum
spin nematic phases of hyperfine spintwo cold atoms. Distinct spin nematic states with
higher symmetries are selected by quantum fluctuations. It also shown that a new type of
coherent spin dynamics can be driven purely by quantum fluctuations. Unlike the usual
meanfield coherent dynamics, quantum fluctuationcontrolled spin dynamics are very
sensitive to variation of quantum fluctuations. They have peculiar dependence of Zeeman
field and potential depths in optical lattices.

February
22, 2010
Scratching the Surface
Prof. Michael Fuhrer
University of Maryland
Graphene, a single atomthick plane of graphite, has recently been
isolated and studied experimentally. In this twodimensional hexagonal
lattice of carbon atoms, the electrons obey the Dirac equation for massless
particles, complete with a twocomponent spinor degree of freedom that
mimics the spin of a relativistic particle. Graphene is also composed
entirely of surface atoms, making the techniques of surface science useful
in studying its properties. In this talk, I will first discuss the
electronic structure of graphene, and its implications for electronic
properties. I will then discuss experiments which combine ultrahigh vacuum
(UHV) surface science with electronic transport measurements. Surface
science techniques can be used to controllably modify graphene's properties:
potassium atoms can be deposited to form charged impurity scatterers; ice
can be deposited to modify the dielectric environment of graphene and tune
the electronelectron interaction strength; and ion irradiation can be used
to create atomic vacancies which act as Kondo impurities, and at high
densities induce strong localization. Graphene may also be used as a
sensitive detector to observe chemical reactions occurring on its surface at
concentrations below 1/1000th of a monolayer.

April
2, 2010
Unusual MagnetoElectric Phenomena in Topological Insulators
Prof. Marcel Franz
University of British Columbia, Canada
Recently discovered topological insulators (TIs) are materials with bulk bandgap and robust gapless surface states protected by topological invariants that characterize their bulk band structure. After a brief introduction to the physics of TIs I will describe two new effects that have been predicted to occur in these materials: the Witten effect and the `wormhole' effect. According to the first a unit magnetic monopole inserted into a TI binds precisely quantized fractional electric charge e/2. According to the second an infinitely thin solenoid carrying half of the magnetic flux quantum inserted into a TI carries onedimensional topologically protected gapless fermionic modes. Both effects depend solely on the topological invariants and are thus universally present in all topological insulators. I will discuss broad physical significance of these findings as well as possibilities for experimental observation of closely related phenomena.

April
12, 2010
Magnetism and Pairing Symmetry in the IronBased Superconductors
Prof. Jiangping Hu
Purdue University
I discuss the existence of strikingly identical paradigms applicable to both cuprates and ironbased superconductors in understanding magnetism, superconductivity and the interplay between the two. The magnetic states and transitions in iron based superconductors are well described by a J1J2Jz magnetic exchange model where J1, J2 and Jz are nearest neighbour, next nearest neighbour and interlayer couplings respectively. Differing from the tJ model for cuprates where dwave pairing symmetry is favored, the magnetic exchange in the iron based superconductors leads to a new prediction that an unconventional swave coskxcosky pairing dominates. I emphasize that the superconducting gaps in different Fermi pockets are determined by a single energy scale parameter which is a distinctive prediction that differs from the weak coupling theories. I will show recent experimental results that support the model and it¡¯s predictions.

April
26, 2010
FRUSTRATION BY DESIGN: Artificial Frustrated Magnets
Prof. Peter Schiffer
Penn. State University
Our group has developed and studied 'artificial frustrated
magnets', model systems based on geometrically frustrated magnetic
materials. Artificial frustrated magnets consist of arrays of
lithographically fabricated singledomain ferromagnetic islands, arranged in
different geometries such that the magnetostatic interactions between the
island moments are frustrated. Magnetic force microscopy imaging of these
arrays allows us to study the accommodation of frustration through the local
correlations between the moments as a function of both the strength of the
interactions and the geometry of the frustration. The results closely mimic
those of the "spin ice" materials, and allow a detailed analysis of the
local correlations in two dimensions. We have also used these arrays to
analyze the process of demagnetization, which is necessary to access low
energy collective states in our arrays and in many other magnetic systems.
Our results shed light on the nature of magnetism in patterned arrays and
provide a rich arena in which to study the physics of frustration.
References: R. F. Wang et al. 439, 303 (2006); C. Nisoli et al. Physical
Review Letters 98, 217203 (2007); X. Ke et al. Physical Review Letters 101,
037205 (2008) and Applied Physics Letters 93 252504 (2008); Li et al.
Physical Review B 81, 092406 (2010).

May
3, 2010
Topological Transitions in Dissipative Quantum Transport
Dr. Mark Rudner
Harvard University
We investigate quantum transport in a family of onedimensional
models where a particle can decay whenever it visits sites on one of two
sublattices. The corresponding nonHermitian tightbinding problem
exhibits distinct topological phases, characterized by a winding number
defined in terms of the Bloch eigenstates in the Brillouin zone. We find
that the mean displacement of a particle initially localized on one of the
nondecaying sites can be expressed in terms of the winding number, and
is therefore quantized as an integer, changing from zero to one at the
critical point. This distinctive and robust feature can be used as an
experimental test for quantum behavior in multilevel systems such as
Josephson arrays, and holds additional implications for photon and
nuclear spin pumping.

May
4, 2010
HOLOGRAPHIC MODELS OF SYMMETRY BREAKING AND QUANTUM PHASE TRANSITIONS
Dr. Nabil Iqbal
MIT
Certain stringtheory inspired ideas allow us to understand strongly coupled field theories by a mapping to a weakly coupled gravity theory in one higher dimension. Recently these techniques have been turned to problems relevant to condensed matter theory. I will review these ideas and discuss a holographic model realizing an "antiferromagnetic" phase in which a global symmetry is broken by the strongly interacting dynamics of a finite charge density. The transition can be driven to zero temperature, when it becomes a quantum phase transition of the BerezinskiiKosterlitzThouless type, and I will explain how this fact can be related to the nearhorizon geometry of a zerotemperature black hole.

June
10, 2010
EVIDENCE FOR NONFERMI LIQUID PHASE IN GeSUBSTITUTED YbRh2Si2
Prof. Silke BühlerPaschen
Technische Universität Wien
The canonical view of heavy fermion quantum criticality assumes a single quantum critical point separating the paramagnet from the antiferromagnet. Recent experiments on quantum critical Ybbased heavy fermion compounds including Yb(Rh{0.94}Ir{0.06}2Si2, YbAgGe, and ßYbAlB4 have tentatively observed the presence of nonFermi liquid behavior over a finite zerotemperature region of the magnetic field or pressuretuned phase diagram, rather than at a single quantum critical point. Our detailed susceptibility and transport measurements show that the ``classic' quantum critical system, Gesubstituted YbRh2Si2, also displays a finite zerotemperature region of nonFermi liquid behavior. I shall discuss possible interpretations of these results, advancing arguments that the nonFermi liquid phase in this material is not a disordersmeared quantum critical point, but a new class of metallic phase.
