Where: HZ 116
When: Mondays at 4:00 p.m.
September
8, 2008 Thermodynamic
Study of the Supersolid Phase
Transition
Dr. Xi Lin
Penn State University
Liquid 4He enters the
superfluid state and flows without friction below 2.176 K. Thin liquid
films adsorbed on solid substrates undergo the same transformation,
although at a lower temperature. When the substrate is subjected to
oscillatory motion a portion of the film, known as the superfluid
fraction, decouples from the oscillation. A similar phenomenon has been
observed in solid 4He, in which a fraction of the solid seems to
decouple from the motion of the surrounding lattice. This observation
has been replicated in various laboratories. In addition, a heat
capacity peak, on top of the phonon contribution, that coincides with
the onset of mass decoupling has been found. The excess specific heat
peaks with a magnitude that decreases with increasing sample quality,
in a way similar to the NCRI sample quality effect.

September
29, 2008 Onedimensional
fermions beyond the Luttinger liquid paradigm
Prof. Michael Pustilnik Georgia Tech In this talk I will review
recent works on dynamic correlation functions in onedimensional
systems of interacting fermions. Due to the energy and momentum
conservation, the correlation functions exhibit a characteristic
threshold behavior. Surprisingly, the conventional harmonic fluid
(a.k.a. Luttinger liquid) framework is not sufficient for the
description of the threshold singularities in the correlation functions.

October
10, 2008 Nonequilibrium Effects Near Itinierant
Electron Quantum Critical Points
Prof. Aditi Mitra New York University An important set of questions
in condensed matter relate to the effect of a nonequilibrium drive on a
system near a quantumcritical point. In this talk results will be
presented for the effect of current flow on two systems: one near a
ferromagneticparamagnetic quantum critical point, and the other near a
superconductormetal quantum critical point. For a ferromagnetic system
it will be shown that current flow has two important effects. One is to
produce decoherence that affects the system in ways rather similar to
temperature. Second, it causes an inversion symmetry breaking which
qualitatively has the effect of producing a drift. For Ising magnets
decoherence is the dominant effect, whereas for Heisenberg magnets the
inversion symmetry breaking due to current flow can lead to dynamical
instabilities of the ordered phase. For a superconducting
orderparameter nonequilibrium effects are even more pronounced as the
order parameter is charged and couples directly to the electric field.
For these systems it will be shown that it is the direct coupling which
has the dominant effect on scaling near a critical point, with the
effect of current induced drift and decoherence being subdominant.

October
20, 2008 Multichannel Kondo Models in nonAbelian
Quantum Hall Droplets
Prof. Gregory Fiete UT Austin The study of topological states
of matter has generated much interest recently on both the theoretical
and experimental fronts. The most well established topological
states are the quantum Hall states. In this talk we study the
coupling between a quantum dot and the edge of a nonAbelian fractional
quantum Hall state which is spatially separated from it by an integer
quantum Hall state. Near a resonance, the physics at energy
scales below the level spacing of the edge states of the dot is
governed by a $k$channel Kondo model when the quantum Hall state is a
ReadRezayi state at filling fraction $\nu=2+k/(k+2)$ or its
particlehole conjugate at $\nu=2+2/(k+2)$. The $k$channel Kondo model
is channel isotropic even without fine tuning in the former state; in
the latter, it is generically channel anisotropic. In the special case
of $k=2$, our results provide a new venue, realized in a mesoscopic
context, to distinguish between the Pfaffian and antiPfaffian states
at filling fraction $\nu=5/2$.

October
27, 2008 Magnetotransport
in Microwaveirradiated Quantum Hall Systems
Prof. Michael Zudov University of Minnesota When a high quality twodimensional electron system is irradiated by microwaves, its magnetoresistance exhibits microwaveinduced resistance oscillations (MIRO) and zeroresistance states. MIRO were discussed in terms of the “displacement” model, which is based on microwaveassisted impurity scattering, and the “inelastic” model, stepping from the oscillatory electron distribution function. It is believed that the “inelastic” contribution greatly exceeds the “displacement” contribution and can also account for MIRO temperature dependence. This talk will discuss our recent experiments focusing on nonlinear response and temperature dependence of MIRO. First, we show that the ‘’displacement’’ mechanism cannot be ignored and might even dominate the response under typical experimental conditions. Further, we find that the MIRO temperature dependence originates primarily from the temperaturedependent quantum lifetime entering the Dingle factor. We suggest that the main source of the modification of the quantum scattering rate is the contribution from electronelectron scattering, a quantity hardly available from other experiments. 
November
10, 2008 FeAs
based
superconductors: A new class of highT_{c}
superconducting
compounds
Prof. Bernd Lorenz University of Houston The discovery of superconductivity in rare earth (R) oxypnictides, ROFeAs, by Hosono et al. has revived the field of hightemperature superconductivity. With transition temperatures of up to 55 K the new class of superconducting compounds has given hope to reach even higher T_{c}’s exceeding those of the copper oxide superconductors. At the same time, questions have been raised concerning possible similarities of the two highT_{c} systems with the perspective that studying the FeAs superconductors might also help to better understand the cuprates. I will present a brief overview of some recent results and discuss examples of FeAsbased superconductors crystallizing in different basic structure types: (i) The PbFCltype structure (LiFeAs) and (ii) the ThCr_{2}Si_{2} type structure (KF e_{2}As_{2}, CsFe_{2}As_{2}, and the solid solution (K/Sr)Fe_{2}As_{2}). The ternary compounds are all selfdoped superconductors. The (K/Sr)Fe_{2}As_{2 } – system reveals an interesting phase diagram, similar to the highT_{c} cuprates, with a maximum T_{c} at an optimal composition and a spin density wave (SDW ) state at the Srrich side. In a narrow composition range the SDW state is followed by a superconducting transition at lower temperatures. The extrapolation of the SDW phase boundary suggests the possible existence of a quantum critical point. 
November
17, 2008 The heat generation by electric current in
nanodevices
Prof. Xincheng Xie Oklahoma State University Over the past two decades, with
the development of micromachining technology, there has been enormous
progress in the operation speed and integrating techniques in the
semiconductor electronics. With these advancements, one critical issue
arises, namely the dissipation of the conducting electrons (i.e., the
thermal generation from electric currents). The dissipation in
nanodevices strongly hinders further development of the semiconductor
electronics. Thus, it is important to uncover the laws of thermal
generation induced by electric currents in nanodevices.
In this work, we study the heat generation in a nanodevice with an electric current flowing through. A general formula for the heat generation is derived by using the nonequilibrium Keldysh Green's functions. This formula can be applied in the linear and nonlinear transport regions, for timedependent cases, and with multiterminal systems. The formula is also valid if the nanodevice contains various interactions. As an application of the formula, the heat generation of the device with leadquantum dotlead is investigated. The dc and the ac biases are studied in detail. We find several interesting behaviors that are unique to nanodevices, revealing significant difference from heat generation in macroscopic systems. 
November
21, 2008 Crystal
Growth at CEA Grenoble
Prof. Gerard Lapertot
CEA Grenoble I will review various material
synthesis techniques and various aspects of a flux growth experiments
at CEA Grenoble. Crystal growth of CeIn3, MnSi, PrOs4Sb12 or YbCu2Si2
will be used as examples. This will be followed by short presentation
about mirror furnace crystal growths, their use and recent
developments. Finally I will outline components of "ideal/optimized"
crystal growth lab, including characterization techniques.

November
24, 2008 Hydrodynamic
Transport in Graphene
Dr. Lars Fritz Harvard Univeristy We study the thermal and
electric transport of a fluid of interacting Dirac fermions using
a quantum Boltzmann approach. We include Coulomb interactions, a dilute
density of charged impurities and the presence of a magnetic
field to describe both the static and the low frequency response
as a function of temperature $T$ and chemical potential $\mu$. In the
quantumcritical regime $\mu\lesssim T$ we find pronounced deviations
from Fermi liquid behavior, such as a collective cyclotron
resonance with an intrinsic, collisionbroadened width, and
significant enhancements of the Mott and WiedemannFranz ratio.
Some of these results have been anticipated by a relativistic
hydrodynamic theory, whose precise range of validity and failure
at large fields and frequencies we determine. The quantum
Boltzmann approach allows us to go beyond the hydrodynamic regime, and
to quantitatively describe the deviations from
magnetohydrodynamics, the crossover to disorder dominated Fermi
liquid behavior at large doping and low temperatures, as well as the
crossover to the ballistic regime at high fields. Finally, we obtain
the full frequency and doping dependence of the single universal
conductivity $\sigma_Q$ which parametrizes the hydrodynamic response.

December
1, 2008 Recent progress in neutron scattering
studies of FeAsbased superconductors
Prof. Pengcheng Dai University of Tennessee In this seminar, I will
summarize our
recent neutron scattering work focused on studying structural and
magnetic properties of FeAsbased superconductors [19]. Forneutron
diffraction, we systematically study the structural and magnetic
phase transitions in various FeAsbased materials and compare them
with recently discovered Fe(Se,Te) family of materials. For
inelastic neutron scattering, we focus on lowenergy spin wave
excitations and discuss magnetic exchange coupling. Crystal field
level excitations sensitive to the spin fluctuations in the FeAs
layer will also be discussed. Comparison will be made between our
neutron scattering work and current theoretical understanding of
these materials.
[1] Clarina de la Cruz et al., Nature 453, 899902 (2008). [2] Jun Zhao et al., Nature Materials (AOP, doi:10.1038/nmat2315). [3] Ying Cheng et al., Phys. Rev. B 78, 064515 (2008). [4] Q. Huang et al., Phys. Rev. B 78, 054529 (2008). [5] Jun Zhao, et al, Phys. Rev. B 78, 140504 (R) (2008). [6] Jun Zhao et al.,
Phys. Rev. B 78, 132504 (2008).
[7] Jun Zhao et al., Phys. Rev. Lett. 101, 167203 (2008) [8] S. Chi et al., Phys. Rev. Lett. (in press). [9] Shiliang Li et al., arXiv:0811.0195. 
January
12, 2009 Local
electronic properties of graphene
Prof. Brian Leroy University of Arizona Combining scanning probe
microscopy with electrical
transport measurements is a powerful approach to probe lowdimensional
systems. The local information provided by scanning probe
microscopy
is invaluable for studying effects such as electronelectron
interactions and scattering. Using this approach, we have probed
the
local electronic properties of graphene with atomic resolution.
We
studied the effect of ripples, charged impurities and defects on the
local density of states. We find that longrange scattering from
ripples and impurities shifts the Dirac point leading to electron and
hole puddles. Shortrange scattering from lattice defects mixes
the two sublattices of graphene and tends to be strongly suppressed
away from the Fermi energy.

January
20, 2009 Timereversal
symmetry breaking and spontaneous anomalous Hall effect in Fermi fluid
Prof. Kai Sun University of Illinois at Urbana Champaign We study the spontaneous
nonmagnetic timereversal symmetry breaking in a 2D Fermi liquid
without breaking either the translational symmetry or the U(1) charge
symmetry. Using a Berry phase approach, we found that for a large class
of models, including all one and twoband models, the timereversal
symmetry breaking states can be classified into two types, dubbed I and
II, depending on the accompanying spatial symmetry breaking patterns.
The properties of each class are studied. In particularly, we show that
the states breaking both timereversal and chiral symmetries (type II)
are described by spontaneously generated Berry phases and exhibit
anomalous Hall effect in the absence of magnetic fields and magnetic
impurities. We also show examples of the timereversal symmetry
breaking phases in several different microscopically motivated models
and calculate their associated Hall conductance within a meanfield
approximation. In particularly, we found a simple lattice structure in
which the timereversal symmetry breaking phases is stabilized by
infinitesimal interactions.

January
23, 2009 Charge
fractionalization in twodimensional Dirac fermions
Dr. ChangYu Hou Boston University Electron fractionalization is
intimately related to topology. In onedimensional systems,
fractionally charged states exist at domain walls between degenerate
vacua. In twodimensional systems, fractionalization exists in quantum
Hall fluids, where timereversal symmetry is broken by a large external
magnetic field. Recently, there has been a tremendous effort in the
search for examples of fractionalization in twodimensional systems
with timereversal symmetry. We will show that quasiparticle
excitations with irrational charge exist in tightbiding systems, such
as graphenelike structure, described, in the continuum approximation,
by the Dirac equation in (2+1)dimensional space and time. These
excitations can be deconfined at zero temperature, but when they are,
the charge rerationalizes to the value 1/2.

January
26, 2009 InteractionInduced
Localization in an Inhomogeneous Quantum Wire
Prof. Harold Baranger Duke University Localization of electrons induced
by electronelectron interactions is
a key issue in strongly interacting systems which dates from the dawn
of solid state physics when
Wigner introduced the notion of an electron crystal. We have studied such interactioninduced localization in several nanoscale settings: a homogeneous quasi1D electron gas, a circular quantum dot, and an inhomogeneous 1D system, a wire with two regions, one at low density and the other high. Quantum Monte Carlo techniques are used in order to treat the strong Coulomb interactions in the low density region. In all three systems, the electrons become localized if the density is sufficiently low. In the homogeneous wire, preliminary results for the "zigzag" transition will be presented. In the inhomogeneous wire, we focus on the separation between the high density and low density regions. If the external potential changes abruptly at the interface, a barrier develops between the two regions, causing Coulomb blockade effects. For a short low density region, as in a quantum point contact, a single localized electron naturally occurs. The picture emerging here is in good agreement with the experimental measurements of tunneling between two wires (the Yacoby group), and has implications for the "0.7 effect" in the conductance of quantum point contacts. 
February
2nd, 2009 Multiferroics:
New (and old) Materials
Dr.
Art Ramirez
Bell Labs The science of both
ferromagnets and ferroelectrics is mature, as are their device
manifestations. Materials possessing both types of order parameters,
"multiferroics", also exist but are much less. Of particular
interest are "induced multiferroics" which exhibit a magnetoelectric
response that becomes large and therefore potentially useful in a
critical region. An additional challenge is to discover systems
that possess a large magnetoelectric response at room temperature. I
will discuss our recent progress in developing useful multiferroics.

February
6th, 2009 Magnetic
BoseEinstein condensation vs. localization in a
sitediluted spin1 antiferromagnet Dr. Rong Yu University of Tennessee In this talk I will report on
the magnetic phase diagram of coupled spin1 chains with site diltion,
modeling the behavior of doped NiCl$_2$tetrakis thiourea (DTN). In
absence of doping, this compound
displays a fieldinduced BoseEinstein condensation of magnons, which has been revealed by the meanfield scaling of the critical temperature, $T_c \sim HH_c^{\phi}$ with $\phi=2/3$. At zero temperature, the system experiences a quantum phase transition (QPT) from a gapped phase to a gapless ordered phase. I show that site dilution introduces a novel gapless quantum disordered phase, corresponding to a Bose glass phase of localized magnons. The existence of the Bose glass phase leads to a change in the universality class of the QPT, and in the scaling of T_c. A crossover temperature is found, above which the meanfield scaling of $T_c$ at finite temperature is observed. But a new universal exponent $\phi\approx 1.2$ is obtained below the crossover temperature. Such a crossover is explained via a scenario of thermal percolation of magnons. 
February
9th, 2009 A tale of
inhomogeneities and lengthscale “hierarchy” in some cuprates
Prof.
Zahirul Islam
XRay Science Division Advanced Photon Source Argonne National Laboratory Scattering studies
have always played a critical role in elucidating microscopic nature of
and intrinsic correlations in complex phenomena such as
hightemperature superconductivity, spincharge stripes, colossal
magnetoresistance and polaronic effects, orbital ordering, etc., in
delectron systems. This talk focuses on xrayscattering studies of
lengthscale “hierarchy” due to incommensurate modulations that may
profoundly influence electronic properties in cuprates.
As a copperoxide
based Mott insulator is doped with charge carriers (“holes”), the
longrangeordered antiferromagnetic state is suppressed, and a
superconducting (SC) ground state emerges above a certain doping level.
Numerous experiments, such as electron microscpic studies, indicate
that cuprates are electronically “inhomogeneous” even in their SC
state. Although the origin of these inhomogeneities may well be
electronic instabilities such as spincharge stripes, a general
consensus remains elusive. Systematic xray diffuse scattering studies
of YBa_{2}Cu_{3}O_{6+x} (YBCO), Bi_{2}Sr_{2}CaCu_{2}O_{8+d},
and (La_{1x}Sr_{x})_{2}CuO_{4 }(LSCO)
have revealed lattice modulations on a “hierarchy” of length
scales. In the case of YBCO, as charge carriers are tuned via
Ostoichiometry variations, shortrange ordered modulations
characterized by a wavevector of the form q=(q_{x}, 0, 0) are
observed. These modulations correspond to correlated atomic
displacements of Ba, Cu, and O atoms, respectively, with correlations
extending several unit cells to form “nanoscopic” patches. They form
well above room temperature, persist down to the lowest temperatures
studied, and, intriguingly, at high doping level appear to be
susceptible to Fermisurface effects. In addition, “bowtie”shape Huang
scattering indicates characteristic coherent strain, signifying an
intrinsically inhomogeneous lattice in both YBCO and LSCO from
mesocopic to microscopic length scales. The role of lattice distortions
in modifying electronpair potential and exchange interactions has been
highlighted recently. It seems that a close interplay between
electronic inhomogeneities and lattice modulations on various length
scales is inevitable in cuprates as well as oxides, in general.
Applications
of high magnetic fields to perturb these systems and to suppress the
superconducting order parameter can have profound implications on our
understanding of their electronic phase diagram. Studies of these
modulations in magnetic fields and future directions are briefly
summarized.
*Use of the Advanced Photon
Source is supported by the U. S. Department of Energy, Office of
Science, under Contract No. DEAC0206CH11357.

February
20th, 2009 Some
Properties of Ironbased SuperconductorsMagnetism, Junctions,
Multiorbital models, etc.
Dr. Daoxin YaoPurdue University In this seminar, I will first
give a brief review about the Ironbased superconductors. Then we study
the magnetism of parent components using a local moment picture.
Through the Heisenberg model, we estimate the exchange couplings from
experimental data. We notice that the interlayer coupling is rather
large, and the system may be very anisotropic along the a and
bdirections in the low temperature orthombic phase. In the third
part, I will talk about the possible superconducting states and how to
use Josephson junctions to distinguish them in the ironbased
superconductors. Within the twoorbital exchange coupling model,
we find the nontrivial ingap states in the signchanged swave
pairing state (cos(kx) cos(ky)). This can be taken as a sharply
distinct feature in contrast to other singlet pairing states. In
addition, we propose a novel trilayer Pijunction as a possible
signature of the signchanged swave. I will also discuss the necessity
of multiorbital models.

February
23rd, 2009 TBA
Dr. Tobias Micklitz Argonne National Laboratory 
March
9th,
2009
Towards
Molecular Modualtion of Electronic Devices.
New and renewed vistas for solar cells? Prof. David Cahen* Dept. of Materials and Interfaces, Weizmann Inst. of Science, Rehovot, Israel 76100 david.cahen@weizmann.ac.il We
explore two main ways to use molecules in molecular
electronics, electrostatically, as dipolar films and
electrodynamically, where
current passes through them. For the former we find that incomplete
partial
dipolar molecular films can control diode behaviour and that this effect extends to poly
and nanocrystalline solar cells. For the latter complete
monolayers are
needed, as ideal as possible to assure current transport through the
molecules.
Such systems with alkyl chain monolayers behave like ideal MIS diodes with significant photovoltaic activity,
an effects that is much more chemical in nature than conventional
wisdom says
(which described everything to the insulating behavior of the
intervening
layer). Recently we are attempting to
combine the two effects by adding dipolar character to the completely
covering
alkyl monolayers, enabling modulating the electronic quality of the
monolayers.
The idea is to use molecular interface modifications to learn about
device
operation and optimizations.
*
Work done. with Ayelet Vilan, Omer Yaffe,
Ariel Biller, Rotem Har Lavan, as well as
D. Gal, H. Haick, G. Hodes, L. Kronik, S. Ruehle, A. Salomon, O. Seitz,
H.
Shpaisman, F. Thieblemont, I. VisolyFisher, and J.
Gooding (UNSW), H. Haick (Technion), A.
Kahn (Princeton U), C. Sukenik, A. Zaban (Bar
Ilan U), R. Tung (CUNY),
E. Umbach (Wuerzburg/Karlsruhe), N.
Ueno (Chiba), H. Zuilhof (Wageningen) and
coworkers. 
March
30th, 2009 Static
and Dynamic Properties of a FermiGas of Cooled Atoms Near a Wide Feshbach Resonance
Prof. Artem G. Abanov
Texas A& M University
I will discuss the static and
dynamic properties of the ultracold fermionic gas near the broad
Feshbach resonance. I will show that the problem of molecular
production, in a singlemode approximation, is
reduced to the linear LandauZener problem for operators. The strong interaction leads to significant renormalization of the gap between adiabatic levels. Two main physical results of our theory is the high sensitivity of molecular production to the initial conditions and generation of a large BCS condensate distributed over a broad range of momenta in a wide range of parameters. 
April
9th, 2009 Nonequilibrium
dynamics of ultracold fermions in optical lattices
Prof. Eugene Demler Havard University 
April
10th,
2009
TBA

April
13th, 2009 TBA
Prof. YeongAh Soh Dartmouth College 
April
20th, 2009 TBA
Dr. Mumtaz Qazilbashb UCSD 
[ Back ]