| Date & time | Speaker & affiliation | Talk title & abstract |
| Jan 7 4:00pm | TBA | |
| Jan 14 4:00pm | David Clarke (UCR) | Domain wall motion in thin ferromagnetic strips
Abstract:
The rich internal structure of domain walls in ferromagnetic
nanostrips greatly affects the motion when an external magnetic
field is applied, leading to reduced mobility when the driving force
is strong. In particular, in the wider strips used in many
experimental studies, the wall is expected to contain a magnetic
vortex at its heart. A linearized two-coordinate model, using just
the position of the vortex. leads to predictions for the average
wall velocity that are in good agreement with numerical simulations
when the field is not too high. In higher fields, additional degrees
of freedom enter the motion. I will show examples of such modes and
discuss the effect that they have on the mobility of the wall.
|
| Jan 21 4:00pm | Lloyd Lumata (Florida State and NHMFL) | Spin Dynamics of Density Wave and Frustrated Spin Systems Probed by
NMR
Abstract: The quasi-one-dimensional organic conductor
(TMTSF)2ClO4 exhibits a
series of field-induced spin-density wave (FISDW) phases at low
temperature and high magnetic fields. Simultaneous 77Se NMR and
electrical transport at magnetic fields up to 30T reveal new
information about the electronic structure of this material. On the
other hand, 69Ga spin-lattice and spin-spin relaxation rate
measurements
on the rare-earth kagome system Pr3Ga5SiO4
suggest the presence of
short-range nanoscale ordering consistent with neutron scattering
results. Detailed field-dependent NMR studies of these two condensed
matter systems will be discussed.
|
| Jan 28 4:00pm | TBA | |
| Feb 4 4:00pm | Cenke Xu (Harvard) | Nematic and Magnetic Orders in Fe-based Superconductors
Abstract:
The Fe-based high temperature superconductors (Fe-SC) discovered a
year ago has created enormous excitements in condensed matter physics
community. In this talk, we will focus on two phenomena shared by all
the Fe-SC materials: tetragonal-orthorhombic lattice distortion (LD),
and spin density wave (SDW). A unified picture for both effects is
proposed, motivated by various evidences from experiments. We will
discuss the nature of classical phase transitions of both LD and SDW
at finite temperature, affected by the elasticity of the lattice, as
well as the quantum phase transitions at zero temperature, driven by
doping and pressure. A global phase diagram for LD and SDW will be
proposed, with predictions for future experiments. Finally, the
quantum nematic transition with a background d-wave superconductor is
studied. The strong coupling between nematic order nodal particle will
modify many experimental observables.
|
| Feb 10 2:00pm (Tuesday) | Ehud Altman (Weizmann Institute) | Quantum steady states and phase transitions in the presence of non equilibrium
noise
Abstract: Systems of ultracold atoms, Polar molecules
or trapped ions are easily driven out of equilibrium by external (classical)
noise sources. I will introduce a new class of quantum dynamical fixed points
that can emerge in such systems and discuss the nature of the phase
transitions between the different steady states. I will also discuss closely
related non-equilibrium transitions in Josephson junctions.
|
| Feb 11 4:00pm | Vivek Aji (UCR) | New Route to Superconductivity: Local Criticality and Time Reversal
Violation
Abstract: The discovery of High Temperature
superconductors has given rise to a
number of new paradigms in Condensed matter physics for the past two
decades. Among competing ideas that attempt to explain the phenomena,
time reversal violation has gained prominence due to recent
experimental observations. In this talk I will present evidence from
polarized neutron scattering data that reveal the existence of
magnetic order in the pseudo-gapped phase. Quantum disordering of this
state results in a quantum critical point which is unconventional. The
critical fluctuations are local in space and power law in time. This
phase transition is driven by the proliferation of a new topological
defect. The coupling of electrons to the quantum fluctuations leads to
an attractive interaction in the d-wave channel providing a novel
mechanism for superconductivity.
|
| Feb 13 (Friday) 4:00pm | Roderich Moessner (MPI Dresden) | Entanglement Skyrmions In Multicomponent Quantum Hall
Systems
Abstract: We discuss charged topological spin
textures in quantum Hall ferromagnets in
which the electrons carry a pseudospin as well as the usual spin degree
of
freedom, as is the case in bilayer GaAs or monolayer graphene
samples. We
develop a theory which treats spin and pseudospin on a manifestly equal
footing, which may also be of help in visualizing the relevant spin
textures. We in particular consider the entanglement of spin and
pseudospin
in the presence of realistic anisotropies. An entanglement operator is
introduced which generates families of degenerate Skyrmions with
differing
entanglement properties. We propose a local characterization of the
latter,
and touch on the role entangled Skyrmions play in the nuclear relaxation
time of quantum Hall ferromagnets.
|
| Feb 18 4:00pm | Maria Vozmediano (ICMM, Madrid)) | Coulomb Interactions and Disorder in
Undoped Graphene
Abstract:
The recent synthesis of a single layer of graphite (graphene) and
the experimental observation of some unusual electronic and
structural properties has prompted a real revolution in the theory
of condensed matter systems. Under a theoretical point of view the
fact, confirmed by the experiments, that the low energy elementary
excitations are well described by massless Dirac fermions implies a
revision of the Landau fermi liquid paradigm. In this talk I will
give a general overview of the graphene physics at the neutrality
point and address the problem of the Coulomb interactions and
disorder. A renormalization group analysis allows to classify
graphene as a strange Fermi liquid described by a Lorentz covariant
infrared fixed point whose effective coupling constant is the fine
structure constant.
|
| Feb 25 4:00pm | Jason Alicea (Caltech) | New Puzzles In Bismuth Subjected To Strong Magnetic Fields
Abstract:
In sharp contrast to most 3D metals, the carrier density in bulk
bismuth is sufficiently small that lowest-Landau-level physics can be
explored with laboratory fields. Very recent experiments probing this
high-field regime have uncovered a number of surprising features which
raise interesting new questions for this material. Specifically,
observations of anomalies in the Hall resistance, Nerst effect, and
magnetization measurements have led to the speculation that
three-dimensional bismuth in the quantum limit hosts novel correlated
states, and possibly even electron fractionalization. In this talk I
will discuss efforts to shed light on these findings by studying an
interacting low-energy theory for bismuth in a magnetic field,
describing Dirac-like electrons coupled to holes. I will argue that an
anomalous Zeeman effect that arises due to strong spin-orbit coupling,
together with various interaction-driven instabilities studied within
a functional renormalization group framework, account for some of the
puzzling observations. Outstanding questions and interesting future
experimental and theoretical directions will also be discussed. |
| Mar 2 (Monday) 4:00pm | Paul Goldbart (UIUC) | Strands of superconductivity at the nanoscale
Abstract:
Superconducting circuitry can now be fabricated at the
nanoscale by depositing suitable materials on to individual molecules,
such as DNA or carbon nanotubes. In this talk I shall examine various
themes that arise when superconductivity is explored in this new
regime, including thermal barrier crossing and quantum tunneling by
the superconducting condensate, as well as the impact of magnetism on
nanosuperconductivity. I shall focus on a fascinating class of
circuits: nanoscale superconducting quantum interference devices (or n-
SQUIDs). After describing how they are made, I shall pay particular
attention to the electrical resistance of these devices and,
especially, its sensitivity to magnetic fields and patterns of
supercurrent. These features hint at possible uses of n-SQUIDs, such
as for mapping the quantum phase of superconducting order and testing
for superconducting correlations in novel materials and settings.
|
| Mar 4 2:10pm | S. Zhao (Penn State & Zhejiang University, Hangzhou) | dc Electric Breakdown in Complex Lead-Based
Perovskites: an
Investigation of Pb(Mg1/3Nb2/3)O3PbTiO3 Relaxor Ferroelectrics
Abstract: Complex lead based perovskite compounds and solid solutions have been of
great interest due to the large piezoelectric coefficients that are
induced on poling these ferroelectric materials, based on which they are
extremely important in piezoelectric applications from aerospace,
automotive, to civil engineering as transducers, actuators, etc.
However, electrochemical processes can give rise to time dependent
electric breakdown. Therefore, owing to the practical interest, it is
important to develop a better understanding of electric degradation in
these materials to improve the reliability and performance of
ferroelectric devices based on lead perovskite compounds and solid
solutions.
In this talk, we shall discuss the dc electric breakdown phenomenon and
the nature of electrical changes on a particular lead-based system,
<001> oriented Pb(Mg1/3Nb2/3)O3PbTiO3 (PMN-PT) single crystal,
which belongs to ferroelectric family and known as relaxor-type
ferroelectrics. Highly Accelerated Lifetime Tests (HALT), Thermally
Stimulated Depolarization Current (TSDC) and Impedance Spectroscopy (IS)
measurements were employed to develop a physical model. It is found that
migration of oxygen vacancies with a low activation energy controls the
degradation process in this particular lead-based system. Activation
energies controlling other conduction species and voltage acceleration
factor were estimated, which are of practical importance to the design
of reliable piezoelectric materials that could operate under high bias
and/or at elevated temperatures for long times.
|
| Mar 4 4:00pm | Tami Pereg-Barnea (Caltech) | Quantum Oscillations from Fermi Arcs in the
Pseudogap Phase of the High-Tc Cuprates
Abstract:
The problem of high-Tc superconductors continues
to provide challenges for the condensed matter community. The key for
determining the mechanism for Cooper pairing is inevitably related to the
nature of the normal state. On the under doped side of the phase diagram the
normal
state is the pseudogap (PSG) which displays a wealth of unusual phenomena and
will be the focus of this talk. Recent experiments in the mixed state of under
doped cuprates revel 1/B oscillations in the Hall resistance and
the magnetization. If these oscillations are to be associated with
quasiparticle motion along a closed Fermi surface then their frequency is a
measure of the Fermi surface size. The observed slow oscillations suggest a
tiny Fermi surface which encloses only a few percents of the Brillouin
zone. Therefore, the experiments are widely perceived as evidence for the
existence of small Fermi pockets. Such a Fermi surface topology is at odds
with the existence of a discontinuous Fermi surface, the so called "Fermi
arcs" which are observed by ARPES.
In this talk I challenge this interpretation and propose a different origin for the observed oscillations. I adopt a phenomenological picture of the PSG as a phase disordered superconductor with a pairing gap that vanishes on the arcs. The model displays periodicity in the density of states as a function of both the energy and magnetic field. The oscillations result from the appearance of states reminiscent of Andreev bound states. |
| Mar 6 12:00 noon | Ying Ran (UC, Berkeley) | Helical Metal Inside A Topological
Band Insulator
Abstract:
Topological defects, such as domain walls and vortices, have long fascinated
physicists. A novel twist is added in quantum systems like superfluid helium
He3, where vortices are associated with low energy excitations in
the
cores. Similarly, cosmic strings which are vortices of the Higgs field, may
be tied to propagating fermion modes. Can analogous phenomena occur in
crystalline solids that host a plethora of topological defects? In this talk
I will show that indeed dislocation lines are associated with one
dimensional fermionic excitations in a `topological insulator', a novel band
insulator believed to be realized in the bulk material
Bi0.9Sb0.1.
In contrast to electrons in a regular quantum wire, these modes are
topologically protected, and not scattered by disorder. Since dislocations
are ubiquitous in real materials, these excitations could dominate spin and
charge transport in topological insulators. Our results provide a novel
route to creating a potentially ideal quantum wire in a bulk solid.
|
| Mar 11 4:00pm | TBA | |
| Mar 18 4:00pm | TBA | (Finals week; APS March Meeting week) |
| Mar 25 4:00pm | TBA | (Spring break) |
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