| Date & time | Speaker & affiliation | Talk title & abstract |
| Sep 24 4:00pm | TBA | |
| Oct 1 4:00pm | Stefan Kehrein (Universität München) | Interaction Quench in the Hubbard Model
Abstract: Motivated by recent experiments in ultracold
atomic gases that explore the quantum dynamics of interacting quantum
many-body systems, I discuss the opposite limit of Landau's Fermi-liquid
paradigm: a Hubbard model with a sudden interaction quench, that is, the
interaction is switched on at time t = 0. Using the flow equation
method, one is able to study the real time dynamics for weak interaction
U in a systematic expansion and find three clearly separated time
regimes: (i) An initial buildup of correlations where the quasiparticles are
formed. (ii) An intermediate quasi-steady regime resembling a zero temperature
Fermi liquid with a nonequilibrium quasiparticle distribution function. (iii)
The long-time limit described by a quantum Boltzmann equation leading to
thermalization of the momentum distribution function. I will then discuss the
implications of these results for experimental realizations of nonequilibrium
dynamics in ultracold atomic gases. |
| Oct 8 4:00pm | Kumar S. Raman (UCR) | Dzyaloshinskii--Moriya interactions in valence bond systems
Abstract:
We investigate the effect of Dzyaloshinskii--Moriya interactions on
the low temperature magnetic susceptibility for a system whose low
energy physics is dominated by short-range valence bonds (singlets).
Our general perturbative approach is applied to specific models
expected to be in this class, including the Shastry--Sutherland model
of the spin-dimer compound SrCu2(BO3)2 and the
antiferromagnetic Heisenberg model of the recently discovered S=1/2
kagomé compound ZnCu3(OH)6Cl2.
|
| Oct 15 4:00pm | TBA | |
| Oct 22 4:00pm | Subroto Mukerjee (Berkeley) | Theory of finite-entanglement scaling at
one-dimensional quantum critical points
Abstract:
Studies of entanglement in many-particle systems suggest that most
quantum critical ground states have infinitely more entanglement
than non-critical states, although a complete understanding of this
property has been obtained only at one-dimensional quantum critical
points with conformal invariance. The diverging entanglement entropy
explains the long-standing difficulty in numerical studies of quantum
criticality: algorithms construct model states with only finite
entanglement, which are a worse approximation to quantum critical states
than to others. Here we present a quantitative theory of this
phenomenon: the scaling theory of finite-entanglement approximations is
only superficially similar to finite-size scaling at critical points,
and has a different physical origin. Finite-entanglement scaling is
governed not by the scaling dimension of an operator but by the central
charge of the critical point, which counts its universal degrees of
freedom. An important ingredient is the recently obtained universal
distribution of density-matrix eigenvalues at a critical point. The
theory is compared to the numerical error scaling of several quantum
critical points, obtained by matrix-product-state methods that extend
the celebrated density-matrix renormalization group (DMRG) algorithm.
|
| Oct 29 4:00pm | Congjun Wu (UCSD) | Novel quantum phases in orbital systems with cold atom optical lattices
Abstract:
Orbital is a degree of freedom independent of charge and spin, which
is characterized by orbital degeneracy and spatial anisotropy. It plays
important roles in magnetism and superconductivity in transition metal
oxides. Recently, cold atom optical lattices have provided a new opportunity
to investigate orbital physics. In this talk, we will present many novel
features in such systems that do not appear in transition metal oxides as
follows. Bosons, as recently demonstrated in experiments, can be pumped into
high orbital bands and stay with a long life time. We will show that such
meta-stable states of bosons exhibit a class of novel superfluid states with
complex-valued wavefunctions spontaneously breaking time reversal symmetry,
thus are beyond Feynman's celebrated argument of the positive-definitiveness
of many-body ground state wavefunctions of bosons. For fermions, we will focus
on the px,y orbital system of the honeycomb lattice,
which exhibits fundamentally different behavior from that in the
pz system of graphene. The interesting physics here
includes the flat band structure, the consequential non-perturbative strong
correlation effects (e.g. Wigner crystal and ferromagnetism), the frustration
in orbital exchange, and the orbital analogy of the quantum anomalous Hall
effect.
|
| Nov 5 4:00pm | Hanoh Lee (LANL) | Pressure effect of single ion Kondo temperature in
dilute CeRhIn5
Abstract: Near a critical pressure
Pc ~ 25 kbar, CeRhIn5 assumes characteristics
of CeCoIn5 at atmospheric pressure: they have comparable
TC, similar a dHvA frequencies, and display
quantum-critical behaviors. Many properties of CeCoIn5 can be
interpreted within a two-fluid phenomenology in which there are
interpenetrating fluids, a localized f-electron Kondo gas (energy
scale TK) and an
interacting Kondo liquid (energy scale T*). We have measured
transport properties of Ce.02La.98 RhIn5
under pressures to 50 kbar to determine
TK(P). A comparison of
TK(P) with T*(P),
determined from the pressure studies of
CeRhIn5, reveals the same correlation between
TK and T* inferred from a two- fluid analysis
of CeCoIn5, further supporting the similarity of
these two compounds and the two-fluid phenomenology.
|
| Nov 12 4:00pm | Brian Leroy (U. Arizona) | Local electronic properties of carbon
nanostructures
Abstract
Combining scanning probe microscopy with electrical transport
measurements is a powerful approach to probe low- dimensional systems.
The local information provided by scanning probe microscopy is
invaluable for studying effects such as electron- electron interactions
and scattering. Using this approach, we have probed the local electronic
properties of carbon nanotubes and graphene with atomic resolution. In
nanotubes, we observe the effect of interactions on their electrical transport
properties. Namely, we study the role of the electron-phonon interaction and
control the excitation of phonons on the nanotube. In graphene, we probe the
effect of scattering on the local density of states. We find that long-range
scattering tends to lead to electron and hole puddles. Short-range scattering
which mix the two sublattices tends to be strongly suppressed away from the
Fermi energy.
|
| Nov 17 4:00pm | Bruno Uchoa (UIUC) | Tailoring magnetic instabilities in graphene
Abstract: Graphene is a two dimensional allotrope of
carbon, whose elementary electronic excitations are massless Dirac
fermions that can propagate ballistically in the sub-micron
scale. Differently of high temperature superconductors and other
strongly correlated materials which also exhibit a vanishing density of
states at the Fermi surface, in graphene the Dirac points are protected
by symmetry and the Dirac particles are very robust against disorder up
to very large energy scales. Despite graphene is considered a strongly
interacting system at half-filling, where the absence of screening is
expected to produce infrared logarithmic singularities to the
self-energy in all orders of perturbation theory, the electrons
nevertheless seem to behave as non-interacting particles. In this
seminar I will focus in possible magnetic instabilities in graphene
coated with adsorbed atoms. I will show that that local magnetic moments
in graphene can be created and controlled with the application of an
electric field, making graphene a promissing candidate for spintronics.
|
| Nov 19 4:00pm | Dmitri Abanine (Princeton) | Charge and spin in graphene
Abstract:
In the first part of the talk, I will focus on Quantum Hall
Effect (QHE) in graphene p-n junctions, which has been recently
observed experimentally. I will explain the observed conductance
quantization which is fractional in the bipolar regime and integer in
the unipolar regime in terms of QH edge modes propagating along and
across the p-n interface. In the bipolar regime the electron and hole
modes can mix at the p-n boundary, leading to current partition and
quantized shot noise plateaus similar to those of conductance, while
in the unipolar regime transport is noiseless. In the second part of
the talk, I will discuss unusual nature of n = 0 QHE state in graphene
and show that electron transport in this regime may be dominated by
counter-propagating edge states. Such states, intrinsic to massless
Dirac quasiparticles, manifest themselves in a large longitudinal
resistivity ρxx~h/e2 in striking contrast
to ρxx behavior in the
standard QHE.
|
| Dec 1 4:00pm | Guo-Xing Miao (MIT) | Interplay Between Spin and Charge Carriers in Superconducting
Spin Valves: An Alternative Approach to Reach Infinite Magnetoresistance
Abstract:
Superconductivity is the collective behavior of a gas of electrons
interacting through the exchange of phonons, and the typical Cooper pair
bonding energy is on the order of 10-3 eV. Bringing a ferromagnet
(FM) in
close proximity to a superconductor (SC) will suppress the
superconductivity because of the presence of strong exchange interactions
(~1eV) that favor the parallel alignment of spins. As a result,
superconductivity in FM/SC/FM hybrid spin valve structures is influenced by
the spin- and super-currents as well as band symmetry, and interface
transparency plays an important role in the spin and charge transports.
Such system shows spin-dependent transition temperatures and infinite
magnetoresistance with clear remanance states [Miao et al., 2007, 2008].
Unlike the traditional spin valve effect (the GMR effect), the SC spin
valve effect does not result from spin dependent scattering, but from the
quenching of superconductivity by the strong FM exchange splitting. The
strength of proximity effect can be tailored with carefully controlled
interfaces. The insertion of an artificial insulating barrier reduces the
transmission probabilities for both the polarized electrons from the FM
side and the Cooper pairs from the SC side, but more so for the latter due
to its two-particle process nature. Spins confined in the Al layer have
very long lifetime and lead to net spin imbalance, which induces weak FFLO
oscillations in this SC layer. In a clean interface system, the interface
transparency is controlled by the intrinsic band matching. Specifically,
the epitaxial interface between bcc-Fe and -V is opaque for Cooper pairs
but nearly transparent for spin minority electrons, thus weakening the SC
spin valve effect in such systems.
|
| Nov 26 4:00pm | TBA | |
| Dec 3 4:00pm | S.-C. Zhang (Stanford) | Effective field theory of topological insulators
Abstract:
Three dimensional topological insulators have surface states
described by the two dimensional massless Dirac equation. In
contrast to graphene, there can be an odd number of Dirac points.
This highly unusual property gives rise to a number of striking
observable effects. The most striking is the topological
magneto-electric effect, where an electric field generates a
magnetic field in the same direction, with an universal constant of
proportionality quantized in odd multiples of the fine structure
constant. We introduce an effective topological field theory
describing all these effect in an unified framework.[1] Xiao-Liang Qi, Taylor Hughes and Shou-Cheng Zhang, "Topological Field Theory of Time-Reversal Invariant Insulators", arXiv:0802.3537. [2] Liu et al, "Magnetic impurities on the surface of a topological insulator", arXiv:0808.2224. |
| Dec 10 4:00pm | TBA | Finals week |
Seminars for Fall 2008
Seminars for Spring 2008
Seminars for Winter 2008
Seminars for Fall 2007
Seminars for Spring 2007
Seminars for Winter 2007
Seminars for Fall 2006
Seminars for Spring 2006
Seminars for Winter 2006
Seminars for Fall 2005