| Date & time | Speaker & affiliation | Talk title & abstract |
| Jan 3 4:00pm | Dr. Vivek Aji (UCR) | Local Criticality, Time Reversal Violation and The Cuprate Phase Diagram
Abstract:
The physics of the cuprates has fascinated us for the past twenty years
primarily because the behavior in the majority of the phase diagram is
unlike any we have encountered before. An organizing principle proposed
is that there exists a quantum critical point near optimal doping and that
the pseudogap is a result of a symmetry broken state. Furthermore the
quantum critical point should give rise to a fluctuation spectrum that is
local in space and power law in time to account for the anomalies
observed. In the first part of my talk I will show that the dissipation
driven transition of a Quantum 2DXY model gives rise to such a spectrum.
In the second part I will introduce the symmetry broken states and the
neutron scattering results that have observed their principle signature.
The long wavelength theory suggested by the symmetries of the time
reversal violating states is indeed the dissipative quantum 2DXY model.
|
| Jan 5 Friday 4:00pm Note the unusual day |
Alberto Cortijo (ICMM-Madrid) | Graphene: High energy physics
in a solid state system
Abstract:
Graphene has become one of the most interesting systems in condensed
matter physics since experimental techniques recently allowed the
isolation of single graphene sheets. Such interest is twofold: Graphene
based devices exhibit promising applicability for the semiconductor
industry. Secondly, from a theoretician's point of view, graphene is an
electronic system that offers the possibility to understand new and
intriguing experimental results using ideas from high energy physics. In
this talk I will review the basic properties of graphene in order to show
how a basic tight binding model yields electronic behavior similar to
that of relativistic particles, and conversely, how ideas from the field
of high energy physics can help us to predict the properties of this
system.
|
| Jan 10 4:00pm | no talk | Abstract: |
| Jan 17 4:00pm | no talk |
Abstract: |
| Jan 24 4:00pm | Alexander Finkel'stein (Weizmann) |
Abstract:
We address a recent experiment in which a strong decrease
of the resistance of a superconducting film has been observed
when a remote unbiased gate was placed above the film.
We show how the response of electrons inside the metallic
gate to a change of vortex positions can reduce drastically
the tunneling rate of the vortices. The magnetic flux of a
vortex penetrating inside the gate scatters the electrons
in a way similar to the Aharonov-Bohm scattering.
The response of the electrons to a sudden change of vortex
positions is related to the Orthogonality Catastrophe that
manifests itself through the vanishing overlap of the two
wave functions describing the macroscopic electron system
before and after the change in the scattering potential.
We interpret the experimentally observed decrease of the
resistance in the gated film as a sort of a "Metal-Insulator"
transition in a system of tunneling vortices induced by the gate. |
| Jan 31 4:00pm | Arkady Shehter (UCR) | Branch-cut singularities in the thermodynamics of Fermi liquid systems
Abstract: The recently measured spin susceptibility of the two dimensional
electron gas exhibits a strong dependence on temperature, which is
incompatible with the standard Fermi liquid phenomenology. Here we
show that the observed temperature behavior is inherent to ballistic
two dimensional electrons.
Besides the single-particle and collective excitations, the thermodynamics of Fermi liquid systems includes effects of the branch-cut singularities originating from the edges of the continuum of pairs of quasiparticles. As a result of the rescattering induced by interactions, the branch-cut singularities generate non-analyticities in the thermodynamic potential which reveal themselves in anomalous temperature dependences. |
| Feb 7 4:00pm | Lijun Zhu (UCR) | Coherence in the Two Kondo Impurity Problem: From
Quantum Dots to Heavy Fermions
Abstract:
We show through a perturbative and an exact calculation using Wilson's
renormalization methods that in the problem of two interacting Kondo
impurities, on-site potential scattering generates a quantum-tunneling
between the two impurities through a marginally relevant operator. The
magnitude of this tunneling V12 depends on the
spin-correlation between the two impurities. For exchange interactions
between the moments comparable to the Kondo energy
TK,
V12 is typically much larger than
TK. This implies that the heavy-fermion mass in
this interesting range is determined by V12 rather
than TK. The importance of these results for
experiments in coupled Kondo dots is also pointed out.
|
| Feb 14 4:00pm | Marcel Franz (UBC) | Anyons in a weakly interacting system
Abstract:
In quantum theory, indistinguishable particles in three-dimensional space
behave in only two distinct ways. Upon interchange, their wavefunction
maps either to itself if they are bosons, or to minus itself if they are
fermions. In two dimensions a more exotic possibility arises: upon
exchange of two particles called ``anyons'' the wave function acquires a
generic complex phase. In this talk I will describe theoretical proposal
for a system whose excitations are anyons with the exchange phase π/4 and
charge -e/2, but, remarkably, can be built by filling a set of
single-particle states of essentially noninteracting electrons. The system
consists of an artificially structured type-II superconducting film
adjacent to a 2D electron gas in the integer quantum Hall regime with unit
filling fraction. The proposal rests on the observation that a vacancy in
an otherwise periodic vortex lattice in the superconductor creates a bound
state in the 2DEG with total charge -e/2. A composite of this fractionally
charged hole and the missing flux due to vacancy behaves as an anyon. The
proposed setup allows for manipulation of these anyons and could prove
useful in various schemes for the fault-tolerant topological quantum
computation.
|
| Feb 16 Friday 4:00pm Note the unusual day |
Carsten Honerkamp (Wuerzburg University) | Fermionic renomalization group flows into phases with
broken symmetries
Abstract: We describe how functional renormalization
group flows for interacting fermions can be continued into phases with
broken symmetries. A symmetry-breaking term in the initial condition for
the self-energy prevents a true divergence of the interactions at the
critical scale. At the same scale, the anomalous self-energy grows
rapidly such that the flow can be followed down to zero scale and all
modes can be integrated out. Within simple mean-field models, we
demonstrate two versions of this idea: one where the initial symmetry
breaking is sent to zero, and another where it is compensated by a
counter-term. The latter scheme is capable of detecting symmetry-broken
phases separated from the symmetric state by an energy barrier. We
discuss generalizations to more realistic models.Refs.: M. Salmhofer et al., Prog. Theor. Phys. 112 943 (2004); R. Gersch et al., Euro. Phys. J B 48 349 (2005); R. Gersch et al., cond-mat/0609520. |
| Feb 21 4:00pm | Assa Auerbach (Technion) | Emergence of Spin-Half Fermion Vortices and The
Vortex Metal
Abstract:
We present two theoretical findings concerning quantum properties of
vortices of two dimensional lattice bosons, at half filling. First, we provide
a rigorous proof of doublet degeneracies for any odd vorticity on finite tori.
The result is made physical by connecting the doublets to a -local- spin half
(v-spin) carried by each vortex. The v-spins have experimental consequences in
the normal superfluid (vortex lattice) phase. Second, building on the first
result, we discover numerically that two vortex wavefunctions obey Fermi-Dirac
statistics: a demonstration of "statistical transmutation" in a
generic
bosonic model. The multi-vortex system undergoes quantum melting at low vortex
density (~10-3/site). The resulting Vortex Metal features a
"Fermi temperature" and unusual transport behavior.
Experiments in cold atoms on optical lattices, low capacitance Josephspon arrays and low superfluid density superconductor films in a magnetic field, could test our predictions. |
| Feb 28 4:00pm | Arik Yochelis (UCLA) | Intertwined by symmetry: spatially localized
oscillations
Abstract:
We have studied the origin and the properties of a new type of
spatially localized oscillations we refer to as reciprocal oscillons.
These oscillations take the form of holes in an oscillating
background, and have been observed in both vibrating granular media
and optically forced chemical reactions but their origin has remained
unclear. Spatially localized structures have been the source of
fascination ever since the discovery of solitons. Like solitons the
oscillations we study exhibit properties that are independent of the
detailed description of the system. However, in contrast to solitons
they are dissipative structures, and (generally) do not propagate. Our
study is motivated by experimental observations but utilizes a general
theoretical framework. We show that the presence of reciprocal
oscillons is intimately related to the existence of interfaces with
both monotonic and nonmonotonic profiles. Our results not only agree
qualitatively with the experiments but the basic scenario described in
our research also suggests their relevance to other media such as
periodically driven reaction - diffusion systems.
|
| Mar 7 4:00pm | APS March meeting | |
| Mar 14 4:00pm | TBA | Abstract: |
| Mar 21 4:00pm | Finals week |
Seminars for Fall 2005
Seminars for Winter 2006
Seminars for Spring 2006
Seminars for Fall 2006