Los Angeles Area Joint Condensed Matter Seminar
on Feb 29, 2:30pm-6:00pm at Caltech, Kirchoff 119
Sponsored by the USC Center for Quantum Information
Science & Technology
2:30-3:00 Steve White (UCI)
"Spectral
Functions from Time Dependent DMRG"
3:10-3:40 Marc Bockrath (Caltech)
"Carbon
nanoelectronics, from interacting electron
systems to devices"
3:45-4:15 Shimul Akhanjee (UCLA)
"Spin-charge
separation in a strongly correlated
spin-polarized chain"
4:15-4:50 coffee break
4:50-5:20 Vivek Aji (UCR)
"Recent
Developments in High Temperature Superconductivity:
Circulating
Current Phase and Local Criticality"
5:30-6:00 Paolo Zanardi (USC)
"(Quantum)
Critical phenomena: The Fidelity approach"
6:00 dinner
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Abstracts:
Spectral Functions from Time Dependent DMRG
Steve White (UCR)
Time dependent DMRG (tDMRG) is a very effective method for obtaining high
resolution dynamical spectra for 1D systems. High accuracy tDMRG
correlation
functions for moderate times can be used to determine the parameters of the
asymptotic long-time behavior, incorporating known analytic exponents and
parameters. This allows the extrapolation of the tDMRG data to very long
times, which is then Fourier transformed to obtain spectral functions.
I will illustrate this approach with a study of the spectral functions of
the S=1 Heisenberg chain and the XXZ S=1/2 chain.
Carbon nanoelectronics, from interacting electron systems to devices
Marc Bockrath (Caltech)
In the absence of disorder, a dilute system of carriers interacting
through long-range Coulomb forces has been predicted to form a periodic
solid called the Wigner crystal. We demonstrate using low-temperature
single-electron transport spectroscopy that a low-density hole system
in low-disorder carbon nanotubes with a band gap is a realization of
the one-dimensional (1D) Wigner crystal. The two inequivalent Dirac
points defining the nanotubesâ^À^Ù band structure yield an isospin degree
of freedom so that the holes act as a spin and isospin chain. We find
three distinct regimes as the applied axial magnetic field and carrier
density are varied : a fully spin and isospin polarized regime, an
isospin-polarized, spin antiferromagnetically-ordered regime, and an
unpolarized regime. These regimes arise from a competition between
exchange and magnetic energy in the spin and isospin chain. We account
for our observations quantitatively using the gapped Luttinger-liquid
model of Levitov and Tsvelik, where carriers are represented by
solitons. We also observe unexpected behavior of the Kondo effect,
which we attribute to the interplay between the spin and isospin
degrees of freedom that yields larger spin states than would be
predicted using a shell-filling picture. Our observation provides a
clean platform for testing theories of interacting electrons in 1D and
also indicates the possibility of using this many-body state for
solid-state quantum information processing.
I will also discuss our recent results in using graphene break
junctions as atomic scale
switches. These state of these switches can be changed reliably for
hundreds of thousands of cycles and may provide the basis for a
graphene-based nonvolatile memory element.
Spin-charge separation in a strongly correlated spin-polarized chain
Shimul Akhanjee and Yaroslav Tserkovnyak (UCLA)
We combine the first-quantized path-integral formalism and bosonization to
develop a phenomenological theory for spin-charge-coupled dynamics in
one-dimensional (1D) ferromagnetic systems with strong interparticle
repulsion, at low temperatures. We assume an effective spin-charge
separation and retain the standard Luttinger-liquid plasmon branch, which
is explicitly coupled to a ferromagnetic spin-wave texture with a
quadratic dispersion. The dynamic spin structure severely suppresses the
plasmon peak in the single-particle propagator, in both fermionic and
bosonic systems. Our analysis provides an effective theory for the new
universality class of 1D ferromagnetic systems, capturing both the trapped
spin and propagating spin-wave regimes of the long-time behavior.
Recent Developments in High Temperature Superconductivity: Circulating
Current Phase and Local Criticality
Vivek Aji (UCR)
Neutron Scattering experiments have provided evidence for the existence
of magnetic order in the underdoped phase of Cuprate superconductors
possessing the symmetries first proposed by Chandra Varma. One of the key
questions in the physics of Cuprates has been the mechanism responsible
for the unusual properties of the normal state at optimal doping that is
well described phenomenologically by the Marginal Fermi Liquid theory. In
this talk I will review the latest experimental results and show how the
Marginal Fermi Liquid arises. The long wavelength theory of the
circulating current phase is described by the dissipative 2DXY model
which posses in its phase space a quantum critical point whose
fluctuation spectrum is local in space and power law in time. These
developments provides a framework to understand the phase diagram of High
Temperature Superconductors.
(Quantum) Critical phenomena: The Fidelity approach
Paolo Zanardi (USC)
The manifold of coupling constants parametrizing a quantum Hamiltonian is
equipped with a natural Riemannian metric with an operational
distinguishability content. The basic idea of the so-called Fidelity
approach
to criticality is that the singularities of this metric are in
correspondence
with the quantum phase transitions featured by the corresponding system.
The
fidelity approach aims at providing a universal conceptual framework to
study
quantum critical phenomena which is differential-geometric and
information-theoretic at the same time. In my talk I will try to illustrate
the main ideas, results and limitations of this novel approach.