6,119 research outputs found
Itinerant quantum critical point with frustration and non-Fermi-liquid
Employing the self-learning quantum Monte Carlo algorithm, we investigate the
frustrated transverse-field triangle-lattice Ising model coupled to a Fermi
surface. Without fermions, the spin degrees of freedom undergoes a second-order
quantum phase transition between paramagnetic and clock-ordered phases. This
quantum critical point (QCP) has an emergent U(1) symmetry and thus belongs to
the (2+1)D XY universality class. In the presence of fermions, spin
fluctuations introduce effective interactions among fermions and distort the
bare Fermi surface towards an interacting one with hot spots and Fermi pockets.
Near the QCP, non-Fermi-liquid behavior are observed at the hot spots, and the
QCP is rendered into a different universality with Hertz-Millis type exponents.
The detailed properties of this QCP and possibly related experimental systems
are also discussed.Comment: 9 pages, 8 figure
An Examination of Some Signi cant Approaches to Statistical Deconvolution
We examine statistical approaches to two significant areas of deconvolution - Blind
Deconvolution (BD) and Robust Deconvolution (RD) for stochastic stationary signals.
For BD, we review some major classical and new methods in a unified framework of
nonGaussian signals. The first class of algorithms we look at falls into the class
of Minimum Entropy Deconvolution (MED) algorithms. We discuss the similarities
between them despite differences in origins and motivations. We give new theoretical
results concerning the behaviour and generality of these algorithms and give evidence
of scenarios where they may fail. In some cases, we present new modifications to the
algorithms to overcome these shortfalls.
Following our discussion on the MED algorithms, we next look at a recently
proposed BD algorithm based on the correntropy function, a function defined as a
combination of the autocorrelation and the entropy functiosn. We examine its BD
performance when compared with MED algorithms. We find that the BD carried
out via correntropy-matching cannot be straightforwardly interpreted as simultaneous
moment-matching due to the breakdown of the correntropy expansion in terms
of moments. Other issues such as maximum/minimum phase ambiguity and computational
complexity suggest that careful attention is required before establishing the
correntropy algorithm as a superior alternative to the existing BD techniques.
For the problem of RD, we give a categorisation of different kinds of uncertainties
encountered in estimation and discuss techniques required to solve each individual
case. Primarily, we tackle the overlooked cases of robustification of deconvolution
filters based on estimated blurring response or estimated signal spectrum. We do
this by utilising existing methods derived from criteria such as minimax MSE with imposed uncertainty bands and penalised MSE. In particular, we revisit the Modified
Wiener Filter (MWF) which offers simplicity and flexibility in giving improved RDs
to the standard plug-in Wiener Filter (WF)
Self-Learning Monte Carlo Method
Monte Carlo simulation is an unbiased numerical tool for studying classical
and quantum many-body systems. One of its bottlenecks is the lack of general
and efficient update algorithm for large size systems close to phase transition
or with strong frustrations, for which local updates perform badly. In this
work, we propose a new general-purpose Monte Carlo method, dubbed self-learning
Monte Carlo (SLMC), in which an efficient update algorithm is first learned
from the training data generated in trial simulations and then used to speed up
the actual simulation. We demonstrate the efficiency of SLMC in a spin model at
the phase transition point, achieving a 10-20 times speedup.Comment: add more refs and correct some typo
Theory of unconventional quantum Hall effect in strained graphene
We show through both theoretical arguments and numerical calculations that
graphene discerns an unconventional sequence of quantized Hall conductivity,
when subject to both magnetic fields (B) and strain. The latter produces
time-reversal symmetric pseudo/axial magnetic fields (b). The single-electron
spectrum is composed of two interpenetrating sets of Landau levels (LLs),
located at , . For , these
two sets of LLs have opposite \emph{chiralities}, resulting in {\em
oscillating} Hall conductivity between 0 and in electron and hole
doped system, respectively, as the chemical potential is tuned in the vicinity
of the neutrality point. The electron-electron interactions stabilize various
correlated ground states, e.g., spin-polarized, quantum spin-Hall insulators at
and near the neutrality point, and possibly the anomalous Hall insulating phase
at incommensurate filling . Such broken-symmetry ground states have
similarities as well as significant differences from their counterparts in the
absence of strain. For realistic strength of magnetic fields and interactions,
we present scaling of the interaction-induced gap for various Hall states
within the zeroth Landau level.Comment: 5 pages and 2 figures + supplementary (3.5 pages and 5 figures);
Published version, cosmetic changes and updated reference
Mottness induced phase decoherence suggests Bose-Einstein condensation in overdoped cuprate high-temperature superconductors
Recent observations of diminishing superfluid phase stiffness in overdoped
cuprate high-temperature superconductors challenges the conventional picture of
superconductivity. Here, through analytic estimation and verified via
variational Monte Carlo calculation of an emergent Bose liquid, we point out
that Mottness of the underlying doped holes dictates a strong phase fluctuation
of the superfluid at moderate carrier density. This effect turns the expected
doping-increased phase stiffness into a dome shape, in good agreement with the
recent observation. Specifically, the effective mass divergence due to
"jamming" of the low-energy bosons reproduces the observed nonlinear relation
between phase stiffness and transition temperature. Our results suggest a new
paradigm, in which the high-temperature superconductivity in the cuprates is
dominated by physics of Bose-Einstein condensation, as opposed to
pairing-strength limited Cooper pairing.Comment: 6+3 pages, 4+1 figure
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