118 research outputs found
Band structure of the Jahn-Teller polaron from Quantum Monte Carlo
A path-integral representation is constructed for the Jahn-Teller polaron
(JTP). It leads to a perturbation series that can be summed exactly by the
diagrammatic Quantum Monte Carlo technique. The ground-state energy, effective
mass, spectrum and density of states of the three-dimensional JTP are
calculated with no systematic errors. The band structure of JTP interacting
with dispersionless phonons, is found to be similar to that of the Holstein
polaron. The mass of JTP increases exponentially with the coupling constant. At
small phonon frequencies, the spectrum of JTP is flat at large momenta, which
leads to a strongly distorted density of states with a massive peak at the top
of the band.Comment: 5 pages of REVTeX, 3 figure
Diagrammatic Monte Carlo for Correlated Fermions
We show that Monte Carlo sampling of the Feynman diagrammatic series (DiagMC)
can be used for tackling hard fermionic quantum many-body problems in the
thermodynamic limit by presenting accurate results for the repulsive Hubbard
model in the correlated Fermi liquid regime. Sampling Feynman's diagrammatic
series for the single-particle self-energy we can study moderate values of the
on-site repulsion () and temperatures down to . We
compare our results with high temperature series expansion and with single-site
and cluster dynamical mean-field theory.Comment: 4 pages, 5 figures, stylistic change
Diagrammatic Quantum Monte Carlo for Two-Body Problem: Exciton
We present a novel method for precise numerical solution of the irreducible
two-body problem and apply it to excitons in solids. The approach is based on
the Monte Carlo simulation of the two-body Green function specified by
Feynman's diagrammatic expansion. Our method does not rely on the specific form
of the electron and hole dispersion laws and is valid for any attractive
electron-hole potential. We establish limits of validity of the Wannier (large
radius) and Frenkel (small radius) approximations, present accurate data for
the intermediate radius excitons, and give evidence for the charge transfer
nature of the monopolar exciton in mixed valence materials.Comment: 4 pages, 5 figure
Effect of the Tunneling Conductance on the Coulomb Staircase
Quantum fluctuations of the charge in the single electron box are
investigated. The rounding of the Coulomb staircase caused by virtual electron
tunneling is determined by perturbation theory up to third order in the
tunneling conductance and compared with precise Monte Carlo data computed with
a new algorithm. The remarkable agreement for large conductance indicates that
presently available experimental data on Coulomb charging effects in metallic
nanostructures can be well explained by finite order perturbative results.Comment: 4 pages, 5 figure
Worm algorithms for classical statistical models
We show that high-temperature expansions may serve as a basis for the novel
approach to efficient Monte Carlo simulations. "Worm" algorithms utilize the
idea of updating closed path configurations (produced by high-temperature
expansions) through the motion of end points of a disconnected path. An amazing
result is that local, Metropolis-type schemes may have dynamical critical
exponents close to zero (i.e., their efficiency is comparable to the best
cluster methods). We demonstrate this by calculating finite size scaling of the
autocorrelation time for various (six) universality classes.Comment: 4 pages, latex, 2 figure
A Distribution of Tunnel Splittings in Mn-Acetate
In magnetic fields applied parallel to the anisotropy axis, the relaxation of
the magnetization of Mn measured for different sweep rates is shown to
collapse onto a single scaled curve. The form of the scaling implies that the
dominant symmetry-breaking process that gives rise to tunneling is a locally
varying second-order anisotropy, forbidden by tetragonal symmetry in the
perfect crystal, which gives rise to a broad distribution of tunnel splittings
in a real crystal of Mn-acetate. Different forms applied to even and
odd-numbered steps provide a distinction between even step resonances
(associated with crystal anisotropy) and odd resonances (which require a
transverse component of magnetic field).Comment: 4 pages, 5 figures. New title; text more clearly writte
Control of electron spin decoherence caused by electron-nuclear spin dynamics in a quantum dot
Control of electron spin decoherence in contact with a mesoscopic bath of
many interacting nuclear spins in an InAs quantum dot is studied by solving the
coupled quantum dynamics. The nuclear spin bath, because of its bifurcated
evolution predicated on the electron spin up or down state, measures the
which-state information of the electron spin and hence diminishes its
coherence. The many-body dynamics of nuclear spin bath is solved with a
pair-correlation approximation. In the relevant timescale, nuclear pair-wise
flip-flops, as elementary excitations in the mesoscopic bath, can be mapped
into the precession of non-interacting pseudo-spins. Such mapping provides a
geometrical picture for understanding the decoherence and for devising control
schemes. A close examination of nuclear bath dynamics reveals a wealth of
phenomena and new possibilities of controlling the electron spin decoherence.
For example, when the electron spin is flipped by a -pulse at , its
coherence will partially recover at as a consequence of quantum
disentanglement from the mesoscopic bath. In contrast to the re-focusing of
inhomogeneously broadened phases by conventional spin-echoes, the
disentanglement is realized through shepherding quantum evolution of the bath
state via control of the quantum object. A concatenated construction of pulse
sequences can eliminate the decoherence with arbitrary accuracy, with the
nuclear-nuclear spin interaction strength acting as the controlling small
parameter
Low-temperature behavior of a Magnetic Impurity in a Heisenberg Chain
Using the bosonization technique, we have studied a spin-1/2 magnetic
impurity in Heisenberg chain, and shown that the impurity specific heat and
spin susceptibility have an anomalous temperature dependence.Comment: 12 pages, Revtex, no figure, to be published in Phys. Rev. Let
Phase diagram of a Disordered Boson Hubbard Model in Two Dimensions
We study the zero-temperature phase transition of a two-dimensional
disordered boson Hubbard model. The phase diagram of this model is constructed
in terms of the disorder strength and the chemical potential. Via quantum Monte
Carlo simulations, we find a multicritical line separating the weak-disorder
regime, where a random potential is irrelevant, from the strong-disorder
regime. In the weak-disorder regime, the Mott-insulator-to-superfluid
transition occurs, while, in the strong-disorder regime, the
Bose-glass-to-superfluid transition occurs. On the multicritical line, the
insulator-to-superfluid transition has the dynamical critical exponent and the correlation length critical exponent ,
that are different from the values for the transitions off the line. We suggest
that the proliferation of the particle-hole pairs screens out the weak disorder
effects.Comment: 4 pages, 4 figures, to be published in PR
A quantum Monte-Carlo method for fermions, free of discretization errors
In this work we present a novel quantum Monte-Carlo method for fermions,
based on an exact decomposition of the Boltzmann operator . It
can be seen as a synthesis of several related methods. It has the advantage
that it is free of discretization errors, and applicable to general
interactions, both for ground-state and finite-temperature calculations. The
decomposition is based on low-rank matrices, which allows faster calculations.
As an illustration, the method is applied to an analytically solvable model
(pairing in a degenerate shell) and to the Hubbard model.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
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