196 research outputs found
Critical entropies for magnetic ordering in bosonic mixtures on a lattice
We perform a numeric study (worm algorithm Monte Carlo simulations) of
ultracold two-component bosons in two- and three-dimensional optical lattices.
At strong enough interactions and low enough temperatures the system features
magnetic ordering. We compute critical temperatures and entropies for the
disappearance of the Ising antiferromagnetic and the xy-ferromagnetic order and
find that the largest possible entropies per particle are ~0.5kB. We also
estimate (optimistically) the experimental hold times required to reach
equilibrium magnetic states to be on a scale of seconds. Low critical entropies
and long hold times render the experimental observations of magnetic phases
challenging and call for increased control over heating sources.Comment: 6 pages, 6 figure
Continuous-Time Quantum Monte Carlo Algorithm for the Lattice Polaron
An efficient continuous-time path-integral Quantum Monte Carlo algorithm for
the lattice polaron is presented. It is based on Feynman's integration of
phonons and subsequent simulation of the resulting single-particle
self-interacting system. The method is free from the finite-size and
finite-time-step errors and works in any dimensionality and for any range of
electron-phonon interaction. The ground-state energy and effective mass of the
polaron are calculated for several models. The polaron spectrum can be measured
directly by Monte Carlo, which is of general interest.Comment: 5 pages, 4 figures, published versio
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
Magnetic long-range order induced by quantum relaxation in single-molecule magnets
Can magnetic interactions between single-molecule magnets (SMMs) in a crystal
establish long-range magnetic order at low temperatures deep in the quantum
regime, where the only electron spin-fluctuations are due to incoherent
magnetic quantum tunneling (MQT)? Put inversely: can MQT provide the
temperature dependent fluctuations needed to destroy the ordered state above
some finite Tc, although it should basically itself be a T-independent process?
Our experiments on two novel Mn4 SMMs provide a positive answer to the above,
showing at the same time that MQT in the SMMs has to involve spin-lattice
coupling at a relaxation rate equaling that predicted and observed recently for
nuclear spin-mediated quantum relaxation.Comment: 4 pages, 3 figure
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
Sharp transition for single polarons in the one-dimensional Su-Schrieffer-Heeger model
We study a single polaron in the Su-Schrieffer-Heeger (SSH) model using four
different techniques (three numerical and one analytical). Polarons show a
smooth crossover from weak to strong coupling, as a function of the
electron-phonon coupling strength , in all models where this coupling
depends only on phonon momentum . In the SSH model the coupling also depends
on the electron momentum ; we find it has a sharp transition, at a critical
coupling strength , between states with zero and nonzero momentum of
the ground state. All other properties of the polaron are also singular at
, except the average number of phonons in the polaronic
cloud. This result is representative of all polarons with coupling depending on
and , and will have important experimental consequences (eg., in ARPES
and conductivity experiments)
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
Nuclear spin driven quantum relaxation in LiY_0.998Ho_0.002F_4
Staircase hysteresis loops of the magnetization of a LiY_0.998Ho_0.002F_4
single crystal are observed at subkelvin temperatures and low field sweep
rates. This behavior results from quantum dynamics at avoided level crossings
of the energy spectrum of single Ho^{3+} ions in the presence of hyperfine
interactions. Enhanced quantum relaxation in constant transverse fields allows
the study of the relative magnitude of tunnel splittings. At faster sweep
rates, non-equilibrated spin-phonon and spin-spin transitions, mediated by weak
dipolar interactions, lead to magnetization oscillations and additional steps.Comment: 5 pages, 5 eps figures, using RevTe
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