82,171 research outputs found
Mean-value identities as an opportunity for Monte Carlo error reduction
In the Monte Carlo simulation of both Lattice field-theories and of models of
Statistical Mechanics, identities verified by exact mean-values such as
Schwinger-Dyson equations, Guerra relations, Callen identities, etc., provide
well known and sensitive tests of thermalization bias as well as checks of
pseudo random number generators. We point out that they can be further
exploited as "control variates" to reduce statistical errors. The strategy is
general, very simple, and almost costless in CPU time. The method is
demonstrated in the two dimensional Ising model at criticality, where the CPU
gain factor lies between 2 and 4.Comment: 10 pages, 2 tables. References updated and typos correcte
Lattice-Spin Mechanism in Colossal Magnetoresistant Manganites
We present a single-orbital double-exchange model, coupled with cooperative
phonons (the so called breathing-modes of the oxygen octahedra in manganites).
The model is studied with Monte Carlo simulations. For a finite range of doping
and coupling constants, a first-order Metal-Insulator phase transition is
found, that coincides with the Paramagnetic-Ferromagnetic phase transition. The
insulating state is due to the self-trapping of every carrier within an oxygen
octahedron distortion.Comment: 4 pages, 5 figures, ReVTeX macro, accepted for publication in PR
Optimized Monte Carlo Method for glasses
A new Monte Carlo algorithm is introduced for the simulation of supercooled
liquids and glass formers, and tested in two model glasses. The algorithm is
shown to thermalize well below the Mode Coupling temperature and to outperform
other optimized Monte Carlo methods. Using the algorithm, we obtain finite size
effects in the specific heat. This effect points to the existence of a large
correlation length measurable in equal time correlation functions.Comment: Proceedings of "X International workshop on Disordered Systems" held
in Molveno (Italy), March 200
Finite size effects in the specific heat of glass-formers
We report clear finite size effects in the specific heat and in the
relaxation times of a model glass former at temperatures considerably smaller
than the Mode Coupling transition. A crucial ingredient to reach this result is
a new Monte Carlo algorithm which allows us to reduce the relaxation time by
two order of magnitudes. These effects signal the existence of a large
correlation length in static quantities.Comment: Proceeding of "3rd International Workshop on Complex Systems". Sendai
(Japan). To appear on AIP Conference serie
On the critical behavior of the specific heat in glass-formers
We show numeric evidence that, at low enough temperatures, the potential
energy density of a glass-forming liquid fluctuates over length scales much
larger than the interaction range. We focus on the behavior of translationally
invariant quantities. The growing correlation length is unveiled by studying
the Finite Size effects. In the thermodynamic limit, the specific heat and the
relaxation time diverge as a power law. Both features point towards the
existence of a critical point in the metastable supercooled liquid phase.Comment: Version to be published in Phys. Rev.
Density Functional Calculations On First-Row Transition Metals
The excitation energies and ionization potentials of the atoms in the first
transition series are notoriously difficult to compute accurately. Errors in
calculated excitation energies can range from 1--4 eV at the Hartree-Fock
level, and errors as high as 1.5eV are encountered for ionization energies. In
the current work we present and discuss the results of a systematic study of
the first transition series using a spin-restricted Kohn-Sham
density-functional method with the gradient-corrected functionals of Becke and
Lee, Yang and Parr. Ionization energies are observed to be in good agreement
with experiment, with a mean absolute error of approximately 0.15eV; these
results are comparable to the most accurate calculations to date, the Quadratic
Configuration Interaction (QCISD(T)) calculations of Raghavachari and Trucks.
Excitation energies are calculated with a mean error of approximately 0.5eV,
compared with \sim 1\mbox{eV} for the local density approximation and 0.1eV
for QCISD(T). These gradient-corrected functionals appear to offer an
attractive compromise between accuracy and computational effort.Comment: Journal of Chemical Physics, 29, LA-UR-93-425
On the evidence for brown-dwarf secondary stars in cataclysmic variables
We present the K-band spectrum of the cataclysmic variable LL And, obtained
using NIRSPEC on Keck-II. The spectrum shows no evidence for the absorption
features observed by Howell & Ciardi (2001), which these authors used to claim
a detection of a brown-dwarf secondary star in LL And. In light of our new
data, we review the evidence for brown-dwarf secondary stars in this and other
cataclysmic variables.Comment: 6 pages, to appear in Monthly Notices, accepte
Measuring correlated electron dynamics with time-resolved photoemission spectroscopy
Time-resolved photoemission experiments can reveal fascinating quantum
dynamics of correlated electrons. However, the thermalization of the electronic
system is typically so fast that very short probe pulses are necessary to
resolve the time evolution of the quantum state, and this leads to poor energy
resolution due to the energy-time uncertainty relation. Although the
photoemission intensity can be calculated from the nonequilibrium electronic
Green functions, the converse procedure is therefore difficult. We analyze a
hypothetical time-resolved photoemission experiment on a correlated electronic
system, described by the Falicov-Kimball model in dynamical mean-field theory,
which relaxes between metallic and insulating phases. We find that the
real-time Green function which describes the transient behavior during the
buildup of the metallic state cannot be determined directly from the
photoemission signal. On the other hand, the characteristic
collapse-and-revival oscillations of an excited Mott insulator can be observed
as oscillating weight in the center of the Mott gap in the time-dependent
photoemission spectrum.Comment: 12 pages, 5 figure
Nonequilibrium dynamical mean-field calculations based on the non-crossing approximation and its generalizations
We solve the impurity problem which arises within nonequilibrium dynamical
mean-field theory for the Hubbard model by means of a self-consistent
perturbation expansion around the atomic limit. While the lowest order, known
as the non-crossing approximation (NCA), is reliable only when the interaction
U is much larger than the bandwidth, low-order corrections to the NCA turn out
to be sufficient to reproduce numerically exact Monte Carlo results in a wide
parameter range that covers the insulating phase and the metal-insulator
crossover regime at not too low temperatures. As an application of the
perturbative strong-coupling impurity solver we investigate the response of the
double occupancy in the Mott insulating phase of the Hubbard model to a
dynamical change of the interaction or the hopping, a technique which has been
used as a probe of the Mott insulating state in ultracold fermionic gases.Comment: 14 pages, 9 figure
On stability of odd-frequency superconducting state
Odd-frequency pairing mechanism of superconductivity has been investigated
for several decades. Nevertheless, its properties, including the thermodynamic
stability, have remained unclear. In particular, it has been argued that the
odd-frequency state is thermodynamically unstable, has an unphysical (anti-)
Meissner effect, and thus can not exist as a homogeneous equilibrium phase. We
argue that this conclusion is incorrect because it implicitly relies on the
inappropriate assumption that the odd-frequency superconductor can be described
by an effective Hamiltonian that breaks the particle conservation symmetry. We
demonstrate that the odd-frequency state can be properly described within the
functional integral approach using non-local-in-time effective action. Within
the saddle point approximation, we find that this phase is thermodynamically
stable, exhibits ordinary Meissner effect, and therefore can be realized as an
equilibrium homogenous state of matter.Comment: 4 pages, no figure
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