18,456 research outputs found
Equation of state of metallic hydrogen from Coupled Electron-Ion Monte Carlo simulations
We present a study of hydrogen at pressures higher than molecular
dissociation using the Coupled Electron-Ion Monte Carlo method. These
calculations use the accurate Reptation Quantum Monte Carlo method to estimate
the electronic energy and pressure while doing a Monte Carlo simulation of the
protons. In addition to presenting simulation results for the equation of state
over a large region of phase space, we report the free energy obtained by
thermodynamic integration. We find very good agreement with DFT calculations
for pressures beyond 600 GPa and densities above . Both
thermodynamic as well as structural properties are accurately reproduced by DFT
calculations. This agreement gives a strong support to the different
approximations employed in DFT, specifically the approximate
exchange-correlation potential and the use of pseudopotentials for the range of
densities considered. We find disagreement with chemical models, which suggests
a reinvestigation of planetary models, previously constructed using the
Saumon-Chabrier-Van Horn equations of state.Comment: 9 pages, 7 figure
OTOC, complexity and entropy in bi-partite systems
There is a remarkable interest in the study of Out-of-time ordered
correlators (OTOCs) that goes from many body theory and high energy physics to
quantum chaos. In this latter case there is a special focus on the comparison
with the traditional measures of quantum complexity such as the spectral
statistics, for example. The exponential growth has been verified for many
paradigmatic maps and systems. But less is known for multi-partite cases. On
the other hand the recently introduced Wigner separability entropy (WSE) and
its classical counterpart (CSE) provide with a complexity measure that treats
equally quantum and classical distributions in phase space. We have compared
the behavior of these measures in a system consisting of two coupled and
perturbed cat maps with different dynamics: double hyperbolic (HH), double
elliptic (EE) and mixed (HE). In all cases, we have found that the OTOCs and
the WSE have essentially the same behavior, providing with a complete
characterization in generic bi-partite systems and at the same time revealing
them as very good measures of quantum complexity for phase space distributions.
Moreover, we establish a relation between both quantities by means of a
recently proven theorem linking the second Renyi entropy and OTOCs.Comment: 6 pages, 5 figure
Quantum Monte Carlo Simulation of the High-Pressure Molecular-Atomic Crossover in Fluid Hydrogen
A first-order liquid-liquid phase transition in high-pressure hydrogen
between molecular and atomic fluid phases has been predicted in computer
simulations using ab initio molecular dynamics approaches. However, experiments
indicate that molecular dissociation may occur through a continuous crossover
rather than a first-order transition. Here we study the nature of molecular
dissociation in fluid hydrogen using an alternative simulation technique in
which electronic correlation is computed within quantum Monte Carlo, the
so-called Coupled Electron Ion Monte Carlo (CEIMC) method. We find no evidence
for a first-order liquid-liquid phase transition.Comment: 4 pages, 5 figures; content changed; accepted for publication in
Phys. Rev. Let
Marcatili's Lossless Tapers and Bends: an Apparent Paradox and its Solution
Numerical results based on an extended BPM algorithm indicate that, in
Marcatili's lossless tapers and bends, through-flowing waves are drastically
different from standing waves. The source of this surprising behavior is
inherent in Maxwell's equations. Indeed, if the magnetic field is correctly
derived from the electric one, and the Poynting vector is calculated, then the
analytical results are reconciled with the numerical ones. Similar
considerations are shown to apply to Gaussian beams in free space.Comment: 4 pages, figures include
Quantum and classical complexity in coupled maps
We study a generic and paradigmatic two-degrees-of-freedom system consisting of two coupled perturbed cat maps with different types of dynamics. The Wigner separability entropy (WSE)âequivalent to the operator space entanglement entropyâand the classical separability entropy (CSE) are used as measures of complexity. For the case where both degrees of freedom are hyperbolic, the maps are classically ergodic and the WSE and the CSE behave similarly, growing to higher values than in the doubly elliptic case. However, when one map is elliptic and the other hyperbolic, the WSE reaches the same asymptotic value than that of the doubly hyperbolic case but at a much slower rate. The CSE only follows the WSE for a few map steps, revealing that classical dynamical features are not enough to explain complexity growth.Fil: Bergamasco, Pablo D.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; ArgentinaFil: Carlo, Gabriel Gustavo. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; ArgentinaFil: Rivas, Alejandro Mariano Fidel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentin
Coherent transport of cold atoms in angle-tuned optical lattices
Optical lattices with a large spacing between the minima of the optical
potential can be created using the angle-tuned geometry where the 1-D periodic
potential is generated by two propagating laser beams intersecting at an angle
different from . The present work analyzes the coherent transport for the
case of this geometry. We show that the potential depth can be kept constant
during the transport by choosing a magic value for the laser wavelength. This
value agrees with that of the counterpropagating laser case, and the magic
wavelength does not depend of the optical lattice geometry. Moreover, we find
that this scheme can be used to implement controlled collision experiments
under special geometric conditions. Finally we study the transport of
hyperfine-Zeeman states of rubidium 87.Comment: 8 pages, 5 figures, one section added, in press in Phys. Rev.
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