2,766 research outputs found
Path Integral Calculations of exchange in solid 4He
Recently there have been experimental indications that solid 4He might be a
supersolid. We discuss the relation of supersolid behavior to ring exchange.
The tunnelling frequencies for ring exchanges in quantum solids are calculated
using Path Integral Monte Carlo by finding the free energy for making a path
that begins with the atoms in one configuration and ends with a permutation of
those positions. We find that the exchange frequencies in solid 4He are
described by a simple lattice model which does not show supersolid behavior.
Thus, the PIMC calculations constrain the mechanism for the supersolid
behavior. We also look at the characteristics of very long exchanges needed for
macroscopic mass transport
Coupled Electron Ion Monte Carlo Calculations of Dense Metallic Hydrogen
We present a new Monte Carlo method which couples Path Integral for finite
temperature protons with Quantum Monte Carlo for ground state electrons, and we
apply it to metallic hydrogen for pressures beyond molecular dissociation. We
report data for the equation of state for temperatures across the melting of
the proton crystal. Our data exhibit more structure and higher melting
temperatures of the proton crystal than Car-Parrinello Molecular Dynamics
results. This method fills the gap between high temperature electron-proton
Path Integral and ground state Diffusion Monte Carlo methods
Metropolis Methods for Quantum Monte Carlo Simulations
Since its first description fifty years ago, the Metropolis Monte Carlo
method has been used in a variety of different ways for the simulation of
continuum quantum many-body systems. This paper will consider some of the
generalizations of the Metropolis algorithm employed in quantum Monte Carlo:
Variational Monte Carlo, dynamical methods for projector monte carlo ({\it
i.e.} diffusion Monte Carlo with rejection), multilevel sampling in path
integral Monte Carlo, the sampling of permutations, cluster methods for lattice
models, the penalty method for coupled electron-ionic systems and the Bayesian
analysis of imaginary time correlation functions.Comment: Proceedings of "Monte Carlo Methods in the Physical Sciences"
Celebrating the 50th Anniversary of the Metropolis Algorith
Characterization of the State of Hydrogen
Fermionic path integral Monte Carlo simulations have been applied to study
the equilibrium properties of the hydrogen and deuterium in the density and
temperature range of 1.6 < rs < 14.0 and 5000K < T < 167000K. We use this
technique to determine the phase diagram by identifying the plasma, the
molecular, atomic and metallic regime. We explain how one can identify the
phases in the path integral formalism and discuss the state of hydrogen for 5
points in the temperature-density plane. Further we will provide arguments for
the nature of the transitions between the regimes.Comment: 4 pages, 2 figures, proceedings of 9th International Workshop on the
Physics of Nonideal Plasmas, Rostock, Germany, September 199
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
Path Integral Monte Carlo Simulations for Fermion Systems: Pairing in the Electron-Hole Plasma
We review the path integral method wherein quantum systems are mapped with
Feynman's path integrals onto a classical system of "ring-polymers" and then
simulated with the Monte Carlo technique. Bose or Fermi statistics correspond
to possible "cross-linking" of polymers. As proposed by Feynman, superfluidity
and Bose condensation result from macroscopic exchange of bosons. To map
fermions onto a positive probability distribution, one must restrict the paths
to lie in regions where the fermion density matrix is positive. We discuss a
recent application to the two-component electron-hole plasma. At low
temperature excitons and bi-excitons form. We have used nodal surfaces
incorporating paired fermions and see evidence of a Bose condensation in the
energy, specific heat and superfluid density. In the restricted path integral
picture, pairing appears as intertwined electron-hole paths. Bose condensation
occurs when these intertwined paths wind around the periodic boundaries.Comment: 14 pages, 7 figures Prepared for the 1999 International Conference on
Strongly Coupled Coulomb Systems, Saint-Malo, Franc
Simulations of Dense Atomic Hydrogen in the Wigner Crystal Phase
Path integral Monte Carlo simulations are applied to study dense atomic
hydrogen in the regime where the protons form a Wigner crystal. The interaction
of the protons with the degenerate electron gas is modeled by Thomas-Fermi
screening, which leads to a Yukawa potential for the proton-proton interaction.
A numerical technique for the derivation of the corresponding action of the
paths is described. For a fixed density of rs=200, the melting is analyzed
using the Lindemann ratio, the structure factor and free energy calculations.
Anharmonic effects in the crystal vibrations are analyzed.Comment: Proceedings article of the Study of Matter at Extreme Conditions
(SMEC) conference in Miami, Florida; submitted to Journal of Physics and
Chemistry of Solids (2005
- …