13,465 research outputs found
Bose Einstein Condensation of incommensurate solid 4He
It is pointed out that simulation computation of energy performed so far
cannot be used to decide if the ground state of solid 4He has the number of
lattice sites equal to the number of atoms (commensurate state) or if it is
different (incommensurate state). The best variational wave function, a shadow
wave function, gives an incommensurate state but the equilibrium concentration
of vacancies remains to be determined. In order to investigate the presence of
a supersolid phase we have computed the one--body density matrix in solid 4He
for the incommensurate state by means of the exact Shadow Path Integral Ground
State projector method. We find a vacancy induced Bose Einstein condensation of
about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk
solid 4He is supersolid at low enough temperature if the exact ground state is
incommensurate.Comment: 5 pages, 2 figure
Dynamic structure factor for 3He in two-dimensions
Recent neutron scattering experiments on 3He films have observed a zero-sound
mode, its dispersion relation and its merging with -and possibly emerging from-
the particle-hole continuum. Here we address the study of the excitations in
the system via quantum Monte Carlo methods: we suggest a practical scheme to
calculate imaginary time correlation functions for moderate-size fermionic
systems. Combined with an efficient method for analytic continuation, this
scheme affords an extremely convincing description of the experimental
findings.Comment: 5 pages, 5 figure
Implementation of the Linear Method for the optimization of Jastrow-Feenberg and Backflow Correlations
We present a fully detailed and highly performing implementation of the
Linear Method [J. Toulouse and C. J. Umrigar (2007)] to optimize
Jastrow-Feenberg and Backflow Correlations in many-body wave-functions, which
are widely used in condensed matter physics. We show that it is possible to
implement such optimization scheme performing analytical derivatives of the
wave-function with respect to the variational parameters achieving the best
possible complexity O(N^3) in the number of particles N.Comment: submitted to the Comp. Phys. Com
Imaginary Time Correlations and the phaseless Auxiliary Field Quantum Monte Carlo
The phaseless Auxiliary Field Quantum Monte Carlo method provides a well
established approximation scheme for accurate calculations of ground state
energies of many-fermions systems. Here we apply the method to the calculation
of imaginary time correlation functions. We give a detailed description of the
technique and we test the quality of the results for static and dynamic
properties against exact values for small systems.Comment: 13 pages, 6 figures; submitted to J. Chem. Phy
Quantum Monte Carlo study of a vortex in superfluid He and search for a vortex state in the solid
We have performed a microscopic study of a straight quantized vortex line in
three dimensions in condensed He at zero temperature using the Shadow Path
Integral Ground State method and the fixed-phase approximation. We have
characterized the energy and the local density profile around the vortex axis
in superfluid He at several densities, ranging from below the equilibrium
density up to the overpressurized regime. For the Onsager-Feynman (OF) phase
our results are exact and represent a benchmark for other theories. The
inclusion of backflow correlations in the phase improves the description of the
vortex with respect to the OF phase by a large reduction of the core energy of
the topological excitation. At all densities the phase with backflow induces a
partial filling of the vortex core and this filling slightly increases with
density. The core size slightly decreases for increasing density and the
density profile has well defined density dependent oscillations whose wave
vector is closer to the wave vector of the main peak in the static density
response function rather than to the roton wave vector. Our results can be
applied to vortex rings of large radius and we find good agreement with the
experimental value of the energy as function of without any free parameter.
We have studied also He above the melting density in the solid phase using
the same functional form for the phase as in the liquid. We found that
off-diagonal properties of the solid are not qualitatively affected by the
velocity field induced by the vortex phase, both with and without backflow
correlations. Therefore we find evidence that a perfect He crystal is not a
marginally stable quantum solid in which rotation would be able to induce
off-diagonal long-range coherence.Comment: 15 pages, 8 figure
Bounds for the Superfluid Fraction from Exact Quantum Monte Carlo Local Densities
For solid 4He and solid p-H2, using the flow-energy-minimizing one-body phase
function and exact T=0 K Monte Carlo calculations of the local density, we have
calculated the phase function, the velocity profile and upper bounds for the
superfluid fraction f_s. At the melting pressure for solid 4He we find that f_s
< 0.20-0.21, about ten times what is observed. This strongly indicates that the
theory for the calculation of these upper bounds needs substantial
improvements.Comment: to be published in Phys. Rev. B (Brief Reports
A first principles simulation of rigid water
We present the results of Car-Parrinello (CP) simulations of water at ambient
conditions and under pressure, using a rigid molecule approximation. Throughout
our calculations, water molecules were maintained at a fixed intramolecular
geometry corresponding to the average structure obtained in fully unconstrained
simulations. This allows us to use larger time steps than those adopted in
ordinary CP simulations of water, and thus to access longer time scales. In the
absence of chemical reactions or dissociation effects, these calculations open
the way to ab initio simulations of aqueous solutions that require timescales
substantially longer than presently feasible (e.g. simulations of hydrophobic
solvation). Our results show that structural properties and diffusion
coefficients obtained with a rigid model are in better agreement with
experiment than those determined with fully flexible simulations. Possible
reasons responsible for this improved agreement are discussed
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