893 research outputs found
Dynamic Structure Function in 3he-4he Mixtures
Relevant features of the dynamic structure function in
He-He mixtures at zero temperature are investigated starting from known
properties of the ground state. Sum rules are used to fix rigorous constraints
to the different contributions to , coming from He and He
elementary excitations, as well as to explore the role of the cross term
. Both the low- (phonon-roton He excitations and
1p-1h He excitations) and high- (deep inelastic scattering) ranges are
discussed.Comment: 29 pages, Plain TeX, 11 figures available by request from
[email protected]
Evaluating the performance of model transformation styles in Maude
Rule-based programming has been shown to be very successful in many application areas. Two prominent examples are the specification of model transformations in model driven development approaches and the definition of structured operational semantics of formal languages. General rewriting frameworks such as Maude are flexible enough to allow the programmer to adopt and mix various rule styles. The choice between styles can be biased by the programmer’s background. For instance, experts in visual formalisms might prefer graph-rewriting styles, while experts in semantics might prefer structurally inductive rules. This paper evaluates the performance of different rule styles on a significant benchmark taken from the literature on model transformation. Depending on the actual transformation being carried out, our results show that different rule styles can offer drastically different performances. We point out the situations from which each rule style benefits to offer a valuable set of hints for choosing one style over the other
Ground state properties and excitation spectrum of a two dimensional gas of bosonic dipoles
We present a quantum Monte Carlo study of two-dimensional dipolar Bose gases
in the limit of zero temperature. The analysis is mainly focused on the
anisotropy effects induced in the homogeneous gas when the polarization angle
with respect to the plane is changed. We restrict our study to the regime where
the dipolar interaction is strictly repulsive, although the strength of the
pair repulsion depends on the vector interparticle distance. Our results show
that the effect of the anisotropy in the energy per particle scales with the
gas parameter at low densities as expected, and that this scaling is preserved
for all polarization angles even at the largest densities considered here. We
also evaluate the excitation spectrum of the dipolar Bose gas in the context of
the Feynman approximation and compare the results obtained with the Bogoliubov
ones. As expected, we find that these two approximations agree at very low
densities, while they start to deviate from each other as the density
increases. For the largest densities studied, we observe a significant
influence of the anisotropy of the dipole-dipole interaction in the excitation
spectrum.Comment: 6 pages, 6 figure
Quadratic diffusion Monte Carlo and pure estimators for atoms
The implementation and reliability of a quadratic diffusion Monte Carlo
method for the study of ground-state properties of atoms are discussed. We show
in the simple yet non-trivial calculation of the binding energy of the Li atom
that the method presented is effectively second-order in the time step. The
fulfilment of the expected quadratic behavior relies on some basic requirements
of the trial wave function used for importance sampling, in the context of the
fixed-node approximation. Expectation values of radial operators are calculated
by means of a pure estimation based on the forward walking methodology. It is
shown that accurate results without extrapolation errors can be obtained with a
pure algorithm that can be easily implemented in any previous diffusion Monte
Carlo program.Comment: RevTex, 20 pages, 3 figures, accepted in J. Chem. Phy
Coherent and Incoherent Dynamic Structure Function of the Free Fermi Gas
A detailed calculation of the coherent and incoherent dynamic structure
functions of the free Fermi gas, starting from their expressions in terms of
the one- and semi-diagonal two-body density matrices, is derived and discussed.
Their behavior and evolution with the momentum transfer is analyzed, and
particular attention is devoted to the contributions that both functions
present at negative energies. Finally, an analysis of the energy weighted sum
rules satisfied by both responses is also performed. Despite of the simplicity
of the model, some of the conclusions can be extended to realistic systems.Comment: LaTeX, 3 figure
Superfluidity versus localization in bulk 4He at zero temperature
We present a zero-temperature quantum Monte Carlo calculation of liquid
He immersed in an array of confining potentials. These external potentials
are centered in the lattice sites of a fcc solid geometry and, by modifying
their well depth and range, the system evolves from a liquid phase towards a
progressively localized system which mimics a solid phase. The superfluid
density decreases with increasing order, reaching a value when the Lindemann's ratio of the model equals the experimental
value for solid He.Comment: 5 pages,5 figure
Single-particle vs. pair superfluidity in a bilayer system of dipolar bosons
We consider the ground state of a bilayer system of dipolar bosons, where
dipoles are oriented by an external field in the direction perpendicular to the
parallel planes. Quantum Monte Carlo methods are used to calculate the
ground-state energy, the one-body and two-body density matrix, and the
superfluid response as a function of the separation between layers. We find
that by decreasing the interlayer distance for fixed value of the strength of
the dipolar interaction, the system undergoes a quantum phase transition from a
single-particle to a pair superfluid. The single-particle superfluid is
characterized by a finite value of both the atomic condensate and the
super-counterfluid density. The pair superfluid phase is found to be stable
against formation of many-body cluster states and features a gap in the
spectrum of elementary excitations.Comment: 4 figure
Composite boson description of a low density gas of excitons
Ground state properties of a fermionic Coulomb gas are calculated using the
fixed-node diffusion Monte Carlo method. The validity of the composite boson
description is tested for different densities. We extract the exciton-exciton
-wave scattering length by solving the four-body problem in a harmonic trap
and mapping the energy to that of two trapped bosons. The equation of state is
consistent with the Bogoliubov theory for composite bosons interacting with the
obtained -wave scattering length. The perturbative expansion at low density
has contributions physically coming from (a) exciton binding energy, (b)
mean-field Gross-Pitaevskii interaction between excitons, (c) quantum depletion
of the excitonic condensate (Lee-Huang-Yang terms for composite bosons). In
addition, for low densities we find a good agreement with the Bogoliubov
bosonic theory for the condensate fraction of excitons. The equation of state
in the opposite limit of large density is found to be well described by the
perturbative theory including (a) mixture of two ideal Fermi gases (b) exchange
energy. We find that for low densities both energetic and coherent properties
are correctly described by the picture of composite bosons (excitons).Comment: 12 pages, 4 figure
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