590 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]
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
High-order Time Expansion Path Integral Ground State
The feasibility of path integral Monte Carlo ground state calculations with
very few beads using a high-order short-time Green's function expansion is
discussed. An explicit expression of the evolution operator which provides
dramatic enhancements in the quality of ground-state wave-functions is
examined. The efficiency of the method makes possible to remove the trial wave
function and thus obtain completely model-independent results still with a very
small number of beads. If a single iteration of the method is used to improve a
given model wave function, the result is invariably a shadow-type wave
function, whose precise content is provided by the high-order algorithm
employed.Comment: 4 page
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
Ground-State Properties of a One-Dimensional System of Hard Rods
A quantum Monte Carlo simulation of a system of hard rods in one dimension is
presented and discussed. The calculation is exact since the analytical form of
the wavefunction is known, and is in excellent agreement with predictions
obtained from asymptotic expansions valid at large distances. The analysis of
the static structure factor and the pair distribution function indicates that a
solid-like and a gas-like phases exist at high and low densities, respectively.
The one-body density matrix decays following a power-law at large distances and
produces a divergence in the low density momentum distribution at k=0 which can
be identified as a quasi-condensate.Comment: 4 pages, 4 figure
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