209 research outputs found
Energy weighted sum rules for mesons in hot and dense matter
We study energy weighted sum rules of the pion and kaon propagator in nuclear
matter at finite temperature. The sum rules are obtained from matching the
Dyson form of the meson propagator with its spectral Lehmann representation at
low and high energies. We calculate the sum rules for specific models of the
kaon and pion self-energy. The in-medium spectral densities of the K and anti-K
mesons are obtained from a chiral unitary approach in coupled channels which
incorporates the S- and P-waves of the kaon-nucleon interaction. The pion
self-energy is determined from the P-wave coupling to particle-hole and
Delta-hole excitations, modified by short range correlations. The sum rules for
the lower energy weights are fulfilled satisfactorily and reflect the
contributions from the different quasi-particle and collective modes of the
meson spectral function. We discuss the sensitivity of the sum rules to the
distribution of spectral strength and their usefulness as quality tests of
model calculations.Comment: 19 pages, 6 figures; one figure added, enhanced discussion, version
to appear in PR
Path Integral Monte Carlo study of phonons in the bcc phase of He
Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate
the dynamic structure factor of solid He in the bcc phase at a finite
temperature of T = 1.6 K and a molar volume of 21 cm. Both the
single-phonon contribution to the dynamic structure factor and the total
dynamic structure factor are evaluated. From the dynamic structure factor, we
obtain the phonon dispersion relations along the main crystalline directions,
[001], [011] and [111]. We calculate both the longitudinal and transverse
phonon branches. For the latter, no previous simulations exist. We discuss the
differences between dispersion relations resulting from the single-phonon part
vs. the total dynamic structure factor. In addition, we evaluate the formation
energy of a vacancy.Comment: 10 figure
Microscopic calculation of the phonon-roton branch in superfluid He
Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk
liquid He at zero temperature are presented. The sign problem associated to
the excited wave function has been dealt both with the fixed-node approximation
and the released-node technique. The upper bounds provided by the fixed-node
approximation are shown to become exact when using the released-node method. An
excellent agreement with experimental data is achieved both at the equilibrium
and near the freezing densities.Comment: 12 pages, RevTex, 3 ps figures include
Ground state properties of a dilute homogeneous Bose gas of hard disks in two dimensions
The energy and structure of a dilute hard-disks Bose gas are studied in the
framework of a variational many-body approach based on a Jastrow correlated
ground state wave function. The asymptotic behaviors of the radial distribution
function and the one-body density matrix are analyzed after solving the Euler
equation obtained by a free minimization of the hypernetted chain energy
functional. Our results show important deviations from those of the available
low density expansions, already at gas parameter values . The
condensate fraction in 2D is also computed and found generally lower than the
3D one at the same .Comment: Submitted to PRA. 7 pages and 8 figure
Dynamics of liquid 4He in Vycor
We have measured the dynamic structure factor of liquid 4He in Vycor using
neutron inelastic scattering. Well-defined phonon-roton (p-r) excitations are
observed in the superfluid phase for all wave vectors 0.3 < Q < 2.15. The p-r
energies and lifetimes at low temperature (T = 0.5 K) and their temperature
dependence are the same as in bulk liquid 4He. However, the weight of the
single p-r component does not scale with the superfluid fraction (SF) as it
does in the bulk. In particular, we observe a p-r excitation between T_c =
1.952 K, where SF = 0, and T_(lambda)=2.172 K of the bulk. This suggests, if
the p-r excitation intensity scales with the Bose condensate, that there is a
separation of the Bose-Einstein condensation temperature and the superfluid
transition temperature T_c of 4He in Vycor. We also observe a two-dimensional
layer mode near the roton wave vector. Its dispersion is consistent with
specific heat and SF measurements and with layer modes observed on graphite
surfaces.Comment: 3 pages, 4 figure
Bose-Einstein Condensation in liquid He near the liquid-solid transition line
We present precision neutron scattering measurements of the Bose-Einstein
condensate fraction, n0(T), and the atomic momentum distribution, n\star(k), of
liquid 4He at pressure p =24 bar. Both the temperature dependence of n0(T) and
of the width of n\star(k) are determined. The n0(T) can be represented by n0(T)
= n0(0)[1-(T/T{\lambda}){\gamma}] with a small n0(0) = 2.80\pm0.20% and large
{\gamma} = 13\pm2 for T < T{\lambda} indicating strong interaction. The onset
of BEC is accompanied by a significant narrowing of the n\star(k). The
narrowing accounts for 65% of the drop in kinetic energy below T{\lambda} and
reveals an important coupling between BEC and k > 0 states. The experimental
results are well reproduced by Path Integral Monte Carlo calculations.Comment: 4 Pages, 5 Figure
Layer- and bulk roton excitations of 4He in porous media
We examine the energetics of bulk and layer-roton excitations of 4He in
various porous medial such as aerogel, Geltech, or Vycor, in order to find out
what conclusions can be drawn from experiments on the energetics about the
physisorption mechanism. The energy of the layer-roton minimum depends
sensitively on the substrate strength, thus providing a mechanism for a direct
measurement of this quantity. On the other hand, bulk-like roton excitations
are largely independent of the interaction between the medium and the helium
atoms, but the dependence of their energy on the degree of filling reflects the
internal structure of the matrix and can reveal features of 4He at negative
pressures. While bulk-like rotons are very similar to their true bulk
counterparts, the layer modes are not in close relation to two-dimensional
rotons and should be regarded as a third, completely independent kind of
excitation
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