104 research outputs found
The Momentum Distribution of Liquid He
We present high-resolution neutron Compton scattering measurements of liquid
He below its renormalized Fermi temperature. Theoretical predictions are in
excellent agreement with the experimental data when instrumental resolution and
final state effects are accounted for. Our results resolve the long-standing
inconsistency between theoretical and experimental estimates of the average
atomic kinetic energy.Comment: 5 pages, 4 figure
Zero-Point Motion of Liquid and Solid Hydrogen
We present an inelastic neutron scattering study of liquid and solid hydrogen
carried out using the wide Angular Range Chopper Spectrometer at Oak Ridge
National Laboratory. From the observed dynamic structure factor, we obtained
empirical estimates of the molecular mean-squared displacement and average
translational kinetic energy. We find that the former quantity increases with
temperature, indicating that a combination of thermal and quantum effects is
important near the liquid-solid phase transition, contrary to previous
measurements. We also find that the kinetic energy drops dramatically upon
melting of the crystals, a consequence of the large increase in molar volume
together with the Heisenberg indeterminacy principle. Our results are compared
with quantum Monte Carlo simulations based on different model potentials. In
general, there is good agreement between our findings and theoretical
predictions based on the Silvera-Goldman and Buck potentials.Comment: 20 pages, 10 figures in color, submitted to Phys. Rev.
Atomic kinetic energy, momentum distribution and structure of solid neon at zero-temperature
We report on the calculation of the ground-state atomic kinetic energy,
, and momentum distribution of solid Ne by means of the diffusion Monte
Carlo method and Aziz HFD-B pair potential. This approach is shown to perform
notably for this crystal since we obtain very good agreement with respect to
experimental thermodynamic data. Additionally, we study the structural
properties of solid Ne at densities near the equilibrium by estimating the
radial pair-distribution function, Lindemann's ratio and atomic density profile
around the positions of the perfect crystalline lattice. Our value for
at the equilibrium density is K, which agrees perfectly with the
recent prediction made by Timms {\it et al.}, K, based on their
deep-inelastic neutron scattering experiments carried out over the temperature
range K, and also with previous path integral Monte Carlo results
obtained with the Lennard-Jones and Aziz HFD-C2 atomic pairwise interactions.
The one-body density function of solid Ne is calculated accurately and found to
fit perfectly, within statistical uncertainty, to a Gaussian curve.
Furthermore, we analyze the degree of anharmonicity of solid Ne by calculating
some of its microscopic ground-state properties within traditional harmonic
approaches. We provide insightful comparison to solid He in terms of the
Debye model, in order to size the relevance of anharmonic effects in Ne.Comment: 20 pages, 7 figures. To be published in Physical Review
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
Direct observation of local Mn-Mn distances in the paramagnetic compound CsMnxMg1-xBr3
We introduce a novel method for local structure determination with a spatial
resolution of the order of 0.01 Angstroem. It can be applied to materials
containing clusters of exchange-coupled magnetic atoms. We use neutron
spectroscopy to probe the energies of the cluster excitations which are
determined by the interatomic coupling strength J. Since for most materials J
is related to the interatomic distance R through a linear relation
dJ/dR={\alpha} (for dR/R<<1), we can directly derive the local distance R from
the observed excitation energies. This is exemplified for the mixed
one-dimensional paramagnetic compound CsMnxMg1 xBr3 (x=0.05, 0.10) containing
manganese dimers oriented along the hexagonal c-axis. Surprisingly, the
resulting Mn-Mn distances R do not vary continuously with increasing internal
pressure, but lock in at some discrete values.Comment: 16 pages, 2 tables, 3 figure
High-momentum dynamic structure function of liquid 3He-4He mixtures: a microscopic approach
The high-momentum dynamic structure function of liquid 3He-4He mixtures has
been studied introducing final state effects. Corrections to the impulse
approximation have been included using a generalized Gersch-Rodriguez theory
that properly takes into account the Fermi statistics of 3He atoms. The
microscopic inputs, as the momentum distributions and the two-body density
matrices, correspond to a variational (fermi)-hypernetted chain calculation.
The agreement with experimental data obtained at \AA is not
completely satisfactory, the comparison being difficult due to inconsistencies
present in the scattering measurements. The significant differences between the
experimental determinations of the 4He condensate fraction and the 3He kinetic
energy, and the theoretical results, still remain unsolved.Comment: 18 pages, 11 figures, to appear in Phys. Rev.
Bose-Einstein Condensate in Solid Helium
We present neutron scattering measurements of the atomic momentum
distribution, n(k), in solid helium under a pressure p = 41 bars and at
temperatures between 80 mK and 500 mK. The aim is to determine whether there is
Bose-Einstein condensation (BEC) below the critical temperature, T_c = 200 mK
where a superfluid density has been observed. Assuming BEC appears as a
macroscopic occupation of the k = 0 state below T_c, we find a condensate
fraction of n_0 = (-0.10 \pm 1.20)% at T = 80 mK and n_0 = (0.08\pm0.78)% at T
= 120 mK, consistent with zero. The shape of n(k) also does not change on
crossing T_c within measurement precision.Comment: 4 pages, 5 figures (in press
- …