Search

We report high-resolution neutron Compton scattering measurements of liquid $^4$He under saturated vapor pressure. There is excellent agreement between the observed scattering and ab initio predictions of its lineshape. Quantum Monte Carlo calculations predict that the Bose condensate fraction is zero in the normal fluid, builds up rapidly just below the superfluid transition temperature, and reaches a value of approximately $7.5\%$ below 1 K. We also used model fit functions to obtain from the scattering data empirical estimates for the average atomic kinetic energy and Bose condensate fraction. These quantities are also in excellent agreement with ab initio calculations. The convergence between the scattering data and Quantum Monte Carlo calculations is strong evidence for a Bose broken symmetry in superfluid $^4$He.

Solid molecular oxygen presents an interesting example of a low-temperature crystal which exists within several different magnetic phases. When solid solutions of argon and oxygen are formed with molar concentrations of oxygen between 60 and 80 %, a new structural and magnetic phase, known as the $$\delta $$ -phase, appears at low temperatures. In order to investigate the nature of the $$\delta $$ -phase, we carried out SQUID magnetometry measurements solid argon-oxygen solutions made up of 74 % oxygen and 26 % argon. In particular, we performed measurements of the magnetic susceptibility of the solid solutions over complete temperature cycles and isothermally as a function of time. Taken together, the experimental data demonstrate that that the $$\delta $$ -phase is not an equilibrium thermodynamic state of the solid solutions, but is instead only a metastable state.

The properties of molecular hydrogen adsorbed in Britesorb were studied through inelastic neutron scattering. We have measured both the rotational energy levels and the momentum distribution at bilayer and nearly full pore fillings. Splitting of the J=1 rotational energy levels is observed for molecular hydrogen adsorbed on the surface, while the rotational properties of the hydrogen adsorbed after monolayer completion is consistent with behavior in the bulk. Additionally, the measurement of the momentum distribution showed that the mean kinetic energy of the molecules in the bilayer is 88 K±7 K. The kinetic energy measured in the nearly full pore was 81 K±6 K, which is consistent with a simple model in which the behavior of the monolayer is dominated by the interaction with the surface of the pore wall but H2 adsorbed after monolayer completion is bulk like.

High-resolution quasi-elastic neutron scattering measurements have been performed on molecular hydrogen in zeolite 13X. Previous NMR measurements suggested that the freezing temperature is suppressed from 14 K down to 8 K. In contrast, previous intermediate resolution quasi-elastic neutron scattering studies suggested freezing occurred between 25 and 35 K. Unfortunately, the limited instrumental resolution available in the previous quasi-elastic neutron scattering study was not sufficient to show this point definitively. We report new quasi-elastic neutron scattering measurements with very high resolution that show no evidence of mobile hydrogen below 25 K, which is well above the bulk liquid-solid transition temperature for hydrogen. A quasi-elastic component appears between 25 and 30 K indicating the presence of mobile H2. However, the width and momentum dependence of the quasi-elastic scattering are much different than would be expected for the diffusive motion of liquid hydrogen in this temperature range. Instead, we find that a slow diffusive component representing jumps between well-defined sites appears first at low temperatures. As the temperature is raised, a faster liquid like diffusive component appears.

New measurements of the roton excitations in superfluid 4He have been made using the IRIS spectrometer (energy resolution ∼20 μeV at the roton energy) at the ISIS spallation neutron source. The roton energy was determined over a temperature range of 0.56 to 1.61 K. High resolution (energy resolution

The diffusion of molecular hydrogen adsorbed in zeolite 13X at high coverage`s has been studied by quasi-elastic neutron scattering for temperatures ranging from 0 to 60 K. No diffusive motion was observed, within instrumental resolution, at temperatures below 20 K, well above the bulk melting point of H{sub 2}, in contrast to recent NMR studies. The diffusive motion of the adsorbed H{sub 2} could be described by a liquid like jump diffusion model above 35 K. The diffusion coefficient demonstrates an Arrhenius behavior: D = D{sub 0} exp(E/kT), with D{sub 0} = 1.2 {times} 10{sup {minus}8} m{sup 2}/s and an activation energy of E = 62 K. The inelastic scattering spectrum has also been studied. The scattering consisted of several peaks superimposed on a broad background extending from 1 meV to above 100 meV and is consistent with rotational transitions for a hindered rotor in a strong orientational potential with a broad distribution of barrier heights.

The orientational properties of n-H2confined in Vycor have been studied via inelastic neutron spectroscopy. We observe two distinct rotational transitions which we ascribe to H2in the center of the pore and H2strongly bound to the surface. The molecules on the surface can be modeled as rotationally hindered rotors, and a distribution of orientational potentials is extracted.

Inelastic neutron scattering has been used to probe the collective excitations of superfluid helium confined to silica aerogel. Confinement in porous aerogel results in a crossover behavior in the temperature dependence of the roton where the energy is relatively temperature independent below 1.9 K and strongly temperature dependent above 1.9 K. As in the bulk liquid, the relationship between the microscopic excitation spectrum and the macroscopic thermodynamics of the confined system are found to be in good agreement.

The elastic, quasi-elastic, and deep inelastic scattering of para-hydrogen in porous vycor glass have been measured in both the solid and liquid phase. At temperatures above 12 K, the H2 in the pores can be described as a bulk-like liquid with a single tightly bound layer at the pore surface. At temperatures below 8 K, the hydrogen in the pores is solidified into a crystalline structure different from that of the bulk. No indication of migration of the hydrogen out of the pores as recently proposed to explain torsional oscillator measurements, is observed. The momentum distribution of the hydrogen in the pores at both low and high temperatures is Gaussian and shows no indication of a Base condensate peak.

Neutron diffraction studies of normal deuterium in porous vycor glass have been carried out at temperatures in the range 4 to 20 K as a function of pore filling. For pore fillings, f, above f = 0.6 we find that the D2 in the pores forms a crystalline solid with a large amorphous component. The appearance of the crystalline phase is characterized by supercooling and hysteresis of the liquid-solid transition in agreement with previous thermodynamic studies. The crystalline phase has a structure which is different from the bulk and exhibits a large correlation length. Pore fillings below f = 0.6 show no sign of crystalline ordering, although enhanced short range ordering is observed.

Deep inelastic neutron scattering measurements on liquid4He have been carried out for temperatures from 0.35 K to 4.2 K and densities from 0.125 to 0.200g/cm3 at a momentum transfer of 23 A−1. These measurements are at large enough momentum transfer that deviations from the Impulse Approximation are accurately described by current theories and information on the single particle momentum distribution may be extracted from the measured scattering. The scattering exhibits non-Gausian behavior in both the normal liquid and superfluid phases. A distinct change in the scattering, marked by a reduction in the width and increased deviations from the classical Gaussian shape, occurs at the suerfluid transition. We present a comparison of our experimental results with recent calculations at a variety of temperatures and densities and show that theory and experiment are in excellent agreement. We also present model scattering functions, obtained by correcting for instrumental resolution and final state effects, that represent the scattering in the IA limit. Finally, we present values for the average kinetic energy and the Bose condensate fraction over a broad range densities and temperatures.

Deep inelastic neutron scattering (DINS) measurements of molecular hydrogen adsorbed in porous vycor glass at 18 K have been carried out for pore fillings ranging from sub-monolayer coverage to nearly full pore. DINS provides direct information on the momentum distribution of hydrogen in the vycor that, due to the light mass of the hydrogen molecule, is determined by the local environment of the molecule. The observed scattering is consistent with the impulse approximation (IA) using a model in which the hydrogen molecules behave as free rotors that are decoupled from the translational motion. The translational kinetic energy is 95 K for monolayer coverage, reflecting the binding of the molecules to the surface, and decreases, as the pore filling is increased, to 73 K for the nearly full pore case.

We report highQ inelastic neutron scattering studies of liquid4He confined inside porous aerogel glass in both the normal and superfluid phases. The scattering consists of a broad component, which we interpret as due to atoms localized on the surface of the glass, and a narrow component, which we interpret as due to the confined liquid inside the pores. The broad component is quite small, and no quantitative analysis is attempted. However, the narrow component due to the confined liquid is sufficiently large that information on the kinetic energy and condensate fraction may be obtained. The measurements are at large enoughQ (23 A−1) that the incoherent approximation may be used to directly extract the kinetic energy. Furthermore, the observed scattering is consistent with the impulse approximation, allowing information on both the momentum distribution and the condensate fraction to be extracted. The kinetic energy of the confined liquid is close to previous measurements of the kinetic energy in the bulk liquid. The condensate fraction of the confined liquid, which is estimated using a modification of a method due to Sears, is close to previous estimates of the condensate fraction in the bulk liquid.

Hydrogen atoms have been produced by RF discharge at the surface of an H2 crystal and studied by ESR techniques. The mean concentration at 4.2 K was estimated to be 1015 atoms per cm3 for about 14 h of source pulsing. This atom concentration was shown to be stable for long times. The results suggest that diffusion of atoms into the interior of the solid takes place very rapidly.