Neutron studies of liquid and solid helium
Abstract
Liquid and solid helium are prototype simple liquids and solids displaying a wide spectrum of quantum properties. Understanding gained in these simple systems can be readily applied to more complicated materials. Neutron scattering studies carried out at DOE supported national facilities and the associated development of theory and models continue to reveal new physics in these fascinating materials. During the past year we have measured the dynamic form factor, S(Q,omega), in superfluid and normal liquid He4 accurately as a function of temperature. This shows that the sharp onephonon excitation, which exists in the superfluid phase below T sub lambda, either disappears at or changes entirely in character at the transition to the normal phase. Thus the sharp onephonon excitation can be associated with superfluidity (or a condensate). However, we find the weight of scattering in the onephonon peak, Z(Q,T), is not porportional to the superfluid density, rho sub S (T). We have also measured S(Q,omega) in the range 3 less than or equal to Q less than or equal to 15 angstrom sup minus 1 in order to obtain the width W(Q) and peak position, E(Q), of S(Q,omega) as a function of Q. We have made microscopic calculations of S(Q,omega) in the range 3 less than or equal to Q less than or equal to 15 angstrom sup minus 1 in both liquid He3 and He4. The aim is a first principles evaluation of W(Q) and E(Q) as a function of Q for comparsion with experiments.
 Publication:

Progress Report
 Pub Date:
 1988
 Bibcode:
 1988dela.rept.....G
 Keywords:

 Form Factors;
 Liquid Helium;
 Neutron Scattering;
 Phase Transformations;
 Superfluidity;
 Liquid Surfaces;
 Momentum Transfer;
 Neutron Diffraction;
 Neutrons;
 Phonons;
 Solids;
 Spin;
 Temperature Dependence;
 SolidState Physics