Experimental studies of the excitation spectrum of³He-⁴He mixtures using neutron inelastic scattering

Abstract

Neutron inelastic scattering techniques have been used to study the excitation spectrum of mixtures containing 6%, 12%, and 25% of³He in⁴He at 0.75 K. The 6% mixture has also been investigated at temperatures of 0.6, 1.0, and 1.5 K. The measurements have been made in the wave-vector transfer rangeQ=0.8−2.3 Å⁻¹. The energy spectra observed contain two principal features, which are interpreted as scattering from (a) phonon-roton excitations arising from the⁴He, and (b) quasiparticle-quasihole excitations arising from the³He. In the dilute³He-⁴He mixture, our results indicate a³He particle-hole continuum which deviates from the quadratic form, proposed by Landau and Pomeranchuk, and which lies below the⁴He phonon-roton dispersion curve. However, we are unable to say whether or not the continuum exhibits a minimum as a function of energy, as has recently been proposed by several authors, because we were unable to resolve the scattering from the particle-hole continuum forQ⪝1.7 Å⁻¹. In this region ofQ space the energy spectra are dominated by the roton excitations. We have also determined the shifts in energy and increases in linewidth of the phonon-roton excitations relative to those of pure⁴He and compared these measurements to several theoretical predictions.