ELEMENTARY EXCITATIONS IN SUPERFLUID HE-3-HE-4 MIXTURES - PRESSURE AND TEMPERATURE-DEPENDENCE

The neutron-scattering function for dilute mixtures of He3 (1% and 5%) in superfluid He4 at pressures of 0, 10, and 18 bars and temperatures in the range from 0.07 to 1.5 K has been determined for wave vectors between 0.4 and 2.2 AÌ-1. This is the first such measurement for -43He mixtures at temperatures below the Fermi temperature. The line shape of the He3 particle-hole excitation is well described by a nearly-free-Fermi-gas model at all temperatures. The He3 quasiparticle spectrum is derived for wave vectors in the range 0.9<Q<1.7 AÌ-1. The results deviate from the quadratic Landau-Pomeranchuk spectrum and are in good agreement with thermodynamical measurements. No rotonlike minimum is observed. We find strong indications of a crossover between the He3 particle-hole band and the He4 roton excitation. The pressure and temperature dependence of the shift and broadening of the He4 phonon-roton peak is derived. The main features are reproduced in a simple model for Landau damping. © 1990 The American Physical Society.