Analysis of line shapes observed in the scattering of thermal neutrons from superfluidHe4

Abstract

The line shapes, energies, and linewidths of elementary excitations in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ have been studied as a function of scattering vector ($q$) and temperature by neutron scattering. Near the roton minimum, a harmonic-oscillator line shape provides a significantly better representation of the observed spectra than the more often employed Lorentzian form. The parameter characterizing the energy of the excitation is found to depend strongly on the analytic form of the dynamical structure factor $S(q, \omega )$ used to analyze the data. Linewidths, on the other hand, appear to be nearly independent of both $q$ and the analytic form used to represent $S(q, \omega )$ between 0.75 and 2.0 ${\mathrm{\AA }}^{-1\mathrm{}}$.