Neutron spectroscopy of low-frequency phonons in solid Hg

Abstract

Extensive measurements of the phonon dispersion curves of trigonal mercury have been carried out by neutron inelastic scattering. Some transverse branches of the phonon spectrum have very low frequencies, as small as 0.9 meV at the zone boundary, compared with a maximum lattice frequency of 15 meV. The data have been accurately fitted with an 8-neighbor Born-von Kármán model, and a reliable phonon density of states F(ω) has been calculated. Comparison with α²(ω)F(ω) derived from tunneling measurements reveals a substantial increase in α²(ω) at low frequencies. Direct calculation of α²(ω)F(ω) from the BvK model, assuming a simple form for the electron-phonon matrix element, gives λ = 2.0 compared with λ = 1.6 from tunneling. We discuss the origin of the frequency dependence of α2 (ω).