Phonon dispersion curves of nonstoichiometricNb3Ge

Abstract

The phonon dispersion curves of nonstoichiometric ${\mathrm{Nb}}_3$Ge ($T_c$=6 K) have been investigated by inelastic neutron scattering. Despite the small size of the sample (0.1 ${\mathrm{cm}}^3$) most of the branches along the main symmetry directions, including the highest-energy modes, have been determined. No drastic anomalies were observed and temperature effects were found to be small. The results can be well described by a Born–von Kármán model with axially symmetric forces up to the seventh-nearest neighbors. In a search for electron-phonon coupling effects our results for ${\mathrm{Nb}}_3$(${\mathrm{Ge}}_{0.8}$ ${\mathrm{Nb}}_{0.2}$) are compared to previous results obtained on the low-$T_c$ compound ${\mathrm{Nb}}_{\mathrm{c}}$Sb ($T_c$≃0.2 K). The major differences between the lattice dynamics of the two compounds were found to be (i) a reduction of the Nb-Ge interaction by 35% compared to the Nb-Sb interaction and (ii) the low frequency of modes in ${\mathrm{Nb}}_3$(${\mathrm{Ge}}_{0.8}$ ${\mathrm{Nb}}_{0.2}$) which involve a ‘‘buckling’’ of the Nb chains.