Lattice Instability of High-Transition-Temperature Superconductors. II. Single-CrystalV3Si Results

Sound-velocity measurements in ${\mathrm{V}}_3$Si have shown that the modulus (${\mathrm{C}}_{11}$-${\mathrm{C}}_{12}$)/2 for [110] shear waves with [$1\overline{1}0$] polarization falls from a value at room temperature typical of the metallic state to near-zero magnitude in the superconducting state, while the bulk modulus (${\mathrm{C}}_{11}$+2${\mathrm{C}}_{12}$)/3 is constant to 0.5% in the same temperature interval. The attenuation of the soft shear mode increases markedly on cooling. A large increase in attenuation for all modes occurs with the structural transformation at 21°K reported by Batterman and Barrett. The transformation, triggered by the small shear stiffness, is arrested with the onset of the superconducting state. The distortions of the structural transformation are found to be comparable in magnitude with the thermal strains resulting from the soft shear mode. The behavior of the high-frequency phonons involved in the superconductivity is not known, although the specific-heat and electrical-resistivity data indicate that they may be stiffer than the near-liquidlike low-frequency phonons. The lattice instability may result from a very strain-sensitive structure in the density-of-states distribution near the Fermi level. The unusual elastic behavior reported here for ${\mathrm{V}}_3$Si is probably characteristic of most high-temperature superconductors.