Phonon density of states in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$

Specific-heat and inelastic neutron scattering studies of the high-$T_c$ superconducting compound ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ are reported. A distinct anomaly of electronic origin in the specific heat is found at the superconducting transition temperature. The phonon density of states up to ∼20 meV is extracted from both experiments and the two sets of results agree well in yielding a sharply enhanced density of states at 10 meV, in accord with a recent theoretical study.