We study collective phonon excitations in ${\mathrm{SrTiO}}_3$ by low-frequency light scattering. We employ extended thermodynamics for phonon gas to construct a theoretical spectral function that is applicable regardless of local thermal equilibrium. Our analysis reveals the temperature dependence of ${\tau }_N$, the relaxation time for the momentum-conserving phonon collisions (normal processes), in ${\mathrm{SrTiO}}_3$. These results indicate that the previously reported anomalous soundlike spectrum originates from second sound, which is a wavelike propagation of heat.

• Received 6 August 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.265502