We report high-resolution inelastic x-ray scattering measurements of the acoustic phonons of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. Our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by a change in phonon dispersions or lifetimes. Hence, the phonon contribution to the thermal conductivity is likely much less significant compared to that of the magnetic excitations in the low temperature phase.

• Received 20 July 2015

DOI:https://doi.org/10.1103/PhysRevB.93.075123