Lattice dynamics of the heavy-fermion compound URu2Si2

We report a comprehensive investigation of the lattice dynamics of URu2Si2 as a function of temperature using Raman scattering, optical conductivity, and inelastic neutron scattering measurements as well as theoretical ab initio calculations. The main effects on the optical phonon modes are related to Kondo physics. The B-1g (Gamma(3) symmetry) phonon mode slightly softens below similar to 100 K, in connection with the previously reported softening of the elastic constant, C-11-C-12, of the same symmetry, both observations suggesting a B-1g symmetry-breaking instability in the Kondo regime. Through optical conductivity, we detect clear signatures of strong electron-phonon coupling, with temperature-dependent spectral weight and Fano line shape of some phonon modes. Surprisingly, the line shapes of two phonon modes, E-u(1) and A(2u)(2), show opposite temperature dependencies. The A(2u)(2) mode loses its Fano shape below 150 K, whereas the E-u(1) mode acquires it below 100 K, in the Kondo crossover regime. This may point to momentum-dependent Kondo physics. By inelastic neutron-scattering measurements we have drawn the full dispersion of the phonon modes between 300 and 2 K. No remarkable temperature dependence has been obtained, including through the hidden order transition. Ab initio calculations with the spin-orbit coupling are in good agreement with the data except for a few low-energy branches with propagation in the (a, b) plane.