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 {\it ab initio} calculations. The main effects on the optical phonon modes are related to Kondo physics. The B1g (Γ3 symmetry) phonon mode slightly softens below ∼100~K, in connection with the previously reported softening of the elastic constant, C11−C12, of the same symmetry, both observations suggesting a B1g 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, Eu(1) and A2u(2), show opposite temperature dependencies. The A2u(2) mode loses its Fano shape below 150 K, whereas the Eu(1) mode acquires it below 100~K, in the Kondo cross-over regime. This may point out to momentum-dependent Kondo physics. By inelastic neutron scattering measurements, we have drawn the full dispersion of the phonon modes between 300~K and 2~K. No remarkable temperature dependence has been obtained including through the hidden order transition. {\it 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.
@article{arxiv.1410.3101,
title = {Lattice dynamics of the heavy fermion compound URu$_2$Si$_2$},
author = {J. Buhot and M. A. Méasson and Y. Gallais and M. Cazayous and A. Sacuto and F. Bourdarot and S. Raymond and G. Lapertot and D. Aoki and L. P. Regnault and A. Ivanov and P. Piekarz and K. Parlinski and D. Legut and C. C. Homes and P. Lejay and R. P. S. M. Lobo},
journal= {arXiv preprint arXiv:1410.3101},
year = {2015}
}