Raman scattering study of the lattice dynamic of URu₂Si₂ and sample's preparation

We report Raman scattering measurements on URu₂Si₂ single crystals as a function of temperature down to 2 K. We probe all the Raman active symmetries. Only when the sample is prepared with a surface perpendicular to a-axis, we observe an extrinsic hardening and broadening of the A_1g and B_1g phonons after polishing, which disappears after annealing. Moreover, a parasitic phase with Si-excess compared to URu₂Si₂ composition appears on the a-axis surface when annealing is at 1075 °C. No parasitic phase is induced when annealing is done at 950 °C. The temperature dependance of the A_1g, and two E _g phonons shows a hardening with decreasing temperature. The B_1g phonon mode's behavior is more unusual, its energy stays stable down to ∼ 30 K before softens at lower temperature. An electron-phonon coupling is certainly at play here.