Raman Scattering in Os: Nonadiabatic Renormalization of the Optical Phonon Self-Energies

The wave vector dependence of the E2g optical phonon frequencies and linewidths in the hexagonal close-packed metal, osmium, was studied at 10 K. A strong anisotropic dispersion, as well as damping thresholds, were observed for two transverse branches in the high-symmetry directions of the Brillouin zone around a phonon wave vector magnitude of 106 cm−;1. Simultaneously, a continuum was found in the Raman spectra, presumably of electronic nature, whose q-dependence correlates with that the phonon self-energies. Numerical simulations of the electronic Raman response in terms of an intraband mechanism were performed with a model energy-band spectrum reproducing the main features of the Fermi surface anisotropy of Os. A large discrepancy was found between the calculated and the measured anisotropy of the electronic scattering with respect to momentum direction and polarization configuration. The possible assignment to a contribution of resonant terms to the Raman vertex is discussed.