Lattice dynamics and the electron-phonon interaction in Ca2RuO4

We present a Raman scattering study of Ca2RuO4, in which we investigate the temperature dependence of the lattice dynamics and the electron-phonon interaction below the metal-insulator transition temperature (TMI). Raman spectra obtained in a backscattering geometry with light polarized in the ab plane reveal nine B1g phonon modes (140, 215, 265, 269, 292, 388, 459, 534, and 683cm-1) and nine Ag phonon modes (126, 192, 204, 251, 304, 322, 356, 395, and 607cm-1) for the orthorhombic crystal structure (Pbca-D2h15). With increasing temperature toward TMI, the observed phonon modes shift to lower energies and exhibit reduced spectral weights, reflecting structural changes associated with the elongation of the RuO6 octahedra. Interestingly, the phonons exhibit significant increases in linewidths and asymmetries as temperature is raised through the Néel temperature (TN). These results indicate that there is an increase in the effective number of electrons and the electron-phonon interaction strengths as the temperature is raised through TN, suggesting the presence of orbital fluctuations in the temperature regime TN<T<TMI.