We present a Raman scattering study of Ca$_2$RuO$_4$, in which we investigate the temperature-dependence of the lattice dynamics and the electron-phonon interaction below the metal-insulator transition temperature ({ \it T}$_{ \rm MI}$). Raman spectra obtained in a backscattering geometry with light polarized in the ab-plane reveal 9 B$_{ 1g}$ phonon modes (140, 215, 265, 269, 292, 388, 459, 534, and 683 cm$^{ -1}$) and 9 A$_g$ phonon modes (126, 192, 204, 251, 304, 322, 356, 395, and 607 cm$^{ -1}$) for the orthorhombic crystal structure (Pbca$-$D$_{ 2h}^{ 15}$). With increasing temperature toward { \it T}$_{ \rm MI}$, the observed phonon modes shift to lower energies and exhibit reduced spectral weights, reflecting structural changes associated with the elongation of the RuO$_6$ octahedra. Interestingly, the phonons exhibit significant increases in linewidths and asymmetries for { \it T} $>$ { \it T}$_{ \rm N}$. 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 { \it T}$_{ \rm N}$, suggesting the presence of orbital fluctuations in the temperature regime { \it T}$_{ \rm N}$ $<$ { \it T} $<$ { \it T}$_{ \rm MI}$.
Categories: cond-mat.str-el