Lattice dynamics and the electron-phonon interaction in Ca${}_2$RuO${}_4$

Abstract

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}${}_{\mathrm{MI}}$). 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}${}_{\mathrm{MI}}$, 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}${}_{\mathrm{N}}$. 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}${}_{\mathrm{N}}$, suggesting the presence of orbital fluctuations in the temperature regime {\it T}${}_{\mathrm{N}}$ $<$ {\it T} $<$ {\it T}${}_{\mathrm{MI}}$.

6 pages, 4 figures