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 (T_MI) . Raman spectra obtained in a backscattering geometry with light polarized in the ab plane reveal nine B_1g phonon modes (140, 215, 265, 269, 292, 388, 459, 534, and 683  cm⁻¹ ) and nine A_g phonon modes (126, 192, 204, 251, 304, 322, 356, 395, and 607  cm⁻¹ ) for the orthorhombic crystal structure (Pbca-D_2h¹⁵) . With increasing temperature toward T_MI , 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 (T_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 T_N , suggesting the presence of orbital fluctuations in the temperature regime T_N<T<T_MI .