Anisotropic properties of the excitation spectrum of Sr₂RuO₄ crystals (T_c = 1.4K) in the normal state investigated by Raman scattering

We have systematically investigated the totally symmetric Raman-scattering spectra of Sr₂RuO₄ crystals in the temperature region between room temperature and 6 K. The electron-phonon interaction for the apical oxygen vibration has been experimentally investigated by a line-shape analysis based on the Fano interference model. It has been found that the electron-phonon interaction strongly depends on not only the polarization geometry but also the phonon-propagating direction. The phonon, which travels along the RuO₂ plane in the (a, a) geometry, shows the strongest electron-phonon interaction. The intensity of the broad background due to the magnetic excitation has been suppressed in the energy region less than 300 cm^-1 at low temperature. Furthermore, the anisotropic formation of the spin gap has been also observed in excitation spectra with the propagation vector parallel to the RuO₂ plane below 60cm^-1 at 7 K. These anisotropic properties have been obtained for a crystal with a high superconductivity transition temperature of 1.4 K. The sample quality is very important, since such an anisotropic property has not been observed in the Raman spectra for samples with T_c=0.97K. The observed magnetic excitation suggests that the magnetic correlation is also important in understanding the mechanism of superconductivity of Sr₂RuO₄.