Anisotropic properties of the excitation spectrum of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ crystals ${(T}_c=1.4\mathrm{}\hspace{0.5em}\mathrm{K})$ in the normal state investigated by Raman scattering

We have systematically investigated the totally symmetric Raman-scattering spectra of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ 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 ${\mathrm{RuO}}_2$ plane in the $\mathrm{}(a,a)\mathrm{}$ 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 ${\mathrm{cm}}^{-1}$ 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 ${\mathrm{RuO}}_2$ plane below $60{\mathrm{cm}}^{-1}$ 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 $T_c=0.97\mathrm{}\mathrm{K}.$ The observed magnetic excitation suggests that the magnetic correlation is also important in understanding the mechanism of superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$