Using high-quality single crystals of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4,$ Raman scattering spectra in the (c,c) geometry have been investigated in different Sr concentrations $\mathrm{}(x=0,0.1,0.113,0,15,0.22\mathrm{})\mathrm{}$ and in the wide temperature region. It has been found that the apical oxygen vibration along the c axis shows the following anomalous behaviors in the dependences of the Sr concentration and temperature. With the increase of the Sr concentration, the peak intensity abruptly decreases from $x=0\mathrm{}$ to 0.1 and its line shape becomes asymmetric. In the temperature dependence, its energy is almost constant or slightly decreases with decreasing temperature, and also its intensity decreases above 100 K universally in the doped samples. In addition, the background intensities increase with increasing x, except for $x=0.113\mathrm{}$ at 6 K, where “1/8 problem” occurs. This increase shows that the background response is caused by an electronic excitation because the magnetic one is forbidden in the (c,c) spectra. The decrease of the background at $x=0.113\mathrm{}$ at 6 K is understood by the formation of charge density wave. The asymmetric line shapes of the apical oxygen vibration are well explained by the Fano model, which is based on the interference effect between the phonon and electronic excitations. Furthermore, the abrupt decrease of the intensity from $x=0\mathrm{}$ to 0.1 is also explained by the introduction of the electron-phonon interaction. The correlation between the obtained electron-phonon interaction and $T_c$ has been found in the Sr-concentration dependence.

• Received 6 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.4142