Phonon dispersion curves for ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$

Using a simple mechanical model we have calculated the bare phonon dispersion curves for (La,Sr${)}_2$ ${\mathrm{CuO}}_4$. Fitting the frequencies of the four Raman modes and one infrared (IR) mode, our calculation reproduces the position of the other IR modes for which experimental evidence exists as well as the shear-wave velocity. This calculation yields information on the phonons which may be important for an understanding of the superconductivity.