Effects of sr doping on the optical properties of polycrystallineLa2−xSrxCuO4−y

We have measured the room-temperature near-normal-incidence reflectivity of polycrystalline pellet samples of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ with $x=0, 0.05, 0.10, 0.15, \mathrm{and} 0.20$ over a wide frequency range (100-50 000 ${\mathrm{cm}}^{-1\mathrm{}}$). The reflectance data have been fitted by an effective-medium approximation that takes into accout the anisotropy in the dielectric functions perpendicular and parallel to the Cu${\mathrm{O}}_2$ planes. Results of our analysis concerning the optical properties of the Cu${\mathrm{O}}_2$ planes agree with reflectance data recently reported by Tajima et al. for the $\mathbf{E}\mathbf{\perp }\hat{c}$ polarization spectrum of single-crystal ${\mathrm{La}}_{1.83}{\mathrm{Sr}}_{0.17}\mathrm{Cu}{\mathrm{O}}_{4-y}$. In addition to the frequencies and symmetries of the ir-active phonons, the plasmons of the system have been studied systematically as a function of Sr concentration.