Effects of Sr doping on the optical properties of polycrystalline La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/

We have measured the room-temperature near-normal-incidence reflectivity of polycrystalline peller samples of La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ with x = 0, 0.05, 0.10, 0.15, and 0.20 over a wide frequency range (100--50 000 cm/sup -1/). The reflectance data have been fitted by an effective-medium approximation that takes into account the anisotropy in the dielectric functions perpendicular and parallel to the CuO2 planes. Results of our analysis concerning the optical properties of the CuO2 planes agree with reflectance data recently reported by Tajima et al. for the Eperpendicularc polarization spectrum of single-crystal La/sub 1.83/Sr/sub 0.17/CuO/sub 4-//sub 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