OPTICAL STUDIES ON BI2SR2XCU2O8 (X = CA,ND) AND TL2BA2CA2CU3O10 SINGLE-CRYSTALS

The reflection spectra were measured on single crystals of Tl2Ba2Ca2Cu3O10 and Bi2Sr2CaCu2O8 in the region of 30-40 000 cm-1. The dielectric functions were calculated by Kramers-Kronig transformation of the reflection spectra. Analysis by the simple harmonic oscillator model reveals that the plasma frequencies of the Drude band are 6400 cm-1 (Tl2Ba2Ca2Cu3O10) and 6000 cm-1 (Bi2Sr2CaCu2O8). Four and three mid-IR exciton bands were found in Tl2Ba2Ca2Cu3O10 and Bi2Sr2CaCu2O8, respectively. The reflectance edges are found at 10 800 cm-1 and 9500 cm-1 for these crystals. The frequency-dependent scattering rate shows strong interaction of free carriers with the mid-IR excitations. Seven phonon bands were observed on Tl2Ba2Ca2Cu3O10, Bi2Sr2CaCu2O8 and Bi2Sr2NdCu2O8 crystals, where the sliding modes of the CuO plane have strong intensity due to coupling with the motion of free carriers. The electronic excitation in the visible region is assigned to the transition from the deep O2p level to the hole near the Fermi level. The loss function is calculated by the dielectric function, which is in excellent agreement with EELS result.