Optical studies on Bi 2Sr 2XCu 2O 8 (X=Ca,Nd) and Tl 2Ba 2Ca 2Cu 3O 10 single crystals

Abstract

The reflection spectra were measured on single crystals of Tl 2Ba 2Ca 2Cu 3O 10 and Bi 2Sr 2CaCu 2O 8 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 (Tl 2Ba 2Ca 2Cu 3O 10) and 6000 cm −1 (Bi 2Sr 2CaCu 2O 8). Four and three mid-IR exciton bands were found in Tl 2Ba 2Ca 2Cu 3O 10 and Bi 2Sr 2CaCu 2O 8, 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 Tl 2Ba 2Ca 2Cu 3O 10, Bi 2Sr 2CaCu 2O 8 and Bi 2Sr 2NdCu 2O 8 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.