Systematics of c-axis phonons in the thallium- and bismuth-based cuprate superconductors

We present grazing incidence reflectivity measurements in the far-infrared region at temperatures above and below T-c for a series of thallium- (Tl2Ba 2CuO6, Tl2Ba2CaCu2O8) and bismuth- (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and Bi2-xPbx Sr2CaCu2O8) based cuprate superconductors. From the spectra, which are domi nated by the c-axis phonons, longitudinal optical (LO) frequencies are dire ctly obtained. The reflectivity curves are well fitted by a series of Loren tz oscillators. In this way the transverse optical (TO) phonon frequencies were accurately determined. On the basis of the comparative study of the Bi - and Tl-based cuprates with a different number of CuO2 layers per unit cel l, we suggest modifications of the assignment of the main oxygen modes. We compare the LO frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained fr om intrinsic Josephson junction characteristics with our measurements, and explain the discrepancy in LO frequencies obtained by the two different met hods. [S0163-1829(99)01938-3].