Study of Mg-doped (Cu_(0.5)Tl_(0.5))Ba_2(Ca_(2-y)Mg_y)(Cu_(2.5)Cd_(0.5)O_(10-δ) (y = 0, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0) superconductors

摘要

(Cu_(0.5)Tl_(0.5))Ba_2(Ca_(2-y)Mg_y)(Cu_(2.5)Cd_(0.5))O_(10-δ) (y - 0.0.05,0.1,0.25,0.5,0.75,1.0) superconductors have been synthesized at normal pressure. The c-axis length (except for the samples with y = 0.05) decreases with the increased Mg-doping, showing that the distance between CuO_2/CdO_2 planes decreases with increased Mg-doping. The onset of superconductivity in the resistivity and ac-susceptibility measurements is sup pressed with the Mg-doping. T_c (R = 0) and magnitude of diamagnetism are also suppressed with the increasing Mg contents. Incorporation of Mg at Ca sites in the unit cell can be evidenced from the mod ification of the FTIR absorption spectra. The CuO_2/CdO_2 planar oxygen modes are not significantly altered, but the apical oxygen modes around 480 and 530 cm~(-1) are softened with the doping of Mg at the Ca sites. It is most likely that softening of the apical oxygen modes is induced due to better inter-plane coupling produced by Mg-doping. Due to the presence of heavier Cd atoms in the CuO_2 planes, the phonon pop ulation is suppressed. The influence of an-harmonic oscillations is further enhanced due to enhancement of inter-CuO_2/CdO_2-plane coupling which further suppresses the phonon density and hence the super-conductivity. These studies show the pivotal role of electron-phonon interactions in the mechanism of high T_c superconductivity.