Thermoelectric power studies of (Nd-Eu-Gd)Ba-Cu-O superconductors

Thermoelectric power measurements of a series of (Nd-Eu-Gd)Ba-Cu-O(123)+x% (Nd-Eu-Gd)Ba-Cu-O(211), where x ranges from 0 to 50, high temperature superconducting materials prepared by the oxygen controlled melt growth (OCMG) process, were carried out over a temperature range 80-225 K. It has been observed that thermoelectric power values are very low but positive and are found to increase almost linearly with decreasing temperature, reaching a maximum value at temperatures close to the superconducting transition temperatures of the materials. On further cooling the materials, the thermopower is found to fall suddenly to zero at their respective T_c values. It has been reported that a number of physical properties and their variation with temperature especially in the normal temperature region (above T_c) influence the behavior of HTSC materials in their superconducting state and as such an attempt has been made to understand the variation of thermopower (in the normal temperature region) with temperature. The experimental thermopower data has been fitted to different theoretical models and it has been observed that the narrow band phenomenological model alone is in a position to explain the variation of thermopower with temperature especially in the normal temperature region. The narrow band model has also been used to understand how the band spectrum changes with varying dopent's concentration. The model has also been used to explain the variation of T_c with increasing concentration of NEG-211.