Low concentration of the superconducting phase and thermoelectric power in 90 K superconductors

Abstract

The thermoelectrical power (TEP) and the electrical resistivity behaviour of three 90 K superconductors viz, Y₁Ba₂Cu₃O_7−x , Sm₁Ba₂Cu₃O_7−x and Gd₁Ba₂Cu₃O_7−x , after the specimens were quenched from the sintering temperature (920°C) to 77 K, are reported. Interestingly the Y₁₂₃ specimen, which has the presence of trace amount of the orthorhombic phase in an otherwise tetragonal phase and does not show a superconducting transition down to 77 K shows zero TEP around 82 K, theT _c for the well oxygenated specimen. The Sm and Gd specimens on the other hand show completely tetragonal structure, semiconducting behaviour in resistivity and no zero TEP up to 77 K. It is argued that the critical concentration of the superconducting phase necessary to make the TEP zero is much smaller than that required for zero resistivity.