We present measurements of specific heat in Mn-doped compounds REBa 2 (Cu 1−x Mn x) 3 O y (RE = Y and Gd) with 0 x 2%. It is found that the transition temperature of Mn-doped YBa 2 (Cu 1−x Mn x) 3 O y (YBaCuMnO) compounds does not change appreciably. On the other hand, in the case of GdBa 2 (Cu 1−x Mn x) 3 O y (GdBaCuMnO) samples, the transition temperature decreases noticeably with the increase in Mn concentration. In REBaCuMnO, a jump in specific heat at the superconducting transition was observed for a low concentration of Mn; however, only a small change in slope was noticed for Mn concentrations above 1%. Replacing 0.5% Cu by Mn has only a marginal effect on the transition temperature, but a threefold suppression of the specific heat jump is observed with such a substitution. This finding clearly demonstrates that these constituents are being incorporated into the superconductors as a whole and not in the form of a local cluster.
Measurement of the linear thermal expansion α has been carried out for Mn substituted YBa 2 (Cu 1−x Mn x ) 3 O y (0 x 2%) using a high resolution dilatometer over the temperature range 10-300 K. Across the superconducting transition, the jump in the coefficient of linear thermal expansion α was found to decrease with increasing Mn content. For the pure sample, we observed a negative jump. While a threefold decrease in α with x = 0.5% was observed, only slight changes in the oxygen content and transition temperature have been noted, as a function of the Mn concentration. The above observations clearly suggest that the substituent Mn is being incorporated into the superconductors as a whole and not in the form of a local cluster. Further, from the Ehrenfest relations, the pressure dependence of T C (dT C /dP) and the discontinuity in the compressibility, K , are expected to decrease with the Mn concentration.
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