Effect of Mn doping on the specific heat of the high T_C superconductor REBa₂Cu₃O_y (RE = Y,Gd)

We present measurements of specific heat in Mn-doped compounds REBa ₂(Cu_1-xMn_x)₃O_y (RE ≤ Y and Gd) with 0≤x≤2%. It is found that the transition temperature of Mn-doped YBa₂(Cu_1-xMn_x)₃O _y (YBaCuMnO) compounds does not change appreciably. On the other hand, in the case of GdBa₂(Cu_1-xMn_x) ₃O_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 three-fold 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.