THE NUCLEAR CONTRIBUTION TO THE SPECIFIC-HEAT OF SINGLE-CRYSTALS OF YBA2CU3O7 AND BI2SR2CACU2O8

Specific heat data below 1 K of Bi2Sr2CaCu2O7 and YBa2Cu3O7 are analyz ed. For YBa2Cu3O7 the nuclear specific heat, C(N), amounts to 38 T-2 m uJ/mol K. C(N) for Bi2Sr2CaCu2O8 exceeds that of YBa2Cu3O7 by a factor of 15. The nuclear quadrupolar specific heat contribution alone is in sufficient to ''plain the data for YBa2Cu3O7, while lack of NQR data d oes not allow such a comparison in Bi2Sr2CaCu2O8 to be made. The contr ibution to C(N) from nuclear spins coupled via the contact hyperfine i nteraction with correlated magnetic spins (in the CuO2 plane) is deriv ed as a function of the correlation length. This contribution can be t reated independently from the quadrupolar term. We show that the exces s specific heat in YBa2Cu3O7 likely originates in a few percentage of an impurity (oxygen deficient) phase with a strong hyperfine field on the Cu nuclei.