Specific-heat measurements on superconducting bi-ca-sr-cu and tl-ca-ba-cu oxides: absence of a linear term in the specific heat of bi-ca-sr-cu oxides

Specific-heat data, extending from 0.4 to 120 K, in both a zero field and at 7 T, are reported for five samples of Bi-Ca-Sr-Cu oxide and one of Tl-Ca-Ba-Cu oxide. The occurrence of bulk superconductivity in all six samples is shown by Meissner-effect measurements, and confirmed for the Tl sample and two Bi samples by the specific-heat data. In a zero field, the Tl sample shows a contribution to the specific heat that is linear in temperature and comparable to those reported for YBa₂{Cu}₃O₇. For all samples of the Bi compound the linear term is zero within the experimental uncertainty, and is at least an order of magnitude smaller.