Nature of the specific heat anomaly in the Bi and Tl high temperature superconductors

Abstract Specific heat, field cooling Meissner effect, isothermal magnetization, AC susceptibility and resistance were measured on sintered pellets of the Bi and Tl high temperature superconductors with nominal compositions BiPb 0.3 SrCaCu 2 O 5.8 and Tl 2 Ba 2 CaCu 2 O 8 , and for comparison on some pellets of the REBa 2 Cu 3 O 6.9 system. With decreasing field, the field cooling Meissner signals of the Bi and Tl systems approach the same magnitude and transition width (Δ T ≈ 2–3 K) as in high quality pellets of the REBa 2 Cu 3 O 6.9 . The field and temperature dependence of the Meissner signals is analysed in detail. At the transition temperatures of five superconducting phases of the Bi and Tl pellets, the specific heat shows no jumps nor maxima, within the accuracy of the measurement (Δ( C/T ) 2 )). There are however sharp breaks of the slope of C/T at the T c values, which are intrinsically as large as in the REBa 2 Cu 3 O 6.9 . The transitions in the Bi and Tl systems look more like third order transitions rather than like the second order transitions of the REBa 2 Cu 3 O 6.9 .