ENTROPY OF MIXED STATE OF SOME LOW-KAPPA TYPE-2 SUPERCONDUCTORS

The entropy of the mixed state of type-II superconductors with low κ(∼1) is studied for the alloys Pb0.96Tl0.04 and In0.88Sn0.12. The current theories are reviewed for approximations valid near Hc1 and near Hc2. The experiments show for the most reversible sample (Pb0.96Tl0.04) that (SB)T of the mixed state depends only on temperature, and that (ST)B increases linearly with B from its value for the superconducting state until it drops discontinuously to its normal state value at Hc2. There is no abrupt increase in the specific heat at constant magnetic induction near Hc1 as is observed in the specific heat at constant applied magnetic field. A method of determining the thermodynamic critical field of an irreversible sample without assuming a parabolic form is discussed and applied to the data. The thermodynamic critical field determined in this way is used to calculate κ1 and κ2. The parameters are very nearly equal for Pb0.96Tl0.04 with a temperature dependence of (1+t2)-12. Measurements of (SB)T are also reported on the type-I superconductors Sn and Pb. The conclusion, as expected, is that (SB)T in the intermediate state depends on the temperature only. © 1969 The American Physical Society.