Low-temperature specific heat of superconductors – I. Historical perspective prior to 1986

Specific heat is a fundamental thermodynamic quantity closely related to internal energy and entropy of a system. In superconductivity research, it has been playing important roles ever since the intriguing critical phenomenon was discovered. Looking back, though, calorimetric measurements, along with the relevant low-temperature techniques, improved slowly over decades. Erroneous conclusions were reached often in the beginning, but eventually several calorimetrically-derived key features formed part of the basis in the microscopic BCS theory and its extended versions. While unable to predict whether or when a given material would become superconducting, theories helped data analysis and guided new superconductors search. Many elements, alloys and compounds joined the long list of known superconductors over the 75-year period between 1911 and 1986. Specific heat data yielded characteristic parameters, and ascertained whether an observed transition was a bulk effect. In the later years, theoretical models or experimental results also revealed a few classes of unconventional superconductors, mostly involving the interplay between superconductivity and magnetism. This article is purposely focused, as historical perspective, on how such activities progressed. Only initial findings and some related reports are cited, leaving out much more detailed and better executed work published later. For the same reason, only select figures and discussions of cited articles, but not their full extent, are included.