Heat capacity, thermal expansion and pressure derivative of critical temperature at the superconducting and charge density wave (CDW) transitions in NbSe2

The results of the study of thermodynamic properties of the anisotropic 2H-NbSe2 compound, in the superconducting to normal state (SN) upper critical field HC2–TC transition line, and in the charge density wave (CDW) transition are presented. Isothermal heat capacity and magnetization measurements were performed to determine the heat capacity jump at the HC2–TC transition line. With these results the validity of the Second Ehrenfest relation for this type II superconductor was checked. The temperature dependences of the heat capacity and the thermal expansion along the crystallographic axis с were measured. Both properties manifest the sharp features of BCS-type at the superconducting transition and more diffuse features at the CDW transitions. These results, together with the First Ehrenfest relation, allowed us to estimate the critical temperature derivative on uniaxial pressure applied along the hexagonal axis с, pIIс, (∂TSN/∂p//c)P→0=+1.45±0.05K/GPa. The same methodology has been applied to the CDW transition, where a positive derivative on uniaxial pressure was deduced.