Interpolation of BCS response functions

This paper considers the problem of finding simple analytical forms for the temperature dependence of thermodynamic and response functions within the weak coupling theory of clean BCS-like superconductors. In particular we derive interpolation formulae for the entropy, specific heat, and spin susceptibility of the normal component, formed by the thermal excitations (Bogoliubov quasiparticles), as well as for the superfluid density and hence the magnetic penetration depth, describing the response of the condensate. The results apply to classical superconductors as well as to unconventionally paired systems like the B-phase of superfluid 3He.