Analytic two-fluid description of unconventional superconductivity

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 unconventional superconductors. In particular we derive interpolation formulae for the entropy, specific heat, spin susceptibility and current 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 unconventional superconductors as well as to unconventionally paired systems like the A-phase of superfluid He-3.