Universal Parameters in the Response of Unconventional Superconductors

This paper is devoted to a unified description of a large class of clean unconventional superconductors within a generalized weak coupling BCS model,applicable to the superfluid phases of liquid 3 He, heavy Fermion and cuprate superconductors as well as the Ruddlesden Popper system Sr 2 RuO 4 . Gap nodes, leading to low energy (nodal) Bogoliubov quasiparticles do not only modify the two universal so called Muhlschlegel BCS parameters, namely the specific heat discontinuity (delta) C/C N at the transition and the gap maximum (delta) 0 (0)/k B T c at zero temperature. They give rise also to the existence ofa certain number of further universal parameters, namely the slopes or curvatures in the temperature dependence of the local response functions the low temperature limit, which vanish altogether in a conventional BCS description of isotropic s wave pairing. All these parameters are calculated analyticallyfor a large variety of model pairing states and discussed with respect to their experimental observability.