Fig. 5
From: Even odder after twenty-three years: the superconducting order parameter puzzle of Sr2RuO4
Sketches illustrating the competition between spin polarisation energy and superconducting condensation energy in classic even parity superconductors with spin singlet pairing. Because a singlet Cooper pair is non-magnetic, the magnetic spin polarisation energy gain is lost when the condensate is fully formed at low temperatures. If the applied field is low (a)) this means that the spin susceptibility is quenched. However, if the applied field is sufficiently high, the spin polarisation energy gain wins out, and the superconductivity is destroyed (b)). Although these sketches are for even parity superconductors in the absence of spin-orbit coupling, and the situation in Sr2RuO4 is much more complicated, the qualitative relationship between the two measurements would naively be expected to persist. It is thus a major discrepancy that critical field limiting is clearly seen in Sr2RuO4 but is not accompanied by a decrease of the Knight shift below T c
