Evidence for even parity unconventional superconductivity in Sr₂RuO₄

Unambiguous identification of the superconducting order parameter symmetry in Sr₂RuO₄ has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid ³He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent ¹⁷O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of < 10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates.