Evidence for even parity unconventional superconductivity in Sr$_2$RuO$_4$

TL;DR

This study provides evidence that the superconducting state in Sr$_2$RuO$_4$ is likely even parity, challenging previous assumptions of odd-parity pairing, based on detailed Knight shift and specific heat measurements.

Contribution

The paper presents combined Knight shift and specific heat data that strongly suggest even parity superconductivity in Sr$_2$RuO$_4$, excluding odd-parity scenarios.

Findings

Knight shift data consistent with even parity pairing

Upper bound on magnetic response excludes odd-parity states

Field-induced quasiparticle response explains shift results

Abstract

Unambiguous identification of the superconducting order parameter symmetry of Sr$_2$RuO$_4$ has remained elusive for more than a quarter century. While a chiral $p$-wave ground state analogue to superfluid $^3$He-$A$ was ruled out only very recently, other proposed $p$-wave scenarios are still viable. Here, field-dependent $^{17}$O Knight shift measurements are compared to corresponding specific heat measurements, previously reported. We conclude that the shift results can be accounted for by the expected field-induced quasiparticle response only. An upper bound for the condensate magnetic response of $<10\%$ of the normal state susceptibility is sufficient to exclude odd-parity candidates.