Pronounced drop of $^{17}$O NMR Knight shift in superconducting state of Sr$_2$RuO$_4$

TL;DR

This study uses $^{17}$O NMR to investigate the superconducting state of Sr$_2$RuO$_4$, revealing a drop in Knight shift consistent with spin polarization reduction, challenging previous models of its order parameter.

Contribution

It provides new NMR evidence of spin polarization changes in Sr$_2$RuO$_4$'s superconducting state, especially under uniaxial pressure, contradicting earlier findings.

Findings

Observed reduction in Knight shift below $T_c$ across all strains

Contradicts previous NMR studies and proposed order parameters

Suggests alternative scenarios for the superconducting pairing mechanism

Abstract

The superconducting state in the quasi-two-dimensional and strongly correlated Sr$_2$RuO$_4$ is uniquely held up as a solid state analog to superfluid $^3$He-$A$, with an odd-parity order parameter that also breaks time reversal symmetry, and for which the vector order parameter has the same direction in spin space for all electron momenta. The recent discovery that uniaxial pressure causes a steep rise and maximum in transition temperature ($T_c$) in strained samples motivated the study of $^{17}$O nuclear magnetic resonance (NMR) that we describe in this article. A reduction of Knight shifts $K$ was observed for all strain values and temperatures $T<T_c$, consistent with a drop in spin polarization in the superconducting state. In unstrained samples, our results are in contradiction with a body of previous NMR work, and with the most prominent previous proposals for the order parameter of Sr$_2$RuO$_4$. Possible alternative scenarios are discussed.