Spin Polarization Enhanced by Spin-Triplet Pairing in Sr$_2$RuO$_4$ Probed by NMR

TL;DR

This study reveals that in Sr$_2$RuO$_4$, spin susceptibility slightly increases in the superconducting state, indicating equal-spin pairing, a phenomenon previously predicted for superfluid helium-3 but not observed in superconductors.

Contribution

The paper provides experimental evidence of spin susceptibility increase in Sr$_2$RuO$_4$, supporting the existence of spin-triplet equal-spin pairing in its superconducting state.

Findings

Spin susceptibility increases by about 2% below Tc.

Spin susceptibility becomes inhomogeneous in the superconducting state.

Supports the presence of equal-spin pairing in Sr$_2$RuO$_4$.

Abstract

We report a novel phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr$_2$RuO$_4$ with nuclear magnetic resonance (NMR) that the spin susceptibility originating from the Ru-4$d$ electron slightly $increases$ by $\sim 2 $\% of total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin-pairing (ESP) in the mixed state. A similar phenomenon was predicted for superfluid $^3$He forty years ago, but had never been demonstrated in any superconductor.