Distinct reduction of Knight shift in superconducting state of Sr$_2$RuO$_4$ under uniaxial strain

TL;DR

This paper investigates how uniaxial strain affects the Knight shift in Sr$_2$RuO$_4$, revealing that inter-orbital spin-triplet pairing with a rotating d-vector explains the observed reduction in spin polarization, distinguishing it from singlet pairing.

Contribution

It introduces a model where uniaxial strain causes a rotation of the d-vector in spin-triplet pairing, accounting for the strain-dependent Knight shift behavior.

Findings

Knight shift drops at T_c under strain

d-vector rotation causes anisotropic spin polarization

Strain effects differentiate triplet from singlet pairing

Abstract

Shortly after the discovery of superconductivity in Sr$_2$RuO$_4$, spin-triplet pairing was proposed and further corroborated by a constant Knight shift ($K$) across the transition temperature (T$_c$). However, a recent experiment observed a drop in $K$ at T$_c$ which becomes larger under uniaxial strain, ruling out several spin-triplet scenarios. Here we show that inter-orbital spin-triplet pairing features a d-vector that rotates when uniaxial strain is applied, leading to a larger drop in the spin polarization perpendicular to the strain direction, distinct from spin-singlet pairing. We propose that anisotropic spin polarization under strain will ultimately differentiate triplet vs.~singlet pairing.