Possibility of mixed helical p-wave pairings in Sr$_2$RuO$_4$

TL;DR

This paper explores complex mixed helical p-wave pairing states in Sr$_2$RuO$_4$, proposing they can explain key experimental signatures like Kerr effect and Knight shift behavior, which previous models struggled to account for.

Contribution

It introduces novel mixed helical p-wave pairing states, specifically $A_{1u}+iA_{2u}$ and $B_{1u}+iB_{2u}$, as potential explanations for experimental phenomena in Sr$_2$RuO$_4$.

Findings

Mixed helical p-wave states support intrinsic Hall and Kerr effects.

These states can explain the absence of spontaneous edge currents.

They account for the observed Knight shift drop.

Abstract

The exact nature of unconventional superconductivity in Sr$_2$RuO$_4$ remains a mystery. At the phenomenological level, no superconducting order parameter proposed thus far seems able to coherently account for all essential experimental signatures. Among the latter is the prominent polar Kerr effect, which implies a nonzero ac anomalous Hall conductivity. Assuming the Kerr effect is intrinsic, it can be accounted for by a bulk chiral Cooper pairing with nonzero orbital angular momentum, such as $p+ip$ or $d+id$, which, however, has difficulties in being reconciled with other experimental results. Given the situation, in this paper, we propose alternative possibilities with complex mixtures of distinct helical p-wave order parameters, namely, $A_{1u}+iA_{2u}$ and $B_{1u}+iB_{2u}$ in the group theory nomenclature. These states essentially consist of two copies of chiral p-wave pairings with opposite chirality and different pairing amplitudes, and therefore support intrinsic Hall and Kerr effects. We further show that these states exhibit salient features that may explain several other key observations in this material, including the absence of spontaneous edge current, a substantial Knight shift drop, and possibly signatures in uniaxial strain and ultrasound measurements.