Magnetic field induced rotation of the d-vector in Sr_2RuO_4

TL;DR

This paper explores how a magnetic field can induce a rotation of the d-vector in Sr_2RuO_4, affecting its spin triplet pairing state and potentially reconciling experimental observations of constant spin susceptibility.

Contribution

It demonstrates that a small external magnetic field can rotate the d-vector from c-axis to the a-b plane, preserving the triplet pairing model.

Findings

Spin-orbit interaction stabilizes the c-axis d-vector.

A small magnetic field can induce d-vector rotation.

The transition affects thermodynamic properties.

Abstract

The superconductor Sr_2RuO_4 is widely believed to be a spin triplet system with a chiral order parameter analogous to the A phase of superfluid helium-3. The best evidence for this pairing state is that the Knight shift or spin susceptibility measured in neutron scattering is constant below T_c, unlike in a spin-singlet superconductor. The original Knight shift and neutron scattering measurements were performed for magnetic fields aligned in the ruthenate a-b plane. These would be consistent with a triplet d-vector d{k} aligned along the c-axis. However recently the Knight shift for fields along c was also found to be constant below T_c, which is not expected for this symmetry state. In this paper we show that while spin-orbit interaction stabilises the c-axis oriented d-vector, it is possible that only a very small external B field may be sufficient to rotate the d-vector into the a-b plane. In this case the triplet pairing model remains valid. We discuss characteristics of the transition and the prospects to detect it in thermodynamic quantities.