Magnetic field induced rotation of the d-vector in Sr₂RuO₄

The superconductor Sr2RuO4 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 Tc, 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-axis was also found to be constant below Tc, 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.