Spin-Orbit Coupling and the in-Plane Anisotropy of the Upper Critical Field in Sr₂RuO₄

Abstract

The anomalous temperature dependence of the in-plane H_c2 anisotropy of Sr₂RuO₄ is discussed based on a model which includes spin-orbit coupling effects. While the basic superconducting state corresponds to a chiral p-wave state, the in-plane magnetic field generates a nonunitary state which leads to the observed anomalous anisotropy. Although this effect is connected with spin-orbit coupling and multi-band effects, it does not affect anisotropy parameters which determine the vortex lattice structure in the mixed state for fields parallel to the c-axis. In this manner the discrepancy between low-temperature in-plane anisotropy and c-axis vortex lattice orientation is resolved.