Spin-orbit coupling and the in-plane anisotropy of the upper critical field in Sr2RuO4

The anomalous temperature dependence of the in-plane H-c2 anisotropy of Sr2 RuO4 is discussed based on a model which includes spin-orbit coupling effec ts. While the basic superconducting state corresponds to a chiral p-wave sl ate, the in-plane magnetic field generates a nonunitary state which leads t o 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 l ow-temperature in-plane anisotropy and c-axis vortex lattice orientation is resolved.