Quasiclassical determination of the in-plane magnetic field phase diagram of superconducting Sr2RuO4 -: art. no. 144528

We have carried out a determination of the magnetic field temperature (H-T) phase diagram for realistic models of the high-field superconducting state of tetragonal Sr2RuO4 with fields oriented in the basal plane. This is done by a variational solution of the Eilenberger equations. This has been carried for spin-triplet gap functions with a d vector along the c axis (the chiral p-wave state) and with a d vector that can rotate easily in the basal plane. We find that, using gap functions that arise from a combination of nearest- and next-nearest-neighbor interactions, the upper critical field can be approximately isotropic as the field is rotated in the basal plane. For the chiral d vector, we find that this theory generically predicts an additional phase transition in the vortex state. For a narrow range of parameters, the chiral d vector gives rise to a tetracritical point in the H-T phase diagram. When this tetracritical point exists, the resulting phase diagram closely resembles the experimentally measured phase diagram for which two transitions are only observed in the high-field regime. For the freely rotating in-plane d vector, we also find that an additional phase transition exists in the vortex phase. However, this phase transition disappears as the in-plane d vector becomes weakly pinned along certain directions in the basal plane.