Phenomenological Theory of the 3 Kelvin Phase in Sr2RuO4

We model the 3 K-phase of Sr 2 RuO 4 with Ru-metal inclusion as interface state with locally enhanced transition temperatures. The resulting 3 K-phase must have a different pairing symmetry than the bulk phase of Sr 2 RuO 4 , because the symmetry at the interface is lower than in the bulk. It is invariant under time reversal and a second transition, in general, above the onset of bulk superconductivity is expected where time reversal symmetry is broken. The nucleation of the 3 K-phase exhibits a capillary effect which can lead to frustration phenomena for the superconducting states on different Ru-inclusions. Furthermore, the phase structure of the pair wave function gives rise to zero-energy quasiparticle states which would be visible in quasiparticle tunneling spectra. Additional characteristic properties are associated with the upper critical field H c 2 . The 3 K-phase has a weaker anisotropy of H c 2 between the inplane and z -axis orientation than the bulk superconducting phase. This is connected wi...