Upper Critical Field of the 3 Kelvin Phase in Sr2RuO4

The inhomogeneous 3 Kelvin phase is most likely a superconducting state nucleating at the interface between micrometer-sized Ru-metal inclusions and Sr2RuO4 above the bulk onset of superconductivity. This filamentary superconducting state yields a characteristic temperature dependence of the upper critical field which is sublinear, i.e., H_{c2} (T) \propto (T^* - T)^{\gamma} with 0.5 \leq \gamma < 1 (T^*: nucleation temperature). The Ginzburg-Landau theory is used to analyze the behavior of the nucleated spin-triplet phase in a field and the characteristic features of H_{c2} observed in the experiment are explained based on a two-component order parameter in the presence of a filament of enhanced superconductivity with a finite width.