Superconducting Double Transition and the Upper Critical Field Limit of Sr₂RuO₄ in Parallel Magnetic Fields

We studied the specific heat and thermal conductivity of the spin-triplet superconductor Sr₂RuO₄ at low temperatures and under oriented magnetic fields H. We resolved a double peak structure of the superconducting transition under magnetic field for the first time, which provides thermodynamic evidence for the existence of multiple superconducting phases. We also found a clear limiting of the upper critical field H _c2 for the field direction parallel to the RuO₂ plane only within ±2°. The limiting of H_c2 occurs in the same H-T domain of the second superconducting phase; we suggest that the two phenomena have the same physical origin.