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 +/-2degrees. 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.