Changes in the superconducting state of Sr₂RuO₄ under magnetic fields probed by specific heat

We report the specific heat of a high-quality single crystal (T_c = 1.48K) of the spin-triplet superconductor Sr₂RuO₄ under magnetic fields precisely parallel to the basal plane. The field-sweep measurements at low temperatures have revealed the existence of two distinct critical fields. We will argue that the dependence of the energy gap on different Fermi-surface sheets in combination with the in-plane anisotropy of the gap can explain such behavior. Furthermore, the temperature-sweep measurements have clearly revealed an unusual entropy release somewhat below T_c(H) only if the field is greater than 1.2T. This behavior corresponds well with the second-transition feature recently observed in the ac susceptibility measurements, and provides thermodynamic evidence for the change in the superconducting state.