Changes in the Superconducting State of Sr_<b>2</b>RuO_<b>4</b>under Magnetic Fields Probed by Specific Heat

Abstract

We report the specific heat of a high-quality single crystal ( T c =1.48 K) of the spin-triplet superconductor Sr 2 RuO 4 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 1.2 T. 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.