2000 年 69 巻 2 号 p. 572-578
We report the specific heat of a high-quality single crystal (Tc=1.48 K) 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 Tc(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.
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