Gap Structure of the Spin-Triplet Superconductor Sr_2RuO_4 Determined from the Field-Orientation Dependence of the Specific Heat

摘要

We report the field-orientation dependent specific heat of the spin-triplet superconductor Sr_2RuO_4 under the magnetic field aligned parallel to the RuO_2 planes with high accuracy. Below about 0.3 K, striking fourfold oscillations of the density of states reflecting the superconducting gap structure have been resolved for the first time. We also obtained strong evidence of multiband superconductivity and concluded that the superconducting gap in the active band, responsible for the superconducting instability, is modulated with a minimum along the [100] direction.