Quasiparticle excitations and evidence for superconducting double transitions in monocrystalline U_(0.97)Th_(0.03)Be_(13)

摘要

Superconducting (SC) gap symmetry and magnetic response of cubic U_(0.97)Th_(0.03)Be_(13) are studied by means of high-precision heat-capacity and dc magnetization measurements using a single crystal, in order to address the long-standing question of its second phase transition at T_(c2) in the SC state below T_(c1). The absence (presence) of an anomaly at T_(c2) in the field-cooling (zero-field-cooling) magnetization indicates that this transition is between two different SC states. There is a qualitative difference in the field variation of the transition temperatures; T_(c2)(H) is isotropic, whereas T_(c1)(H) exhibits a weak anisotropy between the [001] and [111] directions. In the low-temperature phase below T_(c2)(H), the angle-resolved heat capacity C(T,H,Φ) reveals that the gap is fully opened over the Fermi surface, narrowing down the possible gap symmetry.