F–f Magnetic polaron Wigner glass and anomalous superconductivity in U 1− xTh xBe 13

Abstract

Mechanisms of the anomalous properties in the heavy fermion superconductor UBe 13 and its alloys, in particular for the Th dopings, are studied in detail based on the fundamental electronic states to be consistent with all the crucial experimental results. As the reference systems for the magnetic polaron formation, Ce monopnictides, as well as USb and UTe, are mentioned. From detailed systematic studies of the dilute alloy systems, it is postulated that the 5f states in UBe 13 split into the well-localized core 5fΓ 2 7 singlet state and other delocalized 5f states situated around the Fermi energy forming the f–f magnetic polarons through the strong intra-atomic ferromagnetic f–f exchange interaction. The accompanied lattice polarons are also shown to play important roles. In the p–d band states, the f–f exchange interaction and the intersite p–f mixing interactions for the p–f Kondo state are of nearly equal strengths causing a rich variety of delicately balanced states. For the p–f Kondo state, the polarization of the core Γ 2 7 state through the f–f exchange interaction causes novel Kondo effect different from that in Ce compounds. At higher temperatures above 50 K, the conventional Kondo singlet through the p–f mixing dominates. Below 5 K, the f–f magnetic polarons form amorphous types of Wigner crystallization, or Wigner glass, and the p–d band states are strongly scattered forming pseudolocalized states, in particular below the Fermi energy. The complicated unusual anomalies in UBe 13 and its alloys originate fundamentally in the above delicate balance. The anomalous superconducting state was identified as the s-wave pairing through the strong charge fluctuation of the Wigner glass and occurs in competition with the magnetic polaron and the pseudolocalized states with a strongly reduced T c due to the large resistivity at T c causing a double peak structure of T c as a function of Th doping, as well as unusual characteristics of the temperature dependence of H c2. In general, the Wigner glass state persists even in the superconducting state staying in the normal state causing unusual bulk superconducting state.