Strongly correlated electrons in cerium- and ytterbium-intermetallic compounds: The superconducting mixed state of cerium ruthenide and intermediate valence in ytterbium iron antimony

Two investigations of superconductivity and magnetism in novel materials at low temperatures are presented. A common theme running throughout this work is that of strong electronic correlations that can arise in compounds containing elements with unstable f-electron shells, e.g., the lanthanides such as Ce, Pr, Sm, Eu, Tm, and Yb.; The first project concerns the ongoing study of the low-Tc superconductor CeRu2 (Tc ≈ 6.1 K), and reports our investigation of the magnetic field---temperature phase diagram in attempts to understand the nature of the superconductivity. A feature was observed in the superconducting mixed state resembling the "peak effect" magnetic flux line pinning anomaly seen in some low-Tc superconductors. It was proposed, however, that this feature reflected the formation of an exotic magnetic superconducting phase known as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, and our experiments reported herein attempted to elucidate the anomalous behavior. Our results indicate that the dynamics of the flux line lattice in CeRu2, while extremely complex and varied, mirrors that observed in several other low-Tc compounds which are known to exhibit a "conventional" peak effect in the mixed state. Thus, in our view, CeRu2 is not a likely candidate for the formation of the FFLO state.; The second study focuses on an intermetallic compound containing ytterbium, which is at the opposite end of the lanthanide series from cerium, and is often considered the "4f hole" analog to cerium which has at most one 4f electron. The compound YbFe 4Sb12 was synthesized in our laboratory, and a suite of low temperature thermal, magnetic, optical, and transport measurements indicate that the Yb ion exhibits intermediate valence with a coherence temperature of T* ∼ 70 K. In addition, we have made measurements on this compound as well as related chemically doped skutterudites in order to assess their merit as thermoelectric materials, a subject which has drawn considerable attention in the condensed matter community in the past few years.