Metallic and non-metallic duality in the 4<i>d</i> pyrochlore Lu₂Rh₂O₇

The 4d transition metal oxides exist in an intriguing regime where on-site Coulomb repulsion, the band width, and spin-orbit coupling can all have comparable magnitudes. While spin-orbit coupling for these 4d systems is reduced as compared to their 5d counterparts, it is still strong enough to generate exotic magnetic and electronic states. In this talk, we introduce Lu₂Rh₂O₇, a new cubic pyrochlore oxide based on 4d⁵ Rh⁴⁺ with properties that defy simple categorization. Both magnetic susceptibility (χ_P = 7.6×10⁻⁴ emu/mol_Rh) and heat capacity measurements (γ = 21.8(1) mJ/mol_RhK²) indicate that this material is a strongly correlated metal and yet electrical resistivity measurements show decidedly non-metallic behavior. Furthermore, although the magnitude of the resistivity is that of a semiconductor (1 Ω-cm), the temperature dependence does not obey any conventional form. The magnetic properties of this material are also enigmatic; if this material is non-metallic, then one would expect 4d⁵ Rh to carry an S = ½ magnetic moment and yet magnetic susceptibility and muon spin relaxation measurements show no evidence for local moments down to 2 K. We therefore propose that Lu₂Rh₂O₇ may be the prototype for an entirely new class of non-Fermi liquids.