Metal-Nonmetal Changeover in Pyrochlore Iridates

Abstract

We report the low temperature properties of a new series of pyrochlore iridates R₂Ir₂O₇ (R=rare-earth elements). We found that the compounds with R=Pr, Nd, Sm, and Eu exhibit metallic conductivity, whereas those with smaller rare-earth ions are nonmetallic. Such metal-nonmetal changeover is attributable to the importance of electron correlation among the Ir 5d electrons. Reflecting the geometrical frustration in the pyrochlore lattice, the rare-earth moments do not exhibit magnetic ordering to temperatures well below the antiferromagnetic Weiss temperature. We did not find any sign of superconductivity down to 0.3 K in these compounds.