  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, Sm, Nd, 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 have not found any sign of
superconductivity down to 0.3 K in these compounds.
