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.Comment: 4 pages, 5 figures, and submitted to J. Phys. Soc. Jp

O $1s$ x-ray absorption study of the Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ shows that the orbital population of the $4d$ $t_{2g}$ band dramatically changes with temperature. In addition, spin-resolved circularly polarized photoemission study of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ shows that a substantial orbital angular momentum is induced in the Ru $4d$ $t_{2g}$ band. Based on the experimental results and model Hartree-Fock calculations, we argue that the cooperation between the strong spin-orbit coupling in the Ru $4d$ $t_{2g}$ band and the small distortion of the ${\mathrm{RuO}}_6$ octahedra causes the interesting changeover of the spin and orbital anisotropy as a function of temperature.

Investigations of perovskite oxides triggered by the discovery of high-temperature and unconventional superconductors have had crucial roles in stimulating and guiding the development of modern condensed-matter physics. Antiperovskite oxides are charge-inverted counterpart materials to perovskite oxides, with unusual negative ionic states of a constituent metal. No superconductivity was reported among the antiperovskite oxides so far. Here we present the first superconducting antiperovskite oxide Sr_3−xSnO with the transition temperature of around 5 K. Sr₃SnO possesses Dirac points in its electronic structure, and we propose from theoretical analysis a possibility of a topological odd-parity superconductivity analogous to the superfluid ³He-B in moderately hole-doped Sr_3−xSnO. We envision that this discovery of a new class of oxide superconductors will lead to a rapid progress in physics and chemistry of antiperovskite oxides consisting of unusual metallic anions.

We show that the pressure-temperature phase diagram of the Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ features a metal-insulator transition at 0.5 GPa: at 300 K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal, and at $T<~12\hspace{0.5em}\mathrm{K}$ from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. We compare the metallic state to that of the structurally related p-wave superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4,$ and discuss the importance of structural distortions, which are expected to couple strongly to pressure.