Motivated by recent experiments of a novel 5d Mott insulator in Sr2IrO4, we have studied the twodimensional three-orbital Hubbard model with a spin-orbit coupling λ. The variational Monte Carlo method is used to obtain the ground state phase diagram with varying a on-site Coulomb interaction U as well as λ. It is found that the transition from a paramagnetic metal to an antiferromagnetic (AF) insulator occurs at a finite U = UMI, which is greatly reduced by a large λ, characteristic of 5d electrons, and leads to the "spin-orbit-induced" Mott insulator. It is also found that the Hund's coupling induces the anisotropic spin exchange and stabilizes the in-plane AF order. We have further studied the one-particle excitations using the variational cluster approximation, and revealed the internal electronic structure of this novel Mott insulator. These findings are in agreement with experimental observations on Sr2IrO4, and qualitatively different from those of extensively studied 3d Mott insulators.PACS numbers: 71.30.+h, 75.25.Dk, Transition metal oxides have been one of the most fascinating classes of materials in recent years [1]. For the past decays, tremendous amount of efforts have been devoted to explore the nature of 3d transition metal oxides where various exotic states and phenomena have emerged such as high-T c cuprate superconductors, colossal magneto-resistant manganites, multiferroics, and various magnetic orders. It has been established that these states and phenomena are caused by strong Coulomb interactions along with cooperative interactions of spin, charge, and orbital degrees of freedom, which are basically separable in 3d electrons [2].Very recently, 5d transition metal oxides have attracted much attention as a candidate of a novel Mott insulator. Because of the extended nature of 5d orbital, Coulomb interactions are expected to be smaller for 5d electrons (∼ 1-3 eV) than for 3d electrons (∼ 5-7 eV) [3], whereas the spin-orbit coupling (SOC) λ is estimated to be considerably larger in 5d (∼ 0.1-1 eV) than in 3d (∼ 0.01-0.1 eV). Therefore, in 5d transition metal oxides, inherently spin and orbital degrees of freedom are strongly entangled.One of such examples is the layered 5d transition metal oxide Sr 2 IrO 4 . In Sr 2 IrO 4 , t 2g and e g orbitals are well separated by the large crystal field, and the lower t 2g orbital is filled with five electrons, (t 2g )5 . In spite of the large band width and small on-site Coulomb interaction U , Sr 2 IrO 4 is an antiferromagnetic insulator with a weak ferromagnetic moment [4,5]. Neutron diffraction patterns do not detect any superlattice structure that indicates charge order or charge density wave states [6]. It is proposed that the strong SOC is responsible for the insulating mechanism [7]. Indeed, the 4d counterpart of Sr 2 RhO 4 , which has a larger U and a smaller λ than * Electronic address: h-watanabe@riken.jpThe proposed picture of this "spin-orbit-induced" Mott insulator in Sr 2 IrO 4 is as follows. The SOC splits the t 2g orbitals into J eff = 1/2 ...

Based on a microscopic theoretical study, we show that novel superconductivity is induced by carrier doping in layered perovskite Ir oxides where a strong spin-orbit coupling causes an effective total angular momentum J(eff)=1/2 Mott insulator. Using a variational Monte Carlo method, we find an unconventional superconducting state in the ground state phase diagram of a t(2g) three-orbital Hubbard model on the square lattice. This superconducting state is characterized by a d(x(2)-y(2))-wave "pseudospin singlet" formed by the J(eff)=1/2 Kramers doublet, which thus contains interorbital as well as both singlet and triplet components of t(2g) electrons. The superconducting state is found stable only by electron doping, but not by hole doping, for the case of carrier doped Sr2IrO4. We also study an effective single-orbital Hubbard model to discuss the similarities to high-T(c) cuprate superconductors and the multiorbital effects.

Ever proposed descriptors of catalytic activity for oxygen evolution reaction (OER) were systematically investigated. A wide variety of stoichiometric perovskite oxides ABO 3 (A = Ca, Sr, Y, La; B = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were examined as OER catalysts. The simplest descriptor, e g electron number of transition metal ion at B-site, was not applicable for OER overpotentials (η) of the compounds tested in this study. Another descriptor, oxygen 2p band center relative to Fermi energy (ε 2p), was not necessarily adequate for the most part of perovskite oxides. Eventually, a recently proposed descriptor, charge-transfer energy (Δ), displayed a linear relationship with η the most reasonably. Since Δ values were obtained from theoretical calculations, not only by spectroscopic experiments, systematic exploration for a wide range of compounds including hypothetical ones could be allowed. This finding proposes the charge-transfer energy as the most helpful descriptor for design of perovskite oxide catalyst for OER.