We examine the role of spin-orbit coupling in the electronic structure of α-RuCl3, in which Ru ions in 4d 5 configuration form a honeycomb lattice. The measured optical spectra exhibit an optical gap of 220 meV and transitions within the t2g orbitals. The spectra can be described very well with firstprinciples electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. Furthermore, our x-ray absorption spectroscopy measurements at the Ru L edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. We propose that α-RuCl3 is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be relevant for this compound.

Intensive studies of the interplay between spin-orbit coupling (SOC) and electronic correlations in transition metal compounds have recently been undertaken. In particular, j eff = 1/2 bands on a honeycomb lattice provide a pathway to realize Kitaev's exactly solvable spin model. However, since current wisdom requires strong atomic SOC to make j eff = 1/2 bands, studies have been limited to iridium oxides. Contrary to this expectation, we demonstrate how Kitaev interactions arise in 4d-orbital honeycomb α-RuCl3, despite having significantly weaker SOC than the iridium oxides, via assistance from electron correlations. A strong coupling spin model for these correlation-assisted j eff = 1/2 bands is derived, in which large antiferromagnetic Kitaev interactions emerge along with ferromagnetic Heisenberg interactions. Our analyses suggest that the ground state is a zigzagordered phase lying close to the antiferromagnetic Kitaev spin liquid. Experimental implications for angle resolved photoemission spectroscopy, neutron scattering, and optical conductivities are discussed.arXiv:1411.6623v3 [cond-mat.str-el]

The two-dimensional layered perovskite Sr2IrO4 was proposed to be a spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified on a narrow J eff = 1/2 band due to strong spinorbit coupling. On the other hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO3 remains metallic. To understand the physical origin of the metallic state and possible transitions to insulating phases, we construct a tight-binding model for SrIrO3. The band structure possesses a line node made of J eff = 1/2 bands below the Fermi level. As a consequence, instability towards magnetic ordering is suppressed and the system remains metallic. This line node, originating from the underlying crystal structure, turns into a pair of three-dimensional nodal points on the introduction of a staggered potential or spin-orbit coupling strength between alternating layers. Increasing this potential beyond a critical strength induces a transition to a strong topological insulator, followed by another transition to a normal band insulator. We propose that materials constructed with alternating Ir-and Rh-oxide layers along the (001) direction, such as Sr2IrRhO6, are candidates for a strong topological insulator.

Novel interplay of spin-orbit coupling and electron correlations in complexOne of the appealing examples of exotic electronic phases produced by the novel interplay of spin-orbit coupling (SOC) and the other relevant parameters in complex Ir oxides is a novel spin-orbital Mott insulator associated with the half-filled J eff = 1/2 band, lately identified in layered perovskite Sr 2 IrO 4 1,2 . The three dimensional counterpart of Sr 2 IrO 4 , SrIrO 3 , is revealed to be a semimetal close to a band insulator 3,4 , where a line of Dirac nodes generated by combination of SOC and lattice symmetry plays a key role 5 . The large SOC may give rise to intriguing topological phases as well in their sister iridium oxides. It was 4 theoretically predicted that iridium oxides with unique geometry of lattice, such as honeycomb or pyrochlore, may have a topological character [6][7][8] . A family of complex iridium oxides is thus a promising playground to explore rich electronic phases, ranging from semimetal, magnetic insulator and even topological insulator, by the subtle control of lattice.The control of dimensionality and the introduction of interface using superlattice structure have been demonstrated to be a useful technique to control the electronic phase of 3d transition metal oxides including titanium 9 and nickel 10 . The approach using superlattice could be even more powerful in complex 5d Ir oxides than their 3d analogues to explore exotic electronic phases, because of the presence of the large SOC. The emergence of a variety of phases in a narrow materials space, as a consequence of the interplay between SOC and other electronic energy scales, means that only a minute change of the control parameters including dimensionality may totally alter the ground states. To realize topological phases, the modification of local symmetry of lattice, for example breaking inversion symmetry, is often essential, which could be done by introducing interfaces. Indeed, a design of topological insulator was theoretically proposed by utilizing superlattice structure of perovskite oxides 5,11 .In this work, we have successfully grown artificial superlattices [(SrIrO 3 ) m , (T) and M(T)indicate that the magnetism, rather than disorder, is a key ingredient for the (semi-)metal to insulator transition. We also note here that for m = 1 and 2, more significantly for m = 1, (T) shows weak insulating behavior even above magnetic ordering, which might suggest the increased Mott character in the limit of reduced dimensionality. 7The tailored magnetism in the superlattice turns out to be a canted in-plane antiferromagnetism. The weak ferromagnetic moments were observed only for magnetic field parallel to IrO 2 plane as indicated in Fig. 3(a). Using those weak ferromagnetic moment as a marker, we are able to investigate the local lattice distortion in thin films, which in general is very difficult to probe. Since the unit-cell volume of SrIrO 3 is larger than that of SrTiO 3 13,14 , the IrO 6 octahedra should rotate within the plane to match t...

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