Na 2 IrO 3 , a honeycomb 5d 5 oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal-metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na 2 IrO 3 , the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic-structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant 6 New J. Phys. 16 (2014) 013056 V M Katukuri et al ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin-orbit interactions, this strong dependence on distortions of the Ir 2 O 2 plaquettes singles out the honeycomb 5d 5 oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems. IntroductionThe Heisenberg model of magnetic interactions, J S i · S j between spin moments at sites {i, j}, has been successfully used as an effective minimal model to describe the cooperative magnetic properties of both molecular and solid-state many-electron systems. A less conventional spin model-the Kitaev model [1]-has been recently proposed for honeycomb-lattice materials with 90 • metal-oxygen-metal bonds and strong spin-orbit interactions [2]. It has nontrivial topological phases with elementary excitations exhibiting Majorana statistics, which are relevant and much studied in the context of topological quantum computing [1,[3][4][5][6][7]. Candidate materials proposed to host such physics are the honeycomb oxides Na 2 IrO 3 and Li 2 IrO 3 [2]. The magnetically active sites, the Ir 4+ species, display in these compounds a 5d 5 valence electron configuration, octahedral ligand coordination and bonding of nearest-neighbor (NN) Ir ions through two ligands [8,9]. In the simplest approximation, i.e. for sufficiently large t 2g -e g octahedral crystal-field splittings within the Ir 5d shell and degenerate Ir t 2g levels, the ground-state (GS) electron configuration at each Ir site is a t 5 2g effective j = 1/2 spin-orbit doublet [2,[10][11][12]. The anisotropic, Kitaev type coupling then stems from the particular form the superexchange between the Ir j = 1/2 pseudospins takes for 90 • bond angles on the Ir-O 2 -Ir plaquette [2,13,14].Recent measurements on Na 2 IrO 3 [8,9] indicate significant lattice distortions away from the idealized case of cubic IrO 6 octahedra and 90 • Ir-O-Ir bond angles for which the Kitaev-Heis...

Large anisotropic exchange in 5d and 4d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4d5 honeycomb halide α-RuCl3. From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5d5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d5 halides and oxides in general.

The electronic structure of the honeycomb lattice iridates Na(2)IrO(3) and Li(2)IrO(3) has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of j(eff)=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j(eff) physics in A(2)IrO(3). We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na(2)IrO(3) and Li(2)IrO(3) can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr(2)IrO(4).

The electronic structure of Sr3CuIrO6, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering (RIXS) and theoretical calculations. RIXS spectra at the Ir L3-edge reveal an Ir t2g manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large non-cubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the j eff = 1 2 and j eff = 3 2 states and modifies the isotropic wavefunctions on which many theoretical models are based.

Iridium oxides with a honeycomb lattice have been identified as platforms for the much anticipated Kitaev topological spin liquid: the spin-orbit entangled states of Ir4+ in principle generate precisely the required type of anisotropic exchange. However, other magnetic couplings can drive the system away from the spin-liquid phase. With this in mind, here we disentangle the different magnetic interactions in Li2IrO3, a honeycomb iridate with two crystallographically inequivalent sets of adjacent Ir sites. Our ab initio many-body calculations show that, while both Heisenberg and Kitaev nearest-neighbour couplings are present, on one set of Ir–Ir bonds the former dominates, resulting in the formation of spin-triplet dimers. The triplet dimers frame a strongly frustrated triangular lattice and by exact cluster diagonalization we show that they remain protected in a wide region of the phase diagram.

A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.

We report x-ray resonant magnetic scattering and resonant inelastic x-ray scattering studies of epitaxially strained Sr2IrO4 thin films. The films were grown on SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates, under slight tensile and compressive strains, respectively. Although the films develop a magnetic structure reminiscent of bulk Sr2IrO4, the magnetic correlations are extremely anisotropic, with in-plane correlation lengths significantly longer than the out-of-plane correlation lengths. In addition, the compressive (tensile) strain serves to suppress (enhance) the magnetic ordering temperature TN, while raising (lowering) the energy of the zone-boundary magnon. Quantum chemical calculations show that the tuning of magnetic energy scales can be understood in terms of strain-induced changes in bond lengths.

The 5d5 iridate CaIrO3 is isostructural with the post-perovskite phase of MgSiO3, recently shown to occur under extreme pressure in the lower Earth's mantle. It therefore serves as an analogue of post-perovskite MgSiO3 for a wide variety of measurements at ambient conditions or achievable with conventional multianvile pressure modules. By multireference configuration-interaction calculations we here provide essential information on the chemical bonding and magnetic interactions in CaIrO3. We predict a large antiferromagnetic superexchange of 120 meV along the c axis, the same size with the interactions in the cuprate superconductors, and ferromagnetic couplings smaller by an order of magnitude along a. CaIrO3 can thus be regarded as a j = 1/2 quasi-one-dimensional antiferromagnet. While this qualitatively agrees with the stripy magnetic structure proposed by resonant x-ray diffraction, the detailed microscopic picture emerging from our study, in particular, the highly uneven admixture of t2g components, provides a clear prediction for resonant inelastic x-ray scattering experiments.PACS numbers: 71.15. Rf, 71.27.+a, 75.30.Et, 75.10.Dg Iridium oxide compounds are at the heart of intensive experimental and theoretical investigations in solid state physics. The few different structural varieties displaying Ir ions in octahedral coordination and tetravalent 5d5 valence states, in particular, have recently become a fertile ground for studies of new physics driven by the interplay between strong spin-orbit interactions and electron correlations [1]. The spin-orbit couplings (SOC's) are sufficiently large to split the Ir t 5 2g manifold into well separated j = 3/2 and j = 1/2 states. The former are fully occupied while the latter are only half filled. It appears that the widths of the j = 1/2 bands, w ∼ N t, where t is an effective Ir-Ir hopping integral and N is the coordination number, are comparable to the strength of the onsite Coulomb repulsion U . For smaller coordination numbers, such as N = 3 in the layered honeycomb systems Li 2 IrO 3 and Na 2 IrO 3 and N = 4 for square-lattice Sr 2 IrO 4 and Ba 2 IrO 4 and the post-perovskite CaIrO 3 , the undoped compounds are insulating at all temperatures and a j = 1/2 Mott-Hubbard type picture seems to be appropriate [1][2][3]. For larger coordination numbers like N = 6 in the pyrochlore structure, the t 5 2g Ir oxides are metallic at high T's and display transitions to antiferromagnetic (AF) insulating states on cooling [4]. Here, a Slater type picture in which the gap opening is intimately related to the onset of AF order might be more appropriate, as proposed for example for Os oxide perovskite systems with N = 6 [5]. Nevertheless, also more exotic ground states such as a Mott topological insulator [6] or a Weyl semimetal [7] have been proposed for the pyrochlore iridates.Besides the SOC's and Coulomb interactions, one important additional energy scale is the magnitude of the splittings within the Ir t 2g shell. Such splittings may arise from distortions of the O oc...