Novel Jeff=1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4

Charge-orbital density wave and superconductivity in the strong spin-orbit coupled IrTe2:Pd

Using transmission electron microscopy, the anomalies in resistivity and magnetic susceptibility at ~262 K in IrTe2 are found to accompany the superlattice peaks with q[over q=(1/5,0,-1/5). The wave vector is consistent with our theoretical calculation for the Fermi surface nesting vector, indicating that the ~262 K transition is of the charge-orbital density wave (DW) type. We also discovered that both Pd intercalation and substitution induce bulk superconductivity with T(c) up to ~3 K, which competes with DW in a quantum critical pointlike manner.

Subband structure of a two-dimensional electron gas formed at the polar surface of the strong spin-orbit perovskite KTaO3

We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO3. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, our measurements provide a direct upper bound for the potential Rashba spin splitting of only Δk(parallel)}~0.02 Å(-1) at the Fermi level. The polar nature of the KTaO3(100) surface appears to help mediate the formation of the 2DEG as compared to nonpolar SrTiO3(100).

Topological quantum phase transition in 5d transition metal oxide Na2IrO3

We predict a quantum phase transition from normal to topological insulators in the 5d transition metal oxide Na2IrO3, where the transition can be driven by the change of the long-range hopping and trigonal crystal field terms. From the first-principles-derived tight-binding Hamiltonian, we determine the phase boundary through the parity analysis. In addition, our first-principles calculations for Na2IrO3 model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na2IrO3 is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations.

Ab initio studies on the interplay between spin-orbit interaction and Coulomb correlation in Sr2IrO4 and Ba2IrO4

Ab initio analyses of A(2)IrO(4) (A=Sr,Ba) are presented. Effective Hubbard-type models for Ir 5d t(2g) manifolds downfolded from the global band structure are solved based on the dynamical mean-field theory. The results for A=Sr and Ba correctly reproduce paramagnetic metals undergoing continuous transitions to insulators below the Néel temperature T(N). These compounds are classified not into Mott insulators but into Slater insulators. However, the insulating gap opens by a synergy of the Néel order and significant band renormalization, which is also manifested by a 2D bad metallic behavior in the paramagnetic phase near the quantum criticality.

X-ray absorption spectroscopy studies of spin-orbit coupling in 5d transition metal oxides

In order to examine the effects of strong valence band spin-orbit coupling (SOC) in 5d transition metal oxides (TMOs), we have investigated the L(2) and L(3) edge white-line intensities of the x-ray absorption spectra of several 5d TMOs. The white-line intensities at both edges are found to decrease monotonously with increasing 5d electron occupancy, while their ratio showed anomalous behavior for late 5d TMOs (IrO(2), PtO(2), and Au(2)O(3)), deviating significantly from the theoretical value of 2 expected for the case of weak SOC. This observation serves as a clear experimental indication of strong SOC effects in 5d TMOs. We also discussed how the 5d TMOs can have charge transfer effects different from their counterpart 5d elemental metals by making comparative studies. Our works demonstrate the importance of j quantum states due to strong SOC in the 5d system.

Emergent phenomena at oxide interfaces

Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

Reduced effective spin-orbital degeneracy and spin-orbital ordering in paramagnetic transition-metal oxides: Sr2IrO4 versus Sr2RhO4

We discuss the notions of spin-orbital polarization and ordering in paramagnetic materials, and address their consequences in transition-metal oxides. Extending the combined density functional and dynamical mean field theory scheme to the case of materials with large spin-orbit interactions, we investigate the electronic excitations of the paramagnetic phases of Sr(2)IrO(4) and Sr(2)RhO(4). We show that the interplay of spin-orbit interactions, structural distortions and Coulomb interactions suppresses spin-orbital fluctuations. As a result, the room temperature phase of Sr(2)IrO(4) is a paramagnetic spin-orbitally ordered Mott insulator. In Sr(2)RhO(4), the effective spin-orbital degeneracy is reduced, but the material remains metallic, due to both, smaller spin-orbit and smaller Coulomb interactions. The corresponding spectra are in excellent agreement with photoemission data. Finally, we make predictions for the spectra of paramagnetic Sr(2)IrO(4).

Twisted Hubbard model for Sr2IrO4: magnetism and possible high temperature superconductivity

Sr(2)IrO(4) has been suggested as a Mott insulator from a single J(eff)=1/2 band, similar to the cuprates. However, this picture is complicated by the measured large magnetic anisotropy and ferromagnetism. Based on a careful mapping to the J(eff)=1/2 (pseudospin-1/2) space, we propose that the low energy electronic structure of Sr(2)IrO(4) can indeed be described by a SU(2) invariant pseudospin-1/2 Hubbard model very similar to that of the cuprates, but with a twisted coupling to an external magnetic field (a g tensor with a staggered antisymmetric component). This perspective naturally explains the magnetic properties of Sr(2)IrO(4). We also derive several simple facts based on this mapping and the known results about the Hubbard model and the cuprates, which may be tested in future experiments on Sr(2)IrO(4). In particular, we propose that (electron-)doping Sr(2)IrO(4) can potentially realize high-temperature superconductivity.

Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.

The electronic structure of epitaxially stabilized 5d perovskite Ca(1-x)Sr(x)IrO3 (x = 0, 0.5, and 1) thin films: the role of strong spin-orbit coupling

We have investigated the electronic structure of meta-stable perovskite Ca(1 - x)Sr(x)IrO(3)(x = 0, 0.5, and 1) thin films using transport measurements, optical spectroscopy, and first-principles calculations. We artificially fabricated the perovskite phase of Ca(1 - x)Sr(x)IrO(3), which has a hexagonal or post-perovskite crystal structure in bulk form, by growing epitaxial thin films on perovskite GdScO(3) substrates using an epi-stabilization technique. The transport properties of the perovskite Ca(1 - x)Sr(x)IrO(3) films systematically change from nearly insulating (or semi-metallic) for x = 0 to weakly metallic for x = 1. Due to the extended wavefunctions, 5d electrons are usually delocalized. However, the strong spin-orbit coupling in Ca(1 - x)Sr(x)IrO(3) results in the formation of effective total angular momentum J(eff) = 1/2 and 3/2 states, which puts Ca(1 - x)Sr(x)IrO(3) in the vicinity of a metal-insulator phase boundary. As a result, the electrical properties of the Ca(1 - x)Sr(x)IrO(3) films are found to be sensitive to x and strain.

Microscopic study of a spin-orbit-induced Mott insulator in Ir oxides

Motivated by recent experiments of a novel 5d Mott insulator in Sr2IrO4, we have studied the two-dimensional 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 an on-site Coulomb interaction U as well as λ. It is found that the transition from a paramagnetic metal to an antiferromagnetic insulator occurs at a finite U=U(MI), 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 antiferromagnetic order. We have further studied the one-particle excitations by 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.

Orbital magnetism and spin-orbit effects in the electronic structure of BaIrO3

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ~1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L(2,3) absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

Crystal growth and structure and magnetic properties of the 5d oxide Ca3LiOsO6: extended superexchange magnetic interaction in oxide

Crystals of the newly synthesized compound Ca(3)LiOsO(6) were grown by a flux method using LiCl and KCl, followed by single-crystal X-ray diffraction (XRD), low-temperature powder XRD, and measurements of ac and dc magnetic susceptibility and specific heat. The data indicate that Ca(3)LiOsO(6) has a fully opened electronic gap with an antiferromagnetic ordered state, consistent with suggestions from the first-principles study. The observed magnetic transition temperature is 117 K, too high to be caused only by a direct spin-spin interaction. It appears that the original superexchange magnetic path Os-O-Os is absent; thus, the extended superexchange path (Os-O)-(O-Os) can be expected to be responsible for the 117 K magnetic order. If this is true, Ca(3)LiOsO(6) would be highly valuable to study the nature of extended superexchange magnetic interactions in solids.

Electronic structure of double perovskite A2FeReO6 (A = Ba and Ca): interplay between spin-orbit interaction, electron correlation, and lattice distortion

We have investigated the electronic structure of double perovskites, Ba(2)FeReO(6) (metallic) and Ca(2)FeReO(6) (insulating) using optical and x-ray absorption spectroscopy. By comparing the experimental results with the density functional theory calculations, we found that the electronic structure of Ba(2)FeReO(6) could be determined from the interaction of the electron correlation and spin-orbit coupling. On the other hand, for Ca(2)FeReO(6), the lattice distortion and electron correlation are important in determining the electronic structure. Additionally, the insulating gap in Ca(2)FeReO(6) is realized by the spin-orbit coupling. Our work shows that the subtle interplay of the spin-orbit interaction, electron correlation, and lattice distortion should be taken into account to understand the electronic structure of the 5d transition metal oxides.

Quantum spin Hall effect in a transition metal oxide Na2IrO3

We study theoretically the electronic states in a 5d transition metal oxide Na2IrO3, in which both the spin-orbit interaction and the electron correlation play crucial roles. A tight-binding model analysis together with the first-principles band structure calculation predicts that this material is a layered quantum spin Hall system. Because of the electron correlation, an antiferromagnetic order first develops at the edge, and later inside the bulk at low temperatures.

Phase-sensitive observation of a spin-orbital Mott state in Sr2IrO4

Measurement of the quantum-mechanical phase in quantum matter provides the most direct manifestation of the underlying abstract physics. We used resonant x-ray scattering to probe the relative phases of constituent atomic orbitals in an electronic wave function, which uncovers the unconventional Mott insulating state induced by relativistic spin-orbit coupling in the layered 5d transition metal oxide Sr2IrO4. A selection rule based on intra-atomic interference effects establishes a complex spin-orbital state represented by an effective total angular momentum = 1/2 quantum number, the phase of which can lead to a quantum topological state of matter.

Mott insulators in the strong spin-orbit coupling limit: from Heisenberg to a quantum compass and Kitaev models

We study the magnetic interactions in Mott-Hubbard systems with partially filled t_{2g} levels and with strong spin-orbit coupling. The latter entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians that extrapolate from the Heisenberg to a quantum compass model depending on the lattice geometry. This gives way to "engineer" in such Mott insulators an exactly solvable spin model by Kitaev relevant for quantum computation. We, finally, explain "weak" ferromagnetism, with an anomalously large ferromagnetic moment, in Sr2IrO4.

Dimensionality-controlled insulator-metal transition and correlated metallic state in 5d transition metal oxides Sr n+1Ir nO3n+1 (n=1, 2, and infinity)

We investigated the electronic structures of the 5d Ruddlesden-Popper series Sr n+1Ir nO3n+1 (n=1, 2, and infinity) using optical spectroscopy and first-principles calculations. As 5d orbitals are spatially more extended than 3d or 4d orbitals, it has been widely accepted that correlation effects are minimal in 5d compounds. However, we observed a Mott insulator-metal transition with a change of bandwidth as we increased n. In addition, the artificially synthesized perovskite SrIrO3 showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.