The consequences of the Jahn-Teller (JT) orbital-lattice coupling for magnetism of pseudospin J_{eff}=1/2 and J_{eff}=0 compounds are addressed. In the former case, represented by Sr_{2}IrO_{4}, this ...coupling generates, through the so-called pseudo-JT effect, orthorhombic deformations of a crystal concomitant with magnetic ordering. The orthorhombicity axis is tied to the magnetization and rotates with it under magnetic field. The theory resolves a number of puzzles in Sr_{2}IrO_{4} such as the origin of in-plane magnetic anisotropy and magnon gaps, metamagnetic transition, etc. In J_{eff}=0 systems, the pseudo-JT effect leads to spin-nematic transition well above magnetic ordering, which may explain the origin of "orbital order" in Ca_{2}RuO_{4}.
In Mott insulators with the t(2g)4 electronic configuration such as of Re3+, Ru4+, Os4+, and Ir5+ ions, spin-orbit coupling dictates a Van Vleck-type nonmagnetic ground state with an angular momentum ...J=0, and the magnetic response is governed by gapped singlet-triplet excitations. We derive the exchange interactions between these excitons and study their collective behavior on different lattices. In perovskites, a conventional Bose condensation of excitons into a magnetic state is found, while an unexpected one-dimensional behavior supporting spin-liquid states emerges in honeycomb lattices, due to the bond directional nature of exciton interactions in the case of 90° d-p-d bonding geometry.
Over the past few years, Sr
2
IrO
4
, a single-layer member of the Ruddlesden-Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors. ...Although there is not yet firm evidence for superconductivity, a remarkable range of cuprate phenomenology has been reproduced in electron- and hole-doped iridates including pseudogaps, Fermi arcs, and
d
-wave gaps. Furthermore, many symmetry-breaking orders reminiscent of those decorating the cuprate phase diagram have been reported using various experimental probes. We discuss how the electronic structures of Sr
2
IrO
4
through strong spin-orbit coupling leads to the low-energy physics that had long been unique to cuprates, what the similarities and differences between cuprates and iridates are, and how these advance the field of high-temperature superconductivity by isolating essential ingredients of superconductivity from a rich array of phenomena that surround it. Finally, we comment on the prospect of finding a new high-temperature superconductor based on the iridate series.
The combination of strong spin-orbit coupling and correlations, e.g., in ruthenates and iridates, has been proposed as a means to realize quantum materials with nontrivial topological properties. We ...discuss here Mott insulators where on-site spin-orbit coupling favors a local J_{tot}=0 singlet ground state. We investigate excitations into a low-lying triplet, triplons, and find them to acquire nontrivial band topology in a magnetic field. We also comment on magnetic states resulting from triplon condensation, where we find, in addition to the same ordered phases known from the J_{tot}=1/2 Kitaev-Heisenberg model, a triplon liquid taking the parameter space of Kitaev's spin liquid.
Basic mechanisms controlling orbital order and orbital fluctuations in transition metal oxides are discussed. The lattice driven classical orbital picture, e.g. like in manganites LaMnO3, is ...contrasted to the quantum behavior of orbitals in frustrated superexchange models as realised in pseudocubic titanites ATiO3 and vanadates AVO3. In YVO3, the lattice and superexchange effects strongly compete — this explains the extreme sensitivity of magnetic states to temperature and doping. Lifting the t2g orbital degeneracy by a relativistic spin-orbital coupling is considered on example of the layered cobaltates. We find that the spin-orbital mixing of low-energy states leads to unusual magnetic correlations in a triangular lattice of the CoO2 parent compound. Finally, the magnetism of sodium-rich compounds Na1-xCoO2 is discussed in terms of a spin/orbital polaronic liquid.
Abstract
Magnonic devices operating at terahertz frequencies offer intriguing prospects for high-speed electronics with minimal energy dissipation However, guiding and manipulating terahertz magnons ...via external parameters present formidable challenges. Here we report the results of magnetic Raman scattering experiments on the antiferromagnetic spin-orbit Mott insulator Sr
2
IrO
4
under uniaxial stress. We find that the energies of zone-center magnons are extremely stress sensitive: lattice strain of 0.1% increases the magnon energy by 40%. The magnon response is symmetric with respect to the sign of the applied stress (tensile or compressive), but depends strongly on its direction in the IrO
2
planes. A theory based on coupling of the spin-orbit-entangled iridium magnetic moments to lattice distortions provides a quantitative explanation of the Raman data and a comprehensive framework for the description of magnon-lattice interactions in magnets with strong spin-orbit coupling. The possibility to efficiently manipulate the propagation of terahertz magnons via external stress opens up multifold design options for reconfigurable magnonic devices.
The current efforts to find the materials hosting Kitaev model physics have been focused on Mott insulators of d5 pseudospin-1/2 ions Ir4+ and Ru3+ with t2g5(S=1/2,L=1) electronic configuration. Here ...we propose that the Kitaev model can be realized in materials based on d7 ions with t2g5eg2(S=3/2,L=1) configuration such as Co2+, which also host the pseudospin-1/2 magnetism. Considering possible exchange processes, we have derived the d7 pseudospin-1/2 interactions in 90∘ bonding geometry. The obtained Hamiltonian comprises the bond-directional Kitaev K and isotropic Heisenberg J interactions as in the case of d5 ions. However, we find that the presence of additional spin-active eg electrons radically changes the balance between Kitaev and Heisenberg couplings. Most remarkably, we show that the exchange processes involving eg spins are highly sensitive to whether the system is in Mott (U<Δ) or charge-transfer (U>Δ) insulating regime. In the latter case, to which many cobalt compounds do actually belong, the antiferromagnetic Heisenberg coupling J is strongly suppressed and spin-liquid phase can be stabilized. The results suggest cobalt-based materials as promising candidates for the realization of the Kitaev model.
We show how the general and basic asymmetry between two fundamental degrees of freedom present in strongly correlated oxides, spin and orbital, has very profound repercussions on the elementary spin ...and orbital excitations. Whereas the magnons remain largely unaffected, orbitons become inherently coupled with spin fluctuations in spin-orbital models with antiferromagnetic and ferro-orbital ordered ground states. The composite orbiton-magnon modes that emerge fractionalize again in one dimension, giving rise to spin-orbital separation in the peculiar regime where spinons are faster than orbitons.
Using the functional renormalization group approach this paper obtains the rich phase diagram of the Kitaev-Heisenberg model on the honeycomb lattice which describes Na sub(2) IrO sub(3). We study ...the quantum many-body instabilities of the t-J sub(K)-J sub(H) Kitaev-Heisenberg Hamiltonian on the honeycomb lattice as a minimal model for a doped spin-orbit Mott insulator. This spin-1/2 model is believed to describe the magnetic properties of the layered transition-metal oxide Na sub(2) IrO sub(3). We determine the ground state of the system with finite charge-carrier density from the functional renormalization group (fRG) for correlated fermionic systems. To this end, we derive fRG flow equations adapted to the lack of full spin-rotational invariance in the fermionic interactions, here represented by the highly frustrated and anisotropic Kitaev exchange term. Additionally employing a set of the Ward identities for the Kitaev-Heisenberg model, the numerical solution of the flow equations suggests a rich phase diagram emerging upon doping charge carriers into the ground-state manifold (Z sub(2) quantum spin liquids and magnetically ordered phases). We corroborate superconducting triplet p-wave instabilities driven by ferromagnetic exchange and various singlet pairing phases. For filling delta > 1/4, the p-wave pairing gives rise to a topological state with protected Majorana edge modes. For antiferromagnetic Kitaev and ferromagnetic Heisenberg exchanges, we obtain bond-order instabilities at van Hove filling supported by nesting and density-of-states enhancement, yielding dimerization patterns of the electronic degrees of freedom on the honeycomb lattice. Further, our flow equations are applicable to a wider class of model Hamiltonians.
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