Coulomb exchange as source of Kitaev and off-diagonal symmetric anisotropic couplings

Exchange underpins the magnetic properties of quantum matter. In its most basic form, it occurs through the interplay of Pauli’s exclusion principle and Coulomb repulsion, being referred to as Coulomb or direct exchange. Pauli’s exclusion principle combined with inter-atomic electron hopping additionally leads to kinetic exchange and superexchange. Here we disentangle the different exchange channels in anisotropic Kitaev–Heisenberg context. By quantum chemical computations, we show that anisotropic Coulomb exchange, completely neglected so far in the field, may be as large as (or even larger than) other contributions — kinetic exchange and superexchange. This opens new perspectives onto anisotropic exchange mechanisms and sets the proper conceptual framework for further research on tuning Kitaev–Heisenberg magnetism

Ab initio computation of d-d excitation energies in low-dimensional Ti and V oxychlorides

Using a quantum chemical cluster-in-solid computational scheme, we calculate the local d-d excitation energies for two strongly correlated Mott insulators, the oxychlorides TiOCl and VOCl. TiOCl harbors quasi-one-dimensional spin chains made out of S = 1/2 Ti3+ ions while the electronic structure of VOCl displays a more two-dimensional character. We find in both cases that the lowest-energy d-d excitations are within the t2g subshell, starting at 0.34 eV and indicating that orbital degeneracies are significantly lifted. In the vanadium oxychloride, spin triplet to singlet excitations are calculated to be 1 eV higher in energy. For TiOCl, the computed d-level electronic structure and the symmetries of the wavefunctions are in very good agreement with resonant inelastic x-ray scattering results and optical absorption data. For VOCl, future resonant inelastic x-ray scattering experiments will constitute a direct test of the symmetry and energy of about a dozen of different d-d excitations that we predict here

CaIrO3 post-perovskite, a j = 1/2 quasi-one-dimensional antiferromagnet

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

Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

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 Sr$_2$IrO$_4$ by electron spin resonance. While canonical ligand-field theory predicts $g_{\parallel}$-factors $\!<\!2$ for positive tetragonal distortions as present in Sr$_2$IrO$_4$, the experiment indicates $g_{\parallel}\!>\!2$. 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 Sr$_2$IrO$_4$, whereas we find them in Ba$_2$IrO$_4$ 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

Spin-Pure Stochastic-CASSCF via GUGA-FCIQMC Applied to Iron-Sulfur Clusters.

Funder: Max-Planck-GesellschaftIn this work, we demonstrate how to efficiently compute the one- and two-body reduced density matrices within the spin-adapted full configuration interaction quantum Monte Carlo (FCIQMC) method, which is based on the graphical unitary group approach (GUGA). This allows us to use GUGA-FCIQMC as a spin-pure configuration interaction (CI) eigensolver within the complete active space self-consistent field (CASSCF) procedure and hence to stochastically treat active spaces far larger than conventional CI solvers while variationally relaxing orbitals for specific spin-pure states. We apply the method to investigate the spin ladder in iron-sulfur dimer and tetramer model systems. We demonstrate the importance of the orbital relaxation by comparing the Heisenberg model magnetic coupling parameters from the CASSCF procedure to those from a CI-only (CASCI) procedure based on restricted open-shell Hartree-Fock orbitals. We show that the orbital relaxation differentially stabilizes the lower-spin states, thus enlarging the coupling parameters with respect to the values predicted by ignoring orbital relaxation effects. Moreover, we find that, while CASCI results are well fit by a simple bilinear Heisenberg Hamiltonian, the CASSCF eigenvalues exhibit deviations that necessitate the inclusion of biquadratic terms in the model Hamiltonian

NaRuO2_2: Kitaev-Heisenberg exchange in triangular-lattice setting

Kitaev exchange, a new paradigm in quantum magnetism research, occurs for 90$^{\circ}$ metal-ligand-metal links, $t_{2g}^5$ transition ions, and sizable spin-orbit coupling. It is being studied in honeycomb compounds but also on triangular lattices. While for the former it is known by now that the Kitaev intersite couplings are ferromagnetic, for the latter the situation is unclear. Here we pin down the exchange mechanisms and determine the effective coupling constants in the $t_{2g}^5$ triangular-lattice material NaRuO$_2$, recently found to host a quantum spin liquid ground state. We show that, compared to honeycomb compounds, the characteristic triangular-lattice cation surroundings dramatically affect exchange paths and effective coupling parameters, changing the Kitaev interactions to antiferromagnetic. The quantum chemical analysis and subsequent effective spin model computations provide perspective onto the nature of the experimentally observed quantum spin liquid -- it seemingly implies finite longer-range exchange, and the atypical proximity to ferromagnetic order is related to sizable ferromagnetic Heisenberg nearest-neighbor couplings

NaRuO2: Kitaev-Heisenberg exchange in triangular-lattice setting

Kitaev exchange, a new paradigm in quantum magnetism research, occurs for 90° metal-ligand-metal links, t2g5 transition ions, and sizable spin-orbit coupling. It is being studied in honeycomb compounds but also on triangular lattices. While for the former it is known by now that the Kitaev intersite couplings are ferromagnetic, for the latter the situation is unclear. Here we pin down the exchange mechanisms and determine the effective coupling constants in the t2g5 triangular-lattice material NaRuO2, recently found to host a quantum spin liquid ground state. We show that, compared to honeycomb compounds, the characteristic triangular-lattice cation surroundings dramatically affect exchange paths and effective coupling parameters, changing the Kitaev interactions to antiferromagnetic. Quantum chemical analysis combined with subsequent effective spin model simulations provide perspective onto the nature of the experimentally observed quantum spin liquid—it seemingly implies fairly large antiferromagnetic second-neighbor isotropic exchange, and the atypical proximity to ferromagnetic order is related to ferromagnetic nearest-neighbor Heisenberg coupling

Electronic correlations and magnetic interactions in infinite-layer NdNiO2

The large antiferromagnetic exchange coupling in the parent high-Tc cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) NdNiO2 brings to the fore the relevance of magnetic coupling in high-Tc superconductors, particularly because no magnetic ordering is observed in the undoped IL-NdNiO2, unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-NdNiO2 using state-of-the-art many-body quantum chemistry methods. From a systematic comparison of the electronic and magnetic properties with isostructural cuprate IL-CaCuO2, we find that the on-site dynamical correlations are significantly stronger in IL-NdNiO2 compared to the cuprate analog. These dynamical correlations play a critical role in the magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest-neighbor isotropic J of 77 meV between the Ni1+ ions within the ab plane. While we find many similarities in the electronic structure between the nickelate and the cuprate, the role of electronic correlations is profoundly different in the two. We further discuss the implications of our findings in understanding the origin of superconductivity in nickelates. © 2020 authors