Session X37: Complex Oxides: Theory and Computation

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Using determinant quantum Monte Carlo, we study the single-band Hubbard model in two dimensions, with electrons coupled to a constant out-of-plane magnetic field. We present thermodynamic, magnetic and transport properties of this model, including the local moment, compressibility, density of states, magnetoresistance, and hall conductivity, as a function of doping, temperature and Hubbard interaction strength. Our results are compared with experimental data, non-interacting and atomic limits, and other results obtained through alternative theoretical techniques.   

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We theoretically explore how the spin-orbit coupling could give rise to unusual states of spin-orbital or spin-lattice degrees of freedom depending on the local d-electron counting and the lattice geometry. We discuss d¹ and d² transition metal compounds, such as molybdenum oxides with double perovskite and pyrochlore structures, as candidate materials and consider available experimental rezults from this perspective.

References:
[1] M.G. Yamada and G. Jackeli, Phys. Rev. Materials 4, 074007 (2020).
[2] A. Smerald and G. Jackeli, Phys. Rev. Lett. 122,227202 (2019).
[3] J. Romhányi, L. Balents, and G. Jackeli, Phys. Rev. Lett. 118, 217202 (2017).   

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The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms or groups. In this work [1], we show that explicit consideration of magnetic and (leading) bridging orbitals, together with the electron transfer between the former, reveals a strong ferromagnetic kinetic exchange contribution. First-principles calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges such as Fe-Co-Fe complex and quasi 1D Cu chain compound (La₄Ba₂Cu₂O₁₀). In particular, it is responsible for a very large ferromagnetic coupling (−10 meV) between the iron ions in a Fe³⁺−Co³⁺−Fe³⁺ complex. Furthermore, we find that the ferromagnetic exchange interaction turns into antiferromagnetic by substituting the diamagnetic bridge with magnetic one. The phenomenology is observed in two series of materials, supporting the significance of the ferromagnetic kinetic exchange mechanism.

[1] Z. Huang, D. Liu, A. Mansikkamäki, V. Vieru, N. Iwahara, and L. F. Chibotaru, Phys. Rev. Research 2, 033430 (2020).   

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CaCu₃Fe₂Re₂O₁₂, a quadruple perovskite (QP) with the chemical formula AA’₃B₂B’₂O₁₂, shows half-metallic ferrimagnetism with large magnetization and a high Curie temperature (T_C). Here we investigated a series of QPs using density functional theory and Monte Carlo simulations. We found that all compounds are ferrimagnetic with substituted A²⁺ (A=Ca, Sr, Ba, Pb) ions exhibiting high Tc (above 405 K), and the A³⁺ (A=Sc, Y, La ) substituted ones yielding even higher T_C (above 502 K). By examining interatomic exchange parameters, we found that the antiferromagnetic exchange couplings between Re and Cu as well as Re and Fe are responsible for this very high Curie temperature. For the compounds with A³⁺ substitution, electron doping in bands around the Fermi level dominated by the Re ions strengthen the Re-Cu and Re-Fe exchange interactions, which cause an increase in the T_C. This work demonstrates a design strategy of enhancing the spin ordering temperature by replacing A-site non-magnetic ions.   

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The grand challenge in first-principles calculation for magnetic materials is whether we can predict the experimental magnetic structure for a given material. In particular, for noncollinear magnets there is a tremendous amount of possible magnetic configurations that ought to be considered. It is also known that ab initio calculations in the framework of spin-density functional theory (SDFT ) have many local minima. We present an efficient scheme [1] to circumvent these issues by a combination of the cluster multipole (CMP) theory to create an appropriate list of candidate magnetic configurations and SDFT. CMP+SDFT was proven to be reliable in predicting uniform magnetic structures in a high-throughput calculation including 2935 calculations and the effects of strong correlations on the prediction of magnetic structures is investigated by additional 1545 SDFT+U calculations.

[1] M.-T. Huebsch, T. Nomoto, M.-T. Suzuki, and R. Arita, Benchmark for ab initio prediction of magnetic structures based on cluster multipole theory (2020), arXiv:2008.13669 [cond-mat.mtrl-sci].   

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Mean-field band theory, e.g., density functional theory (DFT), is often based on the symmetry-restricted, minimal-cell model, with the smallest number of possible magnetic and structural degrees of freedom. Such symmetry-restricted DFT fails to reproduce many experimental observations, such as local distortions, mass enhancement, and nematicity (in some Fe-based materials). It has been argued that the strong correlation approach is the exclusive theory needed for describing such peculiarities. We report that by removing the constraint of the minimal cell, the energy minimization in DFT finds in enlarged cells (large enough to allow for symmetry breakings) atomic and spin symmetry breakings, which can explain many observed anomalies and provide close agreements with experiments. We show how the energy-lowering symmetry breaking effects in mean-field DFT capture mass enhancement [1] in a range of perovskites e.g. d-electron SrVO3, SrTiO3, BaTiO3, LaMnO3 as well as p-electron CsPbI3 and SrBiO3, and local distortions and nematicity in iron-based compound FeSe [2]. Thus, symmetry breaking in DFT could describe effects that were previously attributed exclusively to complex correlated treatments.
[1] arXiv:2006.10099
[2] arXiv:1911.02670   

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Hexaferrites have been of huge research interest because of their technological applications in permanent magnets, data storage in computer hard disks and tapes, microwave devices, and wireless communications. Finding suitable hexaferrite that can exhibit a strong uniaxial magneto-crystalline anisotropy energy (MAE) and high coercivity is still an open question today for improvement of the device efficiency. We study the electronic structure, magnetic properties, and magneto-structural transition in Sr/La hexaferrites using advanced density functional methods. Our calculations suggest that La-substituted compounds enhance the MAE while transforming from P6$_3$/mmc structure (high temperature phase) to P$\bar{6}m$_2$ structure (low temperature phase) compared to {\it sp}-electron based hexaferrites. The underlying physics in the enhancement of MAE relies in the electronic changes resulting from local crystalline environment of Fe-atoms nearest to the La. Using an anisotropic energy density model, we show that the contribution of higher order anisotropy constants plays significant role in describing the experimentally measured MAE.   

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Very often, the prediction of metallic electronic structure in mean-field theory for compounds known to be insulators (listed below) is attributed to a lack of correlation. We show that the intricate coupling of electronic, spin and lattice degrees of freedom afforded by symmetry broken mean-field theories such as DFT (rather than strong or dynamic correlation) correctly predicts the appropriate insulating or metallic state in a range of compounds. The symmetry-breaking modes that lower the total energy while opening a gap include:
(i) spin ordering as demonstrated in CuBi₂O₄, NiO, and YNiO₃;
(ii) formation of a distribution of local spin (polymorphous) networks as shown in paramagnetic LaTiO₃, and YTiO₃;
(iii) structural symmetry breaking such as (a) bond disproportionation in Li-doped TiO₂, SrBiO₃, and rare earth nickelates, (b) pseudo-Jahn-Teller distortions in LaMnO₃, and (c) spontaneous formation of vacancy arrays in Ba₄As₃;
(iv) allowing spin-orbit coupling in CaIrO₃ and Sr₂IrO₄.
Importantly, the accounting of the above symmetry-breaking modes allows distinguishing real metals (e.g., SrVO₃ and Ca₆Al₇O₁₆) from the above example of false metals.
O.I. Malyi, A. Zunger Applied Physics Reviews, doi: 10.1063/5.0015322.   

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Antiferromagnetism (AF) such as Neel ordering is often closely related to strong local Coulomb repulsion such as onsite Hubbard repulsion in two-dimensional (2D) systems. Whether Neel AF ordering in 2D can be dominantly induced by electron-phonon couplings has not been completely understood. Here, by employing numerically-exact sign-problem-free quantum Monte Carlo (QMC) simulations, we obtain convincing evidences that Su-Schrieffer-Heeger (SSH) phonons can induce Neel AF ordering on the square lattice for a wide range of phonon frequencies. In the limit of vanishing phonon frequency (namely in the adiabatic limit), the SSH phonons favor a valence-bond solid (VBS) ordering. By increasing the phonon frequency, we show that there is a direct (and possibly deconfined) quantum phase transition from the VBS to AF phases.   

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Layered-oxides have been known to host high temperature superconductivity since the discovery of cuprates. The coupling between structure and magnetism has been suggested as important factor for defining the functional properties of these materials. Despite the potential impacts, there are only a few synthesized layered oxides. Recent success in the synthesis of layered structures from three-dimensional perovskites have aimed to fill this knowledge gap by providing novel routes to synthesizing materials with desired properties. We present the feasibility of creating layered ABO₂ structures from ABO₃ (A=Ca, La, Sr, Li, Na, Nd, Pr, Y; B=Fe, Mn, Mo, V, Ni, Ru) perovskites. The oxygen vacancy formation energetics from high-throughput first-principles calculations are used as descriptors for phase transformation. The stability against decomposition into binary oxides and thermodynamic analyses of the formation conditions in terms of chemical potential of the components are presented to guide the experimental synthesis. The magnetic ground state of the ABO₂ structures, the exchange interaction parameters, and the Neel temperature from Monte Carlo simulations are explored as potential indicators of superconductivity.   

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We investigate the coupling of electromagnetic fields to electrons on a lattice with strong on-site Hubbard interactions and calculate the spin and orbital nonlinear electric field susceptibilities. Gauge covariance of the equations of motion for both the photon and matter fields prescribes a minimal coupling procedure in which the fermionic quantum momentum operator is boosted by the electromagnetic vector potential. On a lattice this minimal coupling procedure takes the form of a Peierls substitution whereby the electronic wavefunction acquires a phase upon transport through the electromagnetic field. Strong electron-electron interactions can localize the charge degrees of freedom on a lattice forcing the Peierls phase to vanish. We show how the relativistic corrections in Dirac equation give rise to effective spin-spin interactions that couple directly to an external electric field even in the absence of a Peierls phase.