Lattice distortion and magnetism of3d−t2gperovskite oxides

Abstract

Several puzzling aspects of interplay of the experimental lattice distortion and the the magnetic properties of four narrow t2g-band perovskite oxides (YTiO3, LaTiO3, YVO3, and LaVO3) are clarified using results of first-principles electronic structure calculations. First, we derive parameters of the effective Hubbard-type Hamiltonian for the isolated t2g bands using newly developed downfolding method for the kinetic-energy part and a hybrid approach, based on the combination of the random-phase approximation and the constraint local-density approximation, for the screened Coulomb interaction part. Apart form the above-mentioned approximation, the procedure of constructing the model Hamiltonian is totally parameter-free. The results are discussed in terms of the Wannier functions localized around transition-metal sites. The obtained Hamiltonian was solved using a number of techniques, including the mean-field Hartree-Fock (HF) approximation, the second-order perturbation theory for the correlation energy, and a variational superexchange theory, which takes into account the multiplet structure of the atomic states. We argue that the crystal distortion has a profound effect not only on the values of the crystal-field (CF) splitting, but also on the behavior of transfer integrals and even the screened Coulomb interactions. Even though the CF splitting is not particularly large to quench the orbital degrees of freedom, the crystal distortion imposes a severe constraint on the form of the possible orbital states, which favor the formation of the experimentally observed magnetic structures in YTiO3, YVO3, and LaVO3 even at the level of mean-field HF approximation. It is remarkable that for all three compounds, the main results of all-electron calculations can be successfully reproduced in our minimal model derived for the isolated t2g bands. We confirm that such an agreement is only possible when the nonsphericity of the Madelung potential is explicitly included into the model. Beyond the HF approximation, the correlations effects systematically improve the agreement with the experimental data. Using the same type of approximations we could not reproduce the correct magnetic ground state of LaTiO3. However, we expect that the situation may change by systematically improving the level of approximations for dealing with the correlation effects.

Orbital ordering and magnetic interactions in BiMnO3

“…Our model (2) does not explicitly include the oxygen states. This appears to be a good approximation for the titanium and vanadium perovskite oxides [35], where the transitionmetal and oxygen bands are well separated. However, manganese compounds are much closer to the charge-transfer regime because of the proximity of Mn(3d) and O(2p) bands, and much stronger hybridization, which takes place between these groups of states [45]- [47].…”

“…The origin of these crystal fields is mainly related to the change of the hybridization (or the covalent mixing) in different bonds of the distorted perovskite structure, which after the downfolding gives rise to the site-diagonal elements of the model Hamiltonian. The nonsphericity of the Madelung potential, which plays a crucial role in the t 2g compounds [34,35], is considerably smaller than the effects of the covalent mixing in the e g bands and can be neglected. The alternation of the crystal-field parameters in real space gives rise to the orbital ordering, which is primarily responsible for the magnetic behavior of BiMnO 3 .…”

“…where e R is the direction of the spin magnetic moment at the site R. Generally, the parameters {J RR } are not universal and depend on the magnetic state in which they are calculated (for example, through the change of the orbital ordering [35] or the electronic structure [40] in each magnetic state).…”

Orbital superexchange and crystal field simultaneously at play in YVO3: Resonant inelastic x-ray scattering at the VLedge and the OKedge

“…These are further split by the deviations from cubic symmetry. 21,27,[54][55][56] In cubic approximation, the lowest-lying two-electron state shows . 57 The cubic approximation is sufficient for the overall assignment of all RIXS peaks observed above 1 eV at the V L 3 edge.…”

Orbital Fluctuations in the Different Phases ofLaVO3andYVO3

“…1) with temperature-induced magnetization reversal phenomena [1] and other exotic properties [2,3]. While it is now recognized that the V-t 2g orbital degrees of freedom and the strong Coulomb repulsion are the key ingredients, it is still controversial whether classical (orbital order) [1,4,5,6,7,8] or quantum (orbital fluctuations) [2, 9] effects are responsible for the rich physics of these vanadates.At 300 K, LaVO 3 and YVO 3 are orthorhombic paramagnetic Mott insulators. Their structure (Fig.…”