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Sawada, Hideaki; Terakura, Kiyoyuki

doi:10.1103/physrevb.58.6831

Cited by 118 publications

(101 citation statements)

References 17 publications

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“…The occupied orbitals ͉ i ͘ refer to four sublattices ͑i =1,…, 4͒, with Ti 1 ͑000͒, Ti 2 ͑100͒, Ti 3 ͑001͒, Ti 4 ͑101͒ positions. 43,83 The minus signs in ͉ 3 ͘ and ͉ 4 ͘ reflect a mirror symmetry present in the GdFeO 3 structure. Note that this state resembles G-type OO, and is thus different from the C-type OO encountered for e g orbitals ͓due to the change of sign for the ͉c͘ orbitals in Eqs.…”

Section: B Spin Exchange Constants and Optical Intensitiesmentioning

confidence: 99%

Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

et al. 2005

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The temperature dependence and anisotropy of optical spectral weights associated with different multiplet transitions is determined by the spin and orbital correlations. To provide a systematic basis to exploit this close relationship between magnetism and optical spectra, we present and analyze the spin-orbital superexchange models for a series of representative orbital-degenerate transition metal oxides with different multiplet structure. For each case we derive the magnetic exchange constants, which determine the spin wave dispersions, as well as the partial optical sum rules. The magnetic and optical properties of early transition metal oxides with degenerate t 2g orbitals ͑titanates and vanadates with perovskite structure͒ are shown to depend only on two parameters, viz. the superexchange energy J and the ratio of Hund's exchange to the intraorbital Coulomb interaction, and on the actual orbital state. In e g systems important corrections follow from charge transfer excitations, and we show that KCuF 3 can be classified as a charge transfer insulator, while LaMnO 3 is a Mott insulator with moderate charge transfer contributions. In some cases orbital fluctuations are quenched and decoupling of spin and orbital degrees of freedom with static orbital order gives satisfactory results for the optical weights. On the example of cubic vanadates we describe a case where the full quantum spin-orbital physics must be considered. Thus information on optical excitations, their energies, temperature dependence, and anisotropy, combined with the results of magnetic neutron scattering experiments, provides an important consistency test of the spin-orbital models, and indicates whether orbital and/or spin fluctuations are important in a given compound. I. SUPEREXCHANGE AND OPTICAL EXCITATIONS AT ORBITAL DEGENERACYThe physical properties of Mott ͑or charge transfer͒ insulators are dominated by large on-site Coulomb interactions ϰU which suppress charge fluctuations. Quite generally, the Coulomb interactions lead then to strong electron correlations which frequently involve orbitally degenerate states, such as 3d ͑or 4d͒ states in transition metal compounds, and are responsible for quite complex behavior with often puzzling transport and magnetic properties. 1 The theoretical understanding of this class of compounds, with the colossal magnetoresistance ͑CMR͒ manganites as a prominent example, 2,3 has substantially advanced over the last decade, 4 after it became clear that orbital degrees of freedom play a crucial role in these materials and have to be treated on equal footing with the electron spins, which has led to a rapidly developing field, orbital physics. 5 Due to the strong onsite Coulomb repulsion, charge fluctuations in the undoped parent compounds are almost entirely suppressed, and an adequate description of these strongly correlated insulators appears possible in terms of superexchange. 6 At orbital degeneracy the superexchange interactions have a rather rich structure, represented by the so-called spin-orbita...

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Section: B Spin Exchange Constants and Optical Intensitiesmentioning

confidence: 99%

Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

et al. 2005

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“…19,20,21,22,23,24) as well as the first-principles electronic structure calculations (Refs. 25,26,27,28,29,30,31,32,33). The problem is still far from being understood, and remains to be the subject of numerous contradictions and debates.…”

Section: Introductionmentioning

confidence: 99%

“…For example, at the HF level, using relatively simple model Hamiltonian, which is limited exclusively by the t 2g bands, we will be able to reproduce the main results of all-electron calculations. 27,29 Furthermore, due to the simplicity of the model Hamiltonian we can easily go beyond the HF approximation and include the correlation effects.…”

Section: Introductionmentioning

confidence: 99%

Lattice distortion and magnetism of3d−t2gperovskite oxides

Solovyev

2006

Phys. Rev. B

150

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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.

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“…Y T iO 3 is a ferromagnetic insulator with a Curie temperature of 30 K. It has been investigated theoretically [1][2][3] and by various experimental techniques such as Nuclear Magnetic Resonance [4], polarized and unpolarized neutron diffraction [5 and 6], inelastic neutron scattering [7], resonant X-ray scattering [8], soft X-ray linear dichroism [9], X-ray magnetic diffraction [10], Compton scattering [11] and elastic X-ray scattering [12]. Theoretical studies using unrestricted Hartree-Fock calculation and Density Functional Theory (DFT) with generalized gradient approximation predicted the wave functions of orbital ordering state of the 3d electrons of Ti atoms to be the linear combination of |yz and |xz orbitals of t 2g state [1].…”

Section: Introductionmentioning

confidence: 99%

Spin density in YTiO3 : I. Joint refinement of polarized neutron diffraction and magnetic x-ray diffraction data leading to insights into orbital ordering

et al. 2017

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Orbital ordering below 30 K was previously observed in the ferromagnetic Y T iO3 compound both by polarized neutron diffraction (PND) and X-ray magnetic diffraction (XMD). In this paper we report a new procedure for the joint refinement of a unique spin density model based on both PND and XMD data. The distribution of the unpaired 3d-electron of titanium is clearly seen on the magnetization density reconstructed by the Maximum of Entropy Method from the PND data collection at 5 K. The T i 3+ 3d orbital populations obtained by joint model refinement are discussed in terms of the orbital ordering scheme. Small but significant magnetic moments on apical oxygen O1 and yttrium atoms are found. The agreement between experimental and theoretical spin densities obtained using density functional theory is discussed.

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Orbital and magnetic orderings in localizedt2gsystems,YTiO3and

Cited by 118 publications

References 17 publications

Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

Lattice distortion and magnetism of3d−t2gperovskite oxides

Spin density in YTiO3 : I. Joint refinement of polarized neutron diffraction and magnetic x-ray diffraction data leading to insights into orbital ordering

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