Electronic origin of magnetic and orbital ordering in insulating LaMnO3

Feiner, L. F.; Oleś, Andrzej M.

doi:10.1103/physrevb.59.3295

Cited by 233 publications

(371 citation statements)

References 31 publications

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“…This result is also consistent 104 with the earlier analysis of Ref. 13. Due to the strong dependence of the exchange constants J ab and J c on the angle , one can exclude the OO of alternating directional ͑3x 2 − r 2 ͒ / ͑3y 2 − r 2 ͒ orbitals, obtained with = 120°in Eqs.…”

Section: Acknowledgmentssupporting

confidence: 92%

“…The spin-orbital models have been derived before in several cases, and we refer for these derivations to the original literature. 11,13,15,16 6 at U ӷ t-thus the resulting superexchange interactions will be called U terms. The essential difference which makes it necessary to analyze the excitation energies in each case separately is caused by the existence of several different excitations.…”

Section: ͑21͒mentioning

confidence: 99%

“…The orbital fluctuations are then quenched by the combined effect of the orbital part of the e g superexchange in Eqs. ͑4.2͒-͑4.6͒ and the orbital interactions induced by the JT effect, 13 …”

Section: B Spin Exchange Constants and Optical Intensitiesmentioning

confidence: 99%

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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: Acknowledgmentssupporting

confidence: 92%

Section: ͑21͒mentioning

confidence: 99%

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

et al. 2005

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“…However, in real eg systems such quantum phenomena are usually quenched by finite JH which induces a structural phase transition and thus helps to stabilize a particular ordering of occupied orbitals which supports A-type AF order, as observed when degenerate orbitals are filled either by one hole (KCuF3) [9], or by one electron (LaMnO3) [10]. The coupling to the lattice due to the Jahn-Teller (JT) effect also helps to stabilize the orbital ordering, and quantitative models of the structural transition have to include both these effects [10].The essential feature of the SE described by spin-orbital models is the frustration of magnetic interactions: the FM terms occur next to the AF ones, and it depends on the physical 1 ) …”

mentioning

confidence: 99%

“…Both terms are given explicitly in Ref. [10]. Both the cuprate model (4) and the eg term in the manganite model describe strongly frustrated SE interactions, which take a universal form in the limit of JH → 0,…”

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confidence: 99%

Orbital ordering and orbital fluctuations in transition metal oxides

Oleś

2003

Physica Status Solidi (b)

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We summarize some characteristic features of the frustrated magnetic interactions in spin-orbital models adequate for cubic transition metal oxides with orbital degeneracy. A generic tendency towards dimerization, found already in the degenerate Hubbard model, is confirmed for t 2g but not for eg systems. In the t 2g case the quantum orbital fluctuations are more pronounced and contribute to a stronger competition between different magnetic and orbital states. Therefore the orbital liquid states exist in some undoped t 2g systems, while in the manganites such states can be triggered only by doping. The physical properties of transition metal oxides are dominated by large on-site Coulomb interactions ∝ U which suppress charge fluctuations. Therefore, such systems are either Mott or charge-transfer insulators, and the metallic behavior might occur only as a consequence of doping. Here we will discuss first the undoped systems with localized d electrons which interact by effective superexchange (SE) interactions. An interesting situation occurs when d electrons occupy partly degenerate orbital states, and one has to consider orbital degrees of freedom in the SE at equal footing with electron spins [1]. Competition between different states is then possible, holes may couple to orbital excitations [2], and the quantum effects are enhanced already in undoped systems [3]. The first models of SE in such situations were proposed almost three decades ago [4], either by considering the degenerate Hubbard model [5,6], or for realistic situations encountered in cuprates (KCuF3 and K2CuF4) and in V2O3 [7]. Then it was realized that the SE which is usually antiferromagnetic (AF) might become ferromagnetic (FM) when Hund's exchange interaction JH is finite, but only in recent years the phenomena which originate from the orbital physics are investigated in a more systematic way.The SE which involves the orbital degrees of freedom is described by the so-called spinorbital models [8], and is typically highly frustrated on a cubic lattice where it might even lead to the collapse of magnetic long-range order by strong spin or orbital fluctuations [3]. However, in real eg systems such quantum phenomena are usually quenched by finite JH which induces a structural phase transition and thus helps to stabilize a particular ordering of occupied orbitals which supports A-type AF order, as observed when degenerate orbitals are filled either by one hole (KCuF3) [9], or by one electron (LaMnO3) [10]. The coupling to the lattice due to the Jahn-Teller (JT) effect also helps to stabilize the orbital ordering, and quantitative models of the structural transition have to include both these effects [10].The essential feature of the SE described by spin-orbital models is the frustration of magnetic interactions: the FM terms occur next to the AF ones, and it depends on the physical 1 )

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Toward a Theory of Orbiton Dispersion in LaMnO3

Perebeinos

Allen

1999

phys. stat. sol. (b)

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At 750K, LaMnO3 has a cooperative Jahn-Teller (JT) distortion, with Mn atoms in distorted oxygen octahedra. This lifts the degeneracy of the singly-occupied eg orbitals of the Mn 3+ ions, which then become orbitally ordered. We use a minimal model to describe the ordered phase at T = 0. The on-site Coulomb repulsion U is set to infinity. There are two electronic orbitals and three oxygen vibrational coordinates per unit cell. In addition to spin excitations and phonons, the model has electronic excitations consisting of mis-orienting orbitals on Mn ions. Neglecting coupling to the oxygen displacements, the gap to such excitations is 2∆ = 16g 2 /K where g is the electron-phonon coupling and K is the oxygen spring constant. When static oxygen displacements are coupled, this excitation becomes a self-trapped exciton with energy ∆, half the JT gap. Adding dynamic oxygen displacements in one-phonon approximation introduces dispersion to both the (previously Einsteinlike) phonons and the orbital excitons ("orbitons"). One of the phonon branches has zero frequency at k = (0, 0, π). This is the Goldstone mode of the JT broken symmetry.

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Electronic origin of magnetic and orbital ordering in insulating LaMnO3

Cited by 233 publications

References 31 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

Orbital ordering and orbital fluctuations in transition metal oxides

Toward a Theory of Orbiton Dispersion in LaMnO3

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