Effective Hamiltonian in manganites:mStudy of the orbital and spin structures

Ishihara, Sumio; Inoue, Jun–ichiro; Maekawa, Sadamichi

doi:10.1103/physrevb.55.8280

Cited by 243 publications

(232 citation statements)

References 39 publications

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“…6 At orbital degeneracy the superexchange interactions have a rather rich structure, represented by the so-called spin-orbital models, discovered three decades ago, 7,8 and extensively studied in recent years. [9][10][11][12][13][14][15][16][17][18] Although this field is already quite mature, and the first textbooks have already appeared, 3,4,19 it has been realized only recently that the magnetic and the optical properties of such correlated insulators with partly filled d orbitals are intimately related to each other, being just different experimental manifestations of the same underlying spin-orbital physics. 20,21 While it is clear that the low-energy effective superexchange Hamiltonian decides about the magnetic interactions, it is not immediately obvious that the high-energy optical excitations and their partial sum rules have the same roots and may be described by the superexchange as well.…”

Section: Superexchange and Optical Excitations At Orbital Degeneracymentioning

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: Superexchange and Optical Excitations At Orbital Degeneracymentioning

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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“…Surprisingly, the orbitally ordered insulating groundstate at x = 0 can be understood from a variety of starting points. For strong Hubbard interactions a spin-orbital t-J model has been derived and studied within mean-field approximation [26,27] leading to an orbitally ordered ground-state. The orbitally ordered groundstate also emerges from Hartree-Fock band-structure calculations considering the Mn-3d and O-2p orbitals [28,29].…”

Section: Introductionmentioning

confidence: 99%

Effect of disorder in an orbitally ordered Jahn-Teller insulator

2007

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We study a two dimensional, two-band double-exchange model for eg electrons coupled to JahnTeller distortions in the presence of quenched disorder using a recently developed Monte-Carlo technique. In the absence of disorder the half-filled system at low temperatures is an orbitally ordered ferromagnetic insulator with a staggered pattern of Jahn-Teller distortions. We examine the finite temperature transition to the orbitally disordered phase and uncover a qualitative difference between the intermediate and strongly coupled systems, including a thermally driven insulator to metal crossover in the former case. Long range orbital order is suppressed in the presence of disorder and the system displays a tendency towards metastable states consisting of orbitally disordered stripe-like structures enclosing orbitally ordered domains.

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“…and e 1 g t 6 2g configurations, respectively, the e g orbital is doubly degenerate and the t 2g orbital degree of freedom is quenched. It is widely recognized that the layered-type (A-type) antiferromagnetic (AF) structure in LaMnO 3 is understood from the view point of the anisotropic superexchange (SE) interaction under the directional order of orbital [1,2]. On the other hand, the spin structure in nickelates (R =La) is distinct from the A-type AF; the socalled "up-up-down-down"-type one where two Ni sites of "up" spins are followed by two sites of "down" spins along the principal axes in the cubic unit cell.…”

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confidence: 99%

“…We present the theoretical prescription for the combination effect of the GdFeO 3 -type distortion and the staggered OO. The Hamiltonian adopted here is the spinorbital model which is known to describe well the orbitally degenerate manganites [2]; H=H J +H H +H AF . The main term H J is the exchange interaction between intersite e g spins and orbitals schematically written as (3)].…”

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Distorted perovskite witheg1configuration as a frustrated spin system

et al. 2003

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The evolution of spin-and orbital-ordered states has been investigated for a series of insulating perovskites RMnO3 (R=La,Pr,Nd,...). RMnO3 with a large GdFeO3-type distortion is regarded as a frustrated spin system having ferromagnetic nearest-neighbor and antiferromagnetic (AF) nextnearest-neighbor (NNN) interactions within a MnO2 plane. The staggered orbital order associated with the GdFeO3-type distortion induces the anisotropic NNN interaction, and yields unique sinusoidal and up-up-down-down AF ordered states in the distorted perovskites with e 1 g configuration. and e 1 g t 6 2g configurations, respectively, the e g orbital is doubly degenerate and the t 2g orbital degree of freedom is quenched. It is widely recognized that the layered-type (A-type) antiferromagnetic (AF) structure in LaMnO 3 is understood from the view point of the anisotropic superexchange (SE) interaction under the directional order of orbital [1,2]. On the other hand, the spin structure in nickelates (R =La) is distinct from the A-type AF; the socalled "up-up-down-down"-type one where two Ni sites of "up" spins are followed by two sites of "down" spins along the principal axes in the cubic unit cell. Origin of this unusual magnetic order has been a long-standing question, as well as its relations to metal-insulator transition, orbital order (OO), and charge disproportionation [3][4][5]. Recently, a similar spin structure, i.e. the up-up-down-down order in a MnO 2 plane (E-type AF order in the Wollan-Koehler notation [6]), is found in a manganite, HoMnO 3 [7] with a significantly distorted perovskite structure. This has to be a bridge between the well-understood A-type AF in manganites and the unique magnetic ground state in nickelates.In this Letter, we examine systematically the magnetic and orbital structures in a series of RMnO 3 as a function of the ionic radius (r R ) of R. The most significant effect on the crystal structure by decreasing r R is an enhancement of the cooperative rotation of the MnO 6 octahedra (the GdFeO 3 -type distortion) characterized by the decrease of Mn-O-Mn bond angle φ. Let us first summarize in Fig. 1 the orbital (a) and spin (b) ordering temperatures (T OO and T N , respectively) on Mn sites of RMnO 3 as a function of φ, which is based on both the present and former studies [7][8][9][10]. Here, we adopt the φ at room temperature [11]. The T OO monotonically increases with decreasing r R , while the magnetic transition occurs from the A-type AF to the E-type one through the incommensurate structure. We argue that the combination of OO and next-nearest-neighbor (NNN) SE interaction brings about a nontrivial effect on the magnetic ground state in the systems with the orbital degeneracy and the large GdFeO 3 -type distortion. Microscopic calculation shows that the magnetism in this system is mapped onto the frustrated spin model which well reproduces the phase diagram of RMnO 3 .A series of RMnO 3 (R=La−Dy) crystals were grown by the floating zone method. We made powder x-ray diffraction measurements on th...

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Effective Hamiltonian in manganites:mStudy of the orbital and spin structures

Cited by 243 publications

References 39 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

Effect of disorder in an orbitally ordered Jahn-Teller insulator

Distorted perovskite witheg1configuration as a frustrated spin system

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