Optical conductivity of colossal-magnetoresistance compounds: Role of orbital degeneracy in the ferromagnetic phase

Horsch, Peter; Jaklič, J.; Mack, Frank

doi:10.1103/physrevb.59.6217

Cited by 81 publications

(78 citation statements)

References 51 publications

(72 reference statements)

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“…The purpose of this work is to present a theoretical analysis of the possible outcome of such experiments. The frequency dependence of σ(ω) is interesting since it is expected to reveal the orbital excitations in the doped x 2 -y 2 ordered phase, and may allow to distinguish between the orbital excitation and the Jahn-Teller polaron scenario.The evolution of x 2 -y 2 orbital order upon doping was also found in the study of planar models derived from the degenerate Kondo lattice model in the fully polarized FM phase [10]. Therefore we believe that this model is an appropriate starting point to investigate these questions.…”

mentioning

confidence: 91%

“…The evolution of x 2 -y 2 orbital order upon doping was also found in the study of planar models derived from the degenerate Kondo lattice model in the fully polarized FM phase [10]. Therefore we believe that this model is an appropriate starting point to investigate these questions.…”

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confidence: 91%

“…When neglecting the 3-site terms our Hamiltonian is equivalent to the orbital model studied by Ishihara et al [11]. Since the spectral shape of σ(ω) is only weakly affected by these terms [10], we do not consider them in this study. For the calculation of correlation functions (CF) we use a generalization of the exact diagonalization technique for finite temperature developed by Jaklič and Prelovšek [13].…”

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confidence: 99%

“…What remains is the orbital degree of freedom since there are two nearly degenerate e g orbitals per site. The large intra-atomic repulsive interactions prevent double occupancy of e g orbitals at the same site, which leads to a strong correlation problem and to nontrivial charge and orbital dynamics.For the saturated FM state the restriction to configurations without double occupancy leads to the orbital t-J model [11,10] (1)Here we use a and b (α and β) as orbital-pseudospin indices; whileā denotes the orthogonal e g orbital with respect to orbital a. A convenient basis is | ↑ = d x 2 −y 2 and | ↓ = d 3z 2 −r 2 .…”

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confidence: 99%

“…For the saturated FM state the restriction to configurations without double occupancy leads to the orbital t-J model [11,10] …”

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confidence: 99%

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Optical Conductivity in Doped Manganites with Planarx2-y2Orbita

Mack¹,

Horsch²

1999

Phys. Rev. Lett.

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We investigate a planar model for the ferromagnetic (FM) phase of manganites, which develops orbital order of eg electrons with x 2 -y 2 -symmetry at low temperature. The dynamic structure factor of orbital excitations and the optical conductivity σ(ω) are studied with help of a finite-temperature diagonalization method. Our calculations provide a theoretical prediction for σ(ω) for the 2D FM state and are of possible relevance for the recently found A-type phase of manganites at high doping which consists of FM layers coupled antiferromagnetically. In the x 2 -y 2 ordered regime σ(ω) shows both a Drude peak and a gapped incoherent absorption due to a gap in the orbital excitations. , and as a consequence the conduction should be quasi-2D and the magnetic coupling between the layers small. This broken symmetry state has to be distinguished from the orbital structure of the 3D FM state realized at lower doping, where orbital correlations are expected to be liquid like [4,5]. The latter FM state shows a highly anomalous form of the optical conductivity [6][7][8] with a large incoherent absorption extending up to ω ∼ 1eV. It has been argued that the anomalous incoherent absorption is related to the orbital degree of freedom [4,5,9] and characteristic for an orbital liquid [4,5]. However there are also alternative scenarios which attribute the incoherent feature to Jahn-Teller polarons [7,8]. Comparable optical studies in the high doping regime where the orbital ordered A-phase is the stable ground state have not yet been reported. The purpose of this work is to present a theoretical analysis of the possible outcome of such experiments. The frequency dependence of σ(ω) is interesting since it is expected to reveal the orbital excitations in the doped x 2 -y 2 ordered phase, and may allow to distinguish between the orbital excitation and the Jahn-Teller polaron scenario.The evolution of x 2 -y 2 orbital order upon doping was also found in the study of planar models derived from the degenerate Kondo lattice model in the fully polarized FM phase [10]. Therefore we believe that this model is an appropriate starting point to investigate these questions. The t 2g -spins of the Mn 3+ and Mn 4+ ions as well the e g -electron spins of Mn 3+ are aligned globally in the FM-phase and can be integrated out. What remains is the orbital degree of freedom since there are two nearly degenerate e g orbitals per site. The large intra-atomic repulsive interactions prevent double occupancy of e g orbitals at the same site, which leads to a strong correlation problem and to nontrivial charge and orbital dynamics.For the saturated FM state the restriction to configurations without double occupancy leads to the orbital t-J model [11,10] (1)Here we use a and b (α and β) as orbital-pseudospin indices; whileā denotes the orthogonal e g orbital with respect to orbital a. A convenient basis is | ↑ = d x 2 −y 2 and | ↓ = d 3z 2 −r 2 . The transfer matrix elements are then given bywhich allows for inter-orbital hopping in the xy-plane. The ∓-sign...

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confidence: 91%

mentioning

confidence: 91%

mentioning

confidence: 99%

mentioning

confidence: 99%

“…For the saturated FM state the restriction to configurations without double occupancy leads to the orbital t-J model [11,10] …”

mentioning

confidence: 99%

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Optical Conductivity in Doped Manganites with Planarx2-y2Orbita

Mack¹,

Horsch²

1999

Phys. Rev. Lett.

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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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Orbital Physics in Transition Metal Oxides: Magnetism and Optics

Horsch¹

2007

Handbook of Magnetism and Advanced Magnetic Materials

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Orbital degeneracy and strong correlations in transition‐metal oxides generate spins and orbitals as low‐energy degrees of freedom. Their interplay with both real‐charge motion and virtual‐charge excitations lead to a fascinating richness of spin–charge–orbital‐ordered phases in transition‐metal oxides, to striking phenomena like the colossal magnetoresistance in manganites, the switching of charge‐ordered phases into metallic phases by applied magnetic field, and many other fascinating effects. Central topics of this review are the spin‐orbital superexchange models which provide a concise description of spin and orbital order, effective spin interactions, spin waves, and orbiton excitations in doped and undoped Mott insulators. Particular emphasis is put on recent developments of spin‐orbital superexchange models, the relation of magnetism and optical spectral weights and their temperature dependence, t 2g systems with strong composite spin‐orbital fluctuations, the concepts of orbital liquid, orbital polarons, and the colossal magnetoresistance.

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Optical conductivity of colossal-magnetoresistance compounds: Role of orbital degeneracy in the ferromagnetic phase

Cited by 81 publications

References 51 publications

Optical Conductivity in Doped Manganites with Planarx2-y2Orbita

Optical Conductivity in Doped Manganites with Planarx2-y2Orbita

Toward a Theory of Orbiton Dispersion in LaMnO3

Orbital Physics in Transition Metal Oxides: Magnetism and Optics

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