Antiferromagnetic metallic state in doped manganites

Akimoto, T.; Maruyama, Yusei; Moritomo, Y.; Nakamura, Arao; Hirota, K.; Ohoyama, Kenji; Ohashi, Masayoshi

doi:10.1103/physrevb.57.r5594

Cited by 152 publications

(146 citation statements)

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“…17,18,37) In doped cases, our model exhibits a uniform ordering of d x 2 −y 2 orbital, 44) which has been suggested in recent experiments. 32,33) Our results show that a positive peak of T z (k = 0) grows as δ and has a maximum around δ = 0.5. This ordering presumably comes from an optimization of the kinetic energy in the 2D planes, since it is observed even in the non-interacting 2D model.…”

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confidence: 70%

Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites

Motome

Imada

1999

J. Phys. Soc. Jpn.

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Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in double-exchange models are examined for Mn perovskite oxides. We study the undoped Mott insulator as well as metal-insulator transitions by hole doping, and especially strong incoherence of ferromagnetic metal. We derive models where all the spins are fully polarized in two-dimensional planes as in the experimental indications, and investigate their ground-state properties by quantum Monte Carlo method. At half filling where the number of eg electron is one per site on average, the Coulomb interaction opens a Mott gap and induces a staggered orbital ordering. The opening of the Mott gap is, however, substantially slower than the mean-field results if the Jahn-Teller coupling is absent. The synergy between the strong correlation and the Jahn-Teller coupling largely enhances the Mott gap amplitude and reproduces realistic amplitudes and stabilization energy of the Jahn-Teller distortion. Upon doping, the orbital ordering stabilized by the Coulomb interaction is destroyed immediately. Toward the metal-insulator transition, the short-ranged orbital correlation is critically enhanced in metals, which should be related to strong incoherence of charge dynamics observed in experiments. Our model, moreover, exhibits a uniform ordering of d x 2 −y 2 orbital in a wide region of doping in agreement with experimental indications.KEYWORDS: perovskite manganites, double exchange model, Coulomb interaction, orbital degeneracy, JahnTeller coupling, Mott gap, orbital ordering, Jahn-Teller distortion, metal-insulator transition, ferromagnetic metal, incoherent charge dymanics, quantum Monte Carlo method 'Simple' double-exchange (DE) models, that is, models with a single non-interacting conduction band ferromagnetically coupled with localized spins, have been intensively studied 1, 2, 3, 4, 5) to understand physical properties of Mn perovskite oxides, especially collosal magnetoresistance. 6,7,8,9,10) These models have successfully explained several experimental aspects at least qualitatively; for instance, ferromagnetic metals in a hole-doped region with a transition to a paramagnetic state by raising temperature, and a large negative magnetoresistance near the transition. All these properties are due to the so-called double-exchange mechanism; doped holes tend to gain kinetic energy by aligning localized spins in parallel. 1, 2, 3)However, many open problems still remain. In particular, we focus here the following: (i) All the materials are the Mott insulator with a charge gap of order of eV at half filling (one e g electron per Mn site on average), where two-dimensional (2D) anisotropy realizes in spin and orbital orderings, and a cooperative JahnTeller (JT) distortion. 7,11,12,13,15,14) (ii) Upon doping of holes, a metal-insulator (MI) transition occurs from the anisotropic Mott insulator to an isotropic ferromagnetic metal through disordered ferromagnetic insulator. 6, 9, 16) (iii) In the ferromagnetic metallic state, charge dynamics shows strong inco...

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confidence: 70%

Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites

Motome

Imada

1999

J. Phys. Soc. Jpn.

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“…The observation of the A-phase at high doping concentration by Kawano et al [1] and Akimoto et al [2] seems to be a realization of case (ii) where symmetry is broken spontaneously. Our prediction for this phase are gapped orbital excitations with energy ∼ 3xt.…”

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confidence: 99%

“…Recently Kawano et al [1] and Akimoto et al [2] observed a new ground state in heavily doped 3D manganites Pr 1/2 Sr 1/2 MnO 3 and (La 1−z Nd z ) 1−x Sr x MnO 3 (e.g. x = 0.54 and z = 0.0 − 1.0), which surprisingly show the same antiferromagnetic A-structure known from the undoped LaMnO 3 where ferromagnetic (FM) planes are coupled antiferromagnetically.…”

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confidence: 99%

“…In the presence of hole doping, i.e. for less than one e g -electron per site, both the kinetic energy and the 3-site contributions in H int lead to propagation of the holes and to a frustration of the orbital order prefered by the orbital interaction H (2) int . When neglecting the 3-site terms our Hamiltonian is equivalent to the orbital model studied by Ishihara et al [11].…”

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confidence: 99%

“…x = 0.54 and z = 0.0 − 1.0), which surprisingly show the same antiferromagnetic A-structure known from the undoped LaMnO 3 where ferromagnetic (FM) planes are coupled antiferromagnetically. It was proposed that the occupied orbitals in this phase have x 2 -y 2 symmetry [2] in contrast to the alternating orbital order in LaMnO 3 [3], 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].…”

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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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55Mn NMR study in magnetically ordered state of perovskite manganites

Shimizu¹,

Boujelben²,

Cheikhrouhou³

2004

phys. stat. sol. (a)

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We present NMR results of Pr1−xSrxMnO3 and their lacunar samples, and also present results of the single crystalline layered manganites La1.2Sr1.8Mn2O7 and La1.2Sr1.6Ca0.2Mn2O7. In Pr1−xSrxMnO3, a single resonance line is observed for 0.27 ≤ x ≤ 0.4, which indicates the sample to be in a metallic state. In the Pr and Sr deficient samples, the NMR line arising from Mn2+ is observed. Introducing deficiency to Pr or Sr sites, a charge disproportionation of the type 2Mn3+→ Mn2+ + Mn4+ probably occurs. For La1.2Sr1.8Mn2O7 and La1.2Sr1.6Ca0.2Mn2O7 compounds, the NMR spectra under an external field of 1.5 T are broad and spread with several distinct lines in the frequency range 310–420 MHz. This is different from the results of La1−xSrxMnO3 and Pr1−xSrxMnO3 with a metallic state, where a single line has been observed. The distinct NMR lines are ascribed to Mn4+, Mn3+ and metallic phase, taking account of the results for La1−xSrxMnO3 and Pr1−xSrxMnO3. In addition to the NMR line from the metallic phase, the observation of Mn3+ and Mn4+ lines suggests a phase separation at low temperature in La1.2Sr1.8Mn2O7 and La1.2Sr1.6Ca0.2Mn2O7. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Antiferromagnetic metallic state in doped manganites

Cited by 152 publications

References 18 publications

Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites

Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites

Optical Conductivity in Doped Manganites with Planarx2-y2Orbita

55Mn NMR study in magnetically ordered state of perovskite manganites

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