Temperature effects in a spin‐orbital model for manganites

Daghofer, Maria; Neuber, Danilo R.; Oleś, Andrzej M.; Linden, Wolfgang von der

doi:10.1002/pssb.200562464

Cited by 8 publications

(12 citation statements)

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“…1b. A further increase in temperature promotes excitations to |x orbitals [24], and this experimental finding is also reproduced by the present model [34]. It is interesting to remark that the G-AF order is stabilized in this parameter regime by the core spin AF superexchange J , while the e g part of the superexchange is FM, as the excitations from occupied |z to unoccupied |x orbitals dominate, with the hopping element larger by √ 3 than the one between two |z orbitals, which would give instead the AF coupling.…”

Section: Numerical Resultssupporting

confidence: 85%

“…2 -the magnetic order is lost first in the range of temperature βt ≈ 50, while the AO order is more robust and starts to weaken only above βt ≈ 10. While at κ = 0 the orbital correlations decrease somewhat faster with increasing temperature [34], they are still quite pronounced when the magnetic order is lost. This shows that FM correlations would be typically lost at a lower temperature (T ≈ 150 K) than the orbital correlations (see Fig.…”

Section: Numerical Resultsmentioning

confidence: 95%

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Orbital Correlations in Monolayer Manganites - From Spin t-J Model to Orbital t-J Model

Daghofer

Oleś

2007

Acta Phys. Pol. A

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On the example of monolayer manganites we show that the theoretical ideas developed long ago along the derivation of the spin t−J model from the Hubbard model are nowadays very helpful in strongly correlated oxides with partly filled degenerate orbitals. We analyze a realistic orbital t−J model for eg electrons in La1−xSr1+xMnO4 monolayer manganites, and discuss the evolution of spin and orbital correlations under increasing doping by performing exact diagonalization of finite clusters, with electronic kinetic energy determined self-consistently at a finite temperature by classical Monte Carlo for t2g spins. Several experimental results are reproduced. Spin t−J modelA new perspective to treat strongly correlated electrons was opened by the derivation of the spin t−J model three decades ago [1]. We argue that this derivation opened new routes to understand the complex magnetic and superconducting instabilities in the Hubbard model, and proved to be a very useful paradigm in strongly correlated electron systems. At that time it was already realized that the Hubbard model,where a † iσ is an electron creation operator in spin state σ =↑, ↓ at site i and U is local Coulomb interaction, is perfectly designed to describe electron localization (497)

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Section: Numerical Resultssupporting

confidence: 85%

Section: Numerical Resultsmentioning

confidence: 95%

Orbital Correlations in Monolayer Manganites - From Spin t-J Model to Orbital t-J Model

Daghofer

Oleś

2007

Acta Phys. Pol. A

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“…These data are complemented by the orbital correlations (not shown) which correspond closely to the ground state results. Both spin and orbital correlations weaken with rising temperature, as discussed elsewhere, 45 but for the The situation is somewhat different for the C-AF phase obtained by exact diagonalization at T = 0 for E z −0.2t (see Fig. 2).…”

Section: Numerical Results For Monolayer Manganites a Undoped 2mentioning

confidence: 64%

Doping dependence of spin and orbital correlations in layered manganites

et al. 2006

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We investigate the interplay between spin and orbital correlations in monolayer and bilayer manganites using an effective spin-orbital t-J model which treats explicitly the eg orbital degrees of freedom coupled to classical t2g spins. Using finite clusters with periodic boundary conditions, the orbital many-body problem is solved by exact diagonalization, either by optimizing spin configuration at zero temperature, or by using classical Monte-Carlo for the spin subsystem at finite temperature. In undoped two-dimensional clusters, a complementary behavior of orbital and spin correlations is found -the ferromagnetic spin order coexists with alternating orbital order, while the antiferromagnetic spin order, triggered by t2g spin superexchange, coexists with ferro-orbital order. With finite crystal field term, we introduce a realistic model for La1−xSr1+xMnO4, describing a gradual change from predominantly out-of-plane 3z 2 − r 2 to in-plane x 2 − y 2 orbital occupation under increasing doping. The present electronic model is sufficient to explain the stability of the CE phase in monolayer manganites at doping x = 0.5, and also yields the C-type antiferromagnetic phase found in Nd1−xSr1+xMnO4 at high doping. Also in bilayer manganites magnetic phases and the accompanying orbital order change with increasing doping. Here the model predicts C-AF and G-AF phases at high doping x > 0.75, as found experimentally in La2−2xSr1+2xMn2O7.

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“…This assignment utilizes the points raised above, i.e., the pronounced anisotropy, the difference in spectral weight, and the strong temperature dependence of the spectral weight. Using transmittance measurements, we determine the optical gap ⌬ a = 0.4-0.45 eV at 15 K and 0.1-0.2 eV at 300 K. Our data indicate that the anomalous shrinkage of the c-axis lattice parameter with increasing temperature cannot be attributed to a thermal occupation of local crystal-field levels 22,30,31 but rather to a thermal population of the UHB.…”

Section: Introductionmentioning

confidence: 89%

Mott-Hubbard versus charge-transfer behavior inLaSrMnO4studied via optical conductivity

et al. 2008

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Using spectroscopic ellipsometry, we study the optical conductivity ͑͒ of insulating LaSrMnO 4 in the energy range of 0.75-5.8 eV from 15 to 330 K. The layered structure gives rise to a pronounced anisotropy. A multipeak structure is observed in 1 a ͑͒ ͑ϳ2, 3.5, 4.5, 4.9, and 5.5 eV͒, while only one peak is present at 5.6 eV in 1 c ͑͒. We employ a local multiplet calculation and obtain ͑i͒ an excellent description of the optical data, ͑ii͒ a detailed peak assignment in terms of the multiplet splitting of Mott-Hubbard and charge-transfer absorption bands, and ͑iii͒ effective parameters of the electronic structure, e.g., the on-site Coulomb repulsion U eff = 2.2 eV, the in-plane charge-transfer energy ⌬ a = 4.5 eV, and the crystal-field parameters for the d 4 configuration ͑10Dq = 1.2 eV, ⌬ eg = 1.4 eV, and ⌬ t2g = 0.2 eV͒. The spectral weight of the lowest absorption feature ͑at 1 -2 eV͒ changes by a factor of 2 as a function of temperature, which can be attributed to the change of the nearest-neighbor spin-spin correlation function across the Néel temperature T N = 133 K. Interpreting LaSrMnO 4 effectively as a Mott-Hubbard insulator naturally explains this strong temperature dependence, the relative weight of the different absorption peaks, and the pronounced anisotropy. By means of transmittance measurements, we determine the onset of the optical gap ⌬ opt a = 0.4-0.45 eV at 15 K and 0.1-0.2 eV at 300 K.Our data show that the crystal-field splitting is too large to explain the anomalous temperature dependence of the c-axis lattice parameter by thermal occupation of excited crystal-field levels. Alternatively, we propose that a thermal population of the upper Hubbard band gives rise to the shrinkage of the c-axis lattice parameter.

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Temperature effects in a spin‐orbital model for manganites

Cited by 8 publications

References 10 publications

Orbital Correlations in Monolayer Manganites - From Spin t-J Model to Orbital t-J Model

Orbital Correlations in Monolayer Manganites - From Spin t-J Model to Orbital t-J Model

Doping dependence of spin and orbital correlations in layered manganites

Mott-Hubbard versus charge-transfer behavior inLaSrMnO4studied via optical conductivity

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