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

Abstract

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.

Mott-Hubbard exciton in the optical conductivity ofYTiO3andSmTiO3

“…However, for a quantitative description of σ(ω) and for a reliable peak assignment one has to take all five 3d orbitals into account. 35,36,37,38 A. Multiplet assignment and Hubbard exciton…”

“…4,35,36,37,38,39,40,41,42,43,44,45,46 Therefore, the T dependence of the spectral weight is expected to reflect changes of the spin-spin correlations and/or of the orbital occupation. By considering the nearestneighbor spin-spin correlations, the absolute value of the spectral weight has been calculated for instance for LaMnO 3 [4,35,36] and LaSrMnO 4 [38]. These calculations yield a convincing description of the experimental results, the maximum difference is less than a factor of 2.…”

Ferroelectricity from coupled cooperative Jahn-Teller distortions and octahedral rotations in ordered Ruddlesden-Popper manganates

“…Solid solution LaSrMnO 4 (LSMO) is found to be a Mott-Hubbard insulator, 31,32 crystallizing in the centrosymmetric I4/mmm space group with antiferromagnetic ordering 33 depicted in Fig. 1a.…”

CaCrO3: An Anomalous Antiferromagnetic Metallic Oxide

“…We attribute the peak at 0.35 eV to excitations from the lower (LHB) to the upper (UHB) Hubbard band. Due to the high valence of Cr 4+ , the Cr Hubbard bands shift down towards the fully occupied O-2p band, whereas the pd hybridization between Cr and O bands pushes the LHB back upwards, reducing the effective Coulomb repulsion U eff [22] and admixing O-2p states to the LHB and the UHB in the same way as it was demonstrated for CrO 2 [15]. In insulating Sr 2 CrO 4 with Cr 4+ in a d 2 configuration [16], this LHB-UHB excitation was observed at 1.0 eV.…”