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Register a new userComparative study of the electronic structures of Fe3O4 and Fe2SiO4
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The electronic properties of two spinels Fe$_3$O$_4$ and Fe$_2$SiO$_4$ are studied by the density functional theory. The local Coulomb repulsion $U$ and the Hunds exchange $J$ between the $3d$ electrons on iron are included. For $U=0$, both spinels are half-metals, with the minority $t_{2g}$ states at the Fermi level. Magnetite remains a metal in a cubic phase even at large values of $U$. The metal-insulator transition is induced by the $X_3$ phonon, which lowers the total energy and stabilizes the charge-orbital ordering. Fe$_2$SiO$_4$ transforms to a Mott insulating state for $U>2$ eV with a gap $Delta_gsim U$. The antiferromagnetic interactions induce the tetragonal distortion, which releases the geometrical frustration and stabilizes the long-range order. The differences of electronic structures in the high-symmetry cubic phases and the distorted low-symmetry phases of both spinels are discussed.
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The electronic structure of the transparent semiconductor In2O3 has been studied by angle resolved photoemission spectroscopy upon deposition of metallic indium and also tin on the surface of the semiconductor. By deposition of metallic indium on In2O3 (111) single crystals, we detected the formation of a free-electron like band of effective mass (0.38+-0.05) m0. At low coverages, metallic In shifts the Fermi level of In2O3 to higher energies and a new electronic state forms at the metal/semiconductor interface. This state of two-dimensional character (2D-electron gas) is completely responsible for the electrical conduction in In2O3 (111) at the surface region and has a band dispersion, which does not correspond to the previously found surface accumulation layers in this material. Despite the similarity of the electronic properties of In and Sn, a larger downward banding was observed by Sn coverage, which was not accompanied by the appearance of the surface state.
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We present parameter-free LDA+DMFT (local density approximation + dynamical mean field theory) results for the many-body spectra of cubic SrVO3 and orthorhombic CaVO3. Both systems are found to be strongly correlated metals, but not on the verge of a metal-insulator transition. In spite of the considerably smaller V-O-V bond angle in CaVO3 the LDA+DMFT spectra of the two systems for energies E<E_F are very similar, their quasiparticle parts being almost identical. The calculated spectrum for E>E_F shows more pronounced, albeit still small, differences. This is in contrast to earlier theoretical and experimental conclusions, but in good agreement with recent bulk-sensitive photoemission and x-ray absorption experiments.
The heat capacity of helical magnets Cu2OSeO3 and MnSi has been investigated at high pressures by the ac-calorimetric technique. Despite the differing nature of their magnetic moments, Cu2OSeO3 and MnSi demonstrate a surprising similarity in behavior of their magnetic and thermodynamic properties at the phase transition. Two characteristic features of the heat capacity at the phase transitions of both substances (peak and shoulder) behave also in a similar way at high pressures if analyzed as a function of temperature. This probably implies that the longitudinal spin fluctuations typical of weak itinerant magnets like MnSi contribute little to the phase transition. The shoulders of the heat capacity curves shrink with decreasing temperature suggesting that they arise from classical fluctuations. In case of MnSi the sharp peak and shoulder at the heat capacity disappear simultaneously probably signifying the existence of a tricritical point and confirming the fluctuation nature of the first order phase transition in MnSi as well as in Cu2OSeO3.
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