V. Eyert

The metal‐insulator transitions of VO2: A band theoretical approach

The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide are presented. In addition, the insulating phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal orbitals, which fall into the one-dimensional band, and the isotropically dispersing bands. Hybridization of both types of bands is weak. In the phase splitting of the band due to metal-metal dimerization and upshift of the bands due to increased p-d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal-insulator transition can be understood as a Peierls-like instability of the band in an embedding background of electrons. In the phase, the metal-insulator transition arises as a combined embedded Peierls-like and antiferromagnetic instability. The results for VO2 fit into the general scenario of an instability of the rutile-type transition-metal dioxides at the beginning of the d series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO2 and NbO2. In the compounds, the and bands can be completely separated, which leads to the observed metal-insulator transitions.

Basic Notions and Applications of the Augmented Spherical Wave Method

The basic ideas of modern band theory and the functionality of state of the art calculational schemes are illustrated with the augmented spherical wave (ASW) method. Our description includes a short review of the underlying theory as well as a derivation of the most important formulas. We explain some steps toward the computational implementation and discuss aspects of practical application. Finally, the capabilities offered by the ASW method are demonstrated by an investigation of the electronic structure of FeS2, which belongs to the increasingly exciting class of pyrite-type transition metal dichalcogenides.

Mott-Hubbard Metal-Insulator Transition in ParamagneticV2O3: AnLDA+DMFT(QMC)Study

The electronic properties of paramagnetic V2O3 are investigated by the computational scheme LDA+DMFT(QMC). This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic V2O3 and insulating (V0.962Cr0.038)2O3 we find a Mott-Hubbard transition at a Coulomb interaction U approximately 5 eV. The calculated spectrum is in very good agreement with experiment. Furthermore, the orbital occupation and the spin state S = 1 determined by us agree with recent polarization dependent x-ray-absorption experiments.

The vanadium Magnéli phases VnO2n‐1

To compare the metal-insulator transitions (MITs) of VO2 and V2O3 we analyze the relations between the structural and electronic properties of the vanadium Magneli phases. These materials set up the homologous series VnO2n-1 (3 ≤ n ≤ 9) and have crystal structures comprising typical dioxide-like and sesquioxide-like regions. As the MITs of the vanadium Magneli phases are accompanied by structural transformations, we are able to discuss the effects of characteristic changes in the local atomic environments. The systematic investigation of the transport properties is based on a new and unifying description of the crystal structures of the Magneli phases including VO2 and V2O3. Our results lead to a comprehensive understanding of the MITs in the Magneli class and shed new light on the role of particular electronic states for the MIT of V2O3.

Prominent quasiparticle peak in the photoemission spectrum of the metallic phase of V2O3.

We present the first observation of a prominent quasiparticle peak in the photoemission spectrum of the metallic phase of V2O3 and report new spectral calculations that combine the local-density approximation with the dynamical mean-field theory (using quantum Monte Carlo simulations) to show the development of such a distinct peak with decreasing temperature. The experimental peak width and weight are significantly larger than in the theory.

Embedded Peierls instability and the electronic structure of MoO2

Molybdenum dioxide crystallizes in a monoclinic structure which deviates only slightly from the rutile structure and is characteristic of several early transition metal dioxides. We present results of all-electron electronic structure calculations based on density functional theory within the local density approximation and using the augmented spherical wave method. The electronic properties of MoO2 are dominated by strong hybridization of O 2p and crystal-field-split Mo 4d states with bands near the Fermi energy originating almost exclusively from Mo 4d t2g orbitals. In additional calculations for a hypothetical high-symmetry rutile structure these bands separate into quasi-one-dimensional d∥ states pointing along the rutile c-axis and the rather isotropically dispersing π* bands. On going to the monoclinic structure, the characteristic metal-metal dimerization causes strong splitting of the d∥ bands into bonding and antibonding branches which embrace the nearly inert π* bands at EF. As a consequence, large portions of the Fermi surface are removed. According to our calculations the monoclinic structure of MoO2 thus results from a Peierls-type instability of the d∥ bands in the presence of, but still rather unaffected by, an embedding background of π* states. Our work has strong implications for the current understanding of VO2 and the striking metal-insulator/structural transition displayed by this material.

Two-dimensional electron liquid state at LaAlO 3 -SrTiO 3 interfaces

Using tunneling spectroscopy we have measured the spectral density of states of the mobile, two-dimensional electron system generated at the LaAlO3-SrTiO3 interface. As shown by the density of states the interface electron system differs qualitatively, first, from the electron systems of the materials defining the interface and, second, from the two-dimensional electron gases formed at interfaces between conventional semiconductors.

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Abstract Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic ( T N = 24 K ) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t2g orbitals of Cr3+ and that spin polarization is predicted with 3 μ B per Cr3+. The structural characterizations of CuCr1−xMgxO2 reveal a very limited range of Mg2+ substitution for Cr3+ in this series. As soon as x = 0.02 , a maximum of 1% Cr ions are substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01 . This explains the saturation of the Mg2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01 . Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr4+ “holes”, created by the Mg2+substitution, in the matrix of high spin Cr3+ ( S = 3 / 2 ) are responsible for the transport properties of these compounds.

Electronic structure of paramagnetic V2O3: Strongly correlated metallic and Mott insulating phase

The computation scheme merging the local density approximation and the dynamical mean-field theory (DMFT) is employed to calculate spectra both below and above the Fermi energy and spin and orbital occupations in the correlated paramagnetic metallic and Mott insulating phase of

Origin of magnetic interactions in Ca 3 Co 2 O 6

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