Optical response of metallic and insulating VO2 calculated with the LDA approach

We calculated the optical response of metallic and insulating VO2 using the LDA approach. The band structure calculation was based in the full-potential linear-muffin-tin method. The imaginary part of the dielectric function e2(w) is related to the different optical transitions. The Drude tail in the calculation of the metallic phase corresponds to intra-band d-d transitions. The calculation in the insulating phase is characterized by the transitions to the d||* band. The low frequency features, 0.0-5.0 eV, correspond to V 3d-V 3d transitions, whereas the high frequency structures, 5.0-12 eV, are related to O 2p-V 3d transitions. The calculation helps to explain the imaginary part of the dielectric function e2(w), as well as the electron-energy-loss and reflectance spectra. The results reproduce not only the energy position and relative intensity of the features in the spectra, but also the main changes across the metal-insulator transition and the polarization dependence. The main difference is a shift of about 0.6 eV in the calculation of the insulating phase. This discrepancy arises because the LDA calculation underestimates the value of the band gap

Importance of the V 3d-O 2p hybridization in the Mott-Hubbard material V2O3

We studied the changes in the electronic structure of V2O3 using a cluster model. The calculations included fluctuations from the coherent band in the metallic phase, and non-local Mott-Hubbard fluctuations in the insulating phase. The incoherent structure is mostly related to the usual ligand screening channel (3d2L). The coherent peak in the metallic phase corresponds to coherent band fluctuations (3d2C). The non-local screened state in the insulating phase (3d2D) appears at higher energies, opening the band gap. The photon energy dependence of the spectra is mostly due to the relative V 3d and O 2p cross sections. The present model reproduces also the observed changes in the V 1s core-level spectra. The above results suggest that the Mott-Hubbard transition in V2O3 requires a multi-band model.Comment: 4 pages, 4 figure

Materials Design using Correlated Oxides: Optical Properties of Vanadium Dioxide

Materials with strong electronic Coulomb interactions play an increasing role in modern materials applications. "Thermochromic" systems, which exhibit thermally induced changes in their optical response, provide a particularly interesting case. The optical switching associated with the metal-insulator transition of vanadium dioxide (VO2), for example, has been proposed for use in "intelligent" windows, which selectively filter radiative heat in hot weather conditions. In this work, we develop the theoretical tools for describing such a behavior. Using a novel scheme for the calculation of the optical conductivity of correlated materials, we obtain quantitative agreement with experiments for both phases of VO2. On the example of an optimized energy-saving window setup, we further demonstrate that theoretical materials design has now come into reach, even for the particularly challenging class of correlated electron systems.Comment: 4+x pages, 2 figure

Systematic tight-binding analysis of ARPES spectra of transition-metal oxides

We have performed systematic tight-binding (TB) analyses of the angle-resolved photoemission spectroscopy (ARPES) spectra of transition-metal (TM) oxides A$M$O$_3$ ($M=$ Ti, V, Mn, and Fe) with the perovskite-type structure and compared the obtained parameters with those obtained from configuration-interaction (CI) cluster-model analyses of photoemission spectra. The values of $\epsilon_d-\epsilon_p$ from ARPES are found to be similar to the charge-transfer energy $\Delta$ from O $2p$ orbitals to empty TM 3d orbitals and much larger than $\Delta-U/2$ ($U$: on-site Coulomb energy) expected for Mott-Hubbard-type compounds including SrVO$_3$. $\epsilon_d-\epsilon_p$ values from {\it ab initio} band-structure calculations show similar behaviors to those from ARPES. The values of the $p-d$ transfer integrals to describe the global electronic structure are found to be similar in all the estimates, whereas additional narrowing beyond the TB description occurs in the ARPES spectra of the $d$ band.Comment: 5 pages, 3 figure

Compensation temperatures and exchange bias in La1.5Ca0.5CoIrO6

We report on the study of magnetic properties of the La1.5Ca0.5CoIrO6 double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature the material exhibit exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of Ir4+ ions located between the antiferromagnetically coupled Co ions.Comment: 10 pages, 8 figures and supplemental materia

Structural and functional characterization of (110)-oriented epitaxial La2/3Ca1/3MnO3 electrodes and SrTiO3 tunnel barriers

La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1¿10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic properties are compared to those of (001) LCMO films grown simultaneously on (001) STO substrates It is found that (110) LCMO films present a higher Curie temperature (TC) and a weaker decay of magnetization when approaching TC than their (001) LCMO counterparts. These improved films have been subsequently covered by nanometric STO layers. Conducting atomic-force experiments have shown that STO layers, as thin as 0.8 nm, grown on top of the (110) LCMO electrode, display good insulating properties. We will show that the electric conductance across (110) STO layers, exponentially depending on the barrier thickness, is tunnel-like. The barrier height in STO (110) is found to be similar to that of STO (001). These results show that the (110) LCMO electrodes can be better electrodes than (001) LCMO for magnetic tunnel junctions, and that (110) STO are suitable insulating barriers

Partial contributions to the valence band of MO

We studied the partial contributions to the valence band spectra of MoO2, RuO2 and Rh2O3. The experimental technique of choice was the Cooper minimum method to X-ray photoemission spectroscopy. The spectra were interpreted with an extended cluster model and compared to usual band structure calculations. We conclude that, for the correct description of the valence band, one must include charge transfer, final-state and many-body effects in the description of the electronic structure of these compounds, which can also be extended to other systems