26 research outputs found
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 AO ( 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 from ARPES are found to be similar to the
charge-transfer energy from O orbitals to empty TM 3d orbitals
and much larger than (: on-site Coulomb energy) expected for
Mott-Hubbard-type compounds including SrVO. values
from {\it ab initio} band-structure calculations show similar behaviors to
those from ARPES. The values of the 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 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