Defect states and excitations in a Mott insulator with orbital degrees of freedom: Mott-Hubbard gap versus optical and transport gaps in doped systems (original) (raw)
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Defects, Disorder, and Strong Electron Correlations in Orbital Degenerate, Doped Mott Insulators
Physical review letters, 2015
We elucidate the effects of defect disorder and e-e interaction on the spectral density of the defect states emerging in the Mott-Hubbard gap of doped transition-metal oxides, such as Y(1-x)Ca(x)VO(3). A soft gap of kinetic origin develops in the defect band and survives defect disorder for e-e interaction strengths comparable to the defect potential and hopping integral values above a doping dependent threshold; otherwise only a pseudogap persists. These two regimes naturally emerge in the statistical distribution of gaps among different defect realizations, which turns out to be of Weibull type. Its shape parameter k determines the exponent of the power-law dependence of the density of states at the chemical potential (k-1) and hence distinguishes between the soft gap (k≥2) and the pseudogap (k<2) regimes. Both k and the effective gap scale with the hopping integral and the e-e interaction in a wide doping range. The motion of doped holes is confined by the closest defect poten...
Correlated materials have been studied extensively using photoemission spectroscopy. Their optical properties are instead much less explored. Here we present calculations of the optical absorption spectrum of vanadium dioxide (VO 2 ) in the framework of the Bethe-Salpeter equation (BSE) of many-body perturbation theory. In order to deal with localized electrons we go beyond the standard BSE implementation and extend it to correlated insulators. We show that it is not enough to describe the spectra on the basis of independent electron-hole pairs, even when the electron and hole are separately well described by state-of-the-art one-body Green's functions. Crystal local-field effects are crucial to explain the experimental findings, even qualitatively, and excitonic effects strongly modify the spectra, especially at their onset. In this context, as highighted by the analysis of the BSE results, the quasi-one-dimensional nature of the vanadium-dimer chains plays a prominent role.
Modeling charged defects inside density functional theory band gaps
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2014
Density functional theory (DFT) has emerged as an important tool to probe microscopic behavior in materials. The fundamental band gap defines the energy scale for charge transition energy levels of point defects in ionic and covalent materials. The eigenvalue gap between occupied and unoccupied states in conventional DFT, the Kohn-Sham gap, is often half or less of the experimental band gap, seemingly precluding quantitative studies of charged defects. Applying explicit and rigorous control of charge boundary conditions in supercells, we find that calculations of defect energy levels derived from total energy differences give accurate predictions of charge transition energy levels in Si and GaAs, unhampered by a band gap problem. The GaAs system provides a good theoretical laboratory for investigating band gap effects in defect level calculations: depending on the functional and pseudopotential, the Kohn-Sham gap can be as large as 1.1 eV or as small as 0.1 eV. We find that the effective defect band gap, the computed range in defect levels, is mostly insensitive to the Kohn-Sham gap, demonstrating it is often possible to use conventional DFT for quantitative studies of defect chemistry governing interesting materials behavior in semiconductors and oxides despite a band gap problem.
2011
The Green's functions theory and the bond orbital model are used as a basis for calculations of the electron structure of local defects-specifically, vacancies and their compensated states in III-VI semi conductors. The energy levels in the band gap are established, and the changes induced in the electron den sities in the GaS, GaSe, and InSe semiconductors by anion and cation vacancies and their compensated states are calculated. It is established that, if a vacancy is compensated by an atom of an element from the same subgroup with the same tetrahedral coordination and if the ionic radius of the compensating atom is smaller than that of the substituted atom, the local levels formed by the vacancy completely disappear. It is shown that this mechanism of compensation of vacancies provides a means not only for recovering the parameters of the crystal, but for improving the characteristics of the crystal as well.
Vanadium Dioxide: A Peierls-Mott Insulator Stable against Disorder
Physical Review Letters, 2012
Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we develop and carry out state of the art linear scaling DFT calculations refined with non-local dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M1 phase, and furthermore survives a moderate degree of disorder.
Chemical Physics Letters, 2014
ABSTRACT The metal-insulator transition of VO2 so far has evaded an accurate description by density functional theory. The screened hybrid functional of Heyd, Scuseria and Ernzerhof leads to reasonable solutions for both the low-temperature monoclinic and high-temperature rutile phases only if spin polarization is excluded from the calculations. We explore whether a satisfactory agreement with experiment can be achieved by tuning the fraction of Hartree Fock exchange (alpha) in the density functional. It is found that two branches of locally stable solutions exist for the rutile phase for 12.5% <= alpha <= 20%. One is metallic and has the correct stability as compared to the monoclinic phase, the other is insulating with lower energy than the metallic branch. We discuss these observations based on the V 3d orbital occupations and conclude that alpha = 10% is the best possible choice for spin-polarized VO2 calculations. (c) 2014 Elsevier B.V. All rights reserved.
Mixed-valence states in narrow-gap IV-VI semiconductors with rare-earth ions
Physical Review B, 1999
The mechanism of valence-state formation in the IV-VI alloys doped with rare-earth impurities is analyzed with respect to certain properties of narrow-gap semiconductors. The mean-field approximation in the slaveboson representation is used to account for the strong electron correlation at the impurity. The energy, width, and filling factor of the impurity level are calculated as a function of both the band gap and position of the chemical potential. We also calculate the temperature dependence of the magnetic susceptibility of an impurity. It is shown that mixing to coupled conduction-and valence-band states makes the properties of rare-earth dopants sensitive to the band gap and Fermi level in the narrow-gap semiconductor host. ͓S0163-1829͑99͒08723-8͔
Theoretical investigation of the electrical and optical activity of vanadium in GaAs
Physical review. B, Condensed matter, 1986
%e study the excitation and ionization processes at the isolated substitutional vanadium impurity in GaAs. The electronic structure is solved through the spin-restricted version of the multiplescattering Xa method to obtain the relevant mean-field energies, and correlation effects are evaluated through the Fazzio-Caldas-Zunger multiplet approach. The similar systems GaP:V, InP:V are also investigated. Our results for GaAs:V point to the occurrence of an acceptor level atE ,-0. 16 eV, the donor level appearing very close to or within the valence band. The V-related midgap acceptor should then be related to some complex defect involving vanadium. We also suggest that V2+ in these compounds is present in the low-spin ground state 2E.
Localized in-gap state in a single-electron doped Mott insulator
Physical Review B, 2014
Motivated by the recent atomic-scale scanning tunneling microscope (STM) observation for a spatially localized in-gap state in an electron doped Mott insulator, we evaluate the local electronic state of the Hubbard model on the square lattice using the cluster perturbation theory. An in-gap state is found to exist below the upper Hubbard band around the dopant lattice site, which is consistent with the STM measurements. The emergence of this local in-gap state is accompanied with a rapid reduction of the double occupancy of electrons. A similar in-gap state is also found to exist on the triangular lattice. These results suggest that the in-gap state is an inherent feature of Mott insulators independent of the lattice structure.