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Alex Zunger

Bio: Alex Zunger is an academic researcher from University of Colorado Boulder. The author has contributed to research in topics: Band gap & Quantum dot. The author has an hindex of 128, co-authored 826 publications receiving 78798 citations. Previous affiliations of Alex Zunger include Tel Aviv University & University of Wisconsin-Madison.


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TL;DR: In this paper, Magri and Zunger showed that the band edges and band gaps of superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the InAs barrier thickness when the number of InAs layers exceeds 5$.
Abstract: The band edges and band gaps of ${(\mathrm{In}\mathrm{As})}_{n}∕{(\mathrm{Ga}\mathrm{Sb})}_{m}$ $(n,m=1,20)$ superlattices have been theoretically studied through the plane-wave empirical pseudopotential method for different situations: (i) different substrates, GaSb and InAs; (ii) different point group symmetries, ${C}_{2v}$ and ${D}_{2d}$; and (iii) different growth directions, (001) and (110). We find that (a) the band gaps for the (001) ${C}_{2v}$ superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the GaSb barrier thickness when the number of ${(\mathrm{In}\mathrm{As})}_{n}$ layers exceed $n=5$; (b) substrate effects: compared with the GaSb substrate, the different strain field generated by the InAs substrate leads to a larger variation of the band gaps for the (001) ${C}_{2v}$ superlattices as a function of the InAs well thickness; (c) effect of the type of interfacial bonds: the In-Sb bonds at the interfaces of the (001) ${D}_{2d}$ superlattices partially pin the band edge states, reducing the influence of the confinement effects on electrons and holes, and lowering the band gaps as compared to the (001) ${C}_{2v}$ case. The valence band maximum of the (001) ${D}_{2d}$ superlattices with Ga-As bonds at the interfaces are shifted down, increasing the band gaps as compared to the (001) ${C}_{2v}$ case; (d) effect of layer orientation: the presence of In-Sb bonds at both interfaces of the (110) superlattices pin the band edge states and reduces the band gaps, as compared to the (001) ${C}_{2v}$ case. An anticrossing between the electron and hole levels in the (110) superlattices, for thin GaSb and thick InAs layers, leads to an increase of the band gaps, as a function of the InAs thickness; (e) superlattices vs random alloys: the comparison between the band edges and band gaps of the superlattices on a GaSb substrate and those for random alloys, lattice matched to a GaSb substrate, as a function of the In composition, shows that the random alloys present almost always higher band gaps and give a clear indication of the effect of superlattice's ordering and period on the behavior of the band gaps and band edges. Inclusion of interfacial interdiffusion, using the approach of Magri and Zunger [Phys. Rev. B 65, 165302 (2002)], is shown to significantly increase the band gaps relative to the predictions for abrupt superlattices, bringing the results closer to experiment. It is noteworthy that $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model fit instead measured gaps corresponding to interdiffused interfaces using a chemically abrupt model.

34 citations

Journal ArticleDOI
TL;DR: In this paper, the authors deal with a class of degenerate but gapped metals with the Fermi level inside the conduction band or valence band, yet there is an internal band gap between the principal band edges.
Abstract: This paper deals with a significant family of compounds predicted by simplistic electronic structure theory to be metals but are, in fact, insulators. This false metallic state has been traditionally attributed in the literature to reflect the absence of proper treatment of electron-electron correlation (“Mott insulators”) whereas, in fact, even mean-field like density functional theory describes the insulating phase correctly if the restrictions posed on the simplistic theory are avoided. Such unwarranted restrictions included different forms of disallowing symmetry breaking described in this article. As the science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an “internal band gap” between the principal band edges. The significance of this electronic configuration is that it might be unstable toward the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator. The analogous process also exists for degenerate but gapped metals with the Fermi level inside the valence band, where the energy gain is defined by transfer of electrons from the donor level to the unoccupied part of the valence band. We focus here on the fact that numerous electronic structure methodologies have overlooked some physical factors that could stabilize the insulating alternative, predicting instead false metals that do not really exist (note that this is in general not a physical phase transition, but a correction of a previous error in theory that led to a false prediction of a metal). Such errors include: (i) ignoring spin symmetry breaking, such as disallowing magnetic spin ordering in CuBi2O4 or disallowing the formation of polymorphous spin networks in paramagnetic LaTiO3 and YTiO3; (ii) ignoring structural symmetry breaking, e.g., not enabling energy-lowering bond disproportionation (Li-doped TiO2, SrBiO3, and rare-earth nickelates), or not exploring pseudo-Jahn–Teller-like distortions in LaMnO3, or disallowing spontaneous formation of ordered vacancy compounds in Ba4As3 and Ag3Al22O34; and (iii) ignoring spin–orbit coupling forcing false metallic states in CaIrO3 and Sr2IrO4. The distinction between false metals vs real insulators is important because (a) predicting theoretically that a given compound is metal even though it is found to be an insulator often creates the temptation to invoke high order novel physical effects (such as correlation in d-electron Mott insulators) to explain what was in effect caused by a more mundane artifact in a lower-level mean-field band theory, (b) recent prediction of exotic physical effects such as topological semimetals were unfortunately based on the above compounds that were misconstrued by theory to be metal, but are now recognized to be stable insulators not hosting exotic effects, and (c) practical technological applications based on stable degenerate but gapped metals such as transparent conductors or electrides for catalysis must rely on the systematically correct and reliable theoretical classification of metals vs insulators.

34 citations

Journal ArticleDOI
TL;DR: All 3d impurities observed to date in tetrahedral semiconductors have a high-spin ground state, in agreement with Hund's rule, and it is predicted that the as yet unobserved ground state of GaAs:V/sup 2 +/ is of the low-spin type.
Abstract: All 3d impurities observed to date in tetrahedral semiconductors have a high-spin ground state, in agreement with Hund's rule. Using first-principles self-consistent Green's-function calculations for substitutional GaAs:V within the local-spin-density formalism, we predict that the as yet unobserved ground state of GaAs:V/sup 2 +/ is of the low-spin type. The origin of this unusual ground state is explained.

34 citations

Posted Content
TL;DR: In this paper, the mixing energy of disordered Ni-Au alloys has been investigated using Monte Carlo simulations, and it was shown that using inverse Monte Carlo to extract interaction energies from the measured/calculated short-range order in NiAu would result in interactions which would produce ordering-type mixing energies, in contradiction with both experimental measurements and precise LDA energies.
Abstract: The theory of phase stability in the Ni-Au alloy system is a popular topic due to the large size mismatch between Ni and Au, which makes the effects of atomic relaxation critical, and also the fact that Ni-Au exhibits a phase separation tendency at low temperatures, but measurements at high-temperature show an ordering-type short-range order. We have clarified the wide disparity which exists in the previously calculated values of mixing energies and thermodynamic properties by computing ``state-of-the-art'' energetics (full-potential, fully-relaxed LDA total energies) combined with ``state-of-the-art'' statistics (k-space cluster expansion with Monte Carlo simulations) for the Ni-Au system. We find: (i) LDA provides accurate mixing energies of disordered Ni_{1-x}Au_x alloys (\Delta H_{mix} < +100 meV/atom) provided that both atomic relaxation (a ~100 meV/atom effect) and short-range order (~25 meV/atom) are taken into account properly. (ii) Previous studies using empirical potentials or approximated LDA methods often underestimate the formation energy of ordered compounds, and hence also underestimate the mixing energy of random alloys. (iii) Measured values of the total entropy of mixing combined with calculated values of the configurational entropy demonstrate that the non-configurational entropy in Ni-Au is large, and leads to a significant reduction in miscibillity gap temperature. (iv) The calculated short-range order agrees well with measurements, and both predict ordering in the disordered phase. (v) Consequently, using inverse Monte Carlo to extract interaction energies from the measured/calculated short-range order in Ni-Au would result in interactions which would produce ordering-type mixing energies, in contradiction with both experimental measurements and precise LDA energies.

34 citations

Journal ArticleDOI
TL;DR: The special quasirandom structures construct is applied to Cu{sub 1{minus}{ital x}Pd{sub {ital x}} alloys in the context of local-density total-energy minimization, finding a {ital distribution} of Cu-Cu, Cu-Pd, and Pd-PD bonds whose lengths deviate significantly from the single, unrelaxed value assumed in mean-field models.
Abstract: Mean-field theories of unrelaxed ${\mathrm{Cu}}_{0.75}$${\mathrm{Pd}}_{0.25}$ alloys exhibit a deep (${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{F}}$-5.5 eV) Pd bonding state at the bottom of the Cu d band that does not show up in photoemission experiments. We have applied the ``special quasirandom structures'' construct to ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pd}}_{\mathit{x}}$ alloys in the context of local-density total-energy minimization, finding a distribution of Cu-Cu, Cu-Pd, and Pd-Pd bonds whose lengths deviate significantly from the single, unrelaxed value assumed in mean-field models. Such lattice distortions are found to induce a \ensuremath{\sim}1-eV shift in the Pd bonding state to lower binding energies, thus removing much of the discrepancy with experiment.

33 citations


Cited by
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Journal ArticleDOI
TL;DR: A detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set is presented in this article. But this is not a comparison of our algorithm with the one presented in this paper.

47,666 citations

Journal ArticleDOI
TL;DR: The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition, and a detailed analysis of the local structural properties and their changes induced by an annealing process is reported.
Abstract: We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

16,744 citations

Journal ArticleDOI
TL;DR: In this paper, the self-interaction correction (SIC) of any density functional for the ground-state energy is discussed. But the exact density functional is strictly selfinteraction-free (i.e., orbitals demonstrably do not selfinteract), but many approximations to it, including the local spin-density (LSD) approximation for exchange and correlation, are not.
Abstract: The exact density functional for the ground-state energy is strictly self-interaction-free (i.e., orbitals demonstrably do not self-interact), but many approximations to it, including the local-spin-density (LSD) approximation for exchange and correlation, are not. We present two related methods for the self-interaction correction (SIC) of any density functional for the energy; correction of the self-consistent one-electron potenial follows naturally from the variational principle. Both methods are sanctioned by the Hohenberg-Kohn theorem. Although the first method introduces an orbital-dependent single-particle potential, the second involves a local potential as in the Kohn-Sham scheme. We apply the first method to LSD and show that it properly conserves the number content of the exchange-correlation hole, while substantially improving the description of its shape. We apply this method to a number of physical problems, where the uncorrected LSD approach produces systematic errors. We find systematic improvements, qualitative as well as quantitative, from this simple correction. Benefits of SIC in atomic calculations include (i) improved values for the total energy and for the separate exchange and correlation pieces of it, (ii) accurate binding energies of negative ions, which are wrongly unstable in LSD, (iii) more accurate electron densities, (iv) orbital eigenvalues that closely approximate physical removal energies, including relaxation, and (v) correct longrange behavior of the potential and density. It appears that SIC can also remedy the LSD underestimate of the band gaps in insulators (as shown by numerical calculations for the rare-gas solids and CuCl), and the LSD overestimate of the cohesive energies of transition metals. The LSD spin splitting in atomic Ni and $s\ensuremath{-}d$ interconfigurational energies of transition elements are almost unchanged by SIC. We also discuss the admissibility of fractional occupation numbers, and present a parametrization of the electron-gas correlation energy at any density, based on the recent results of Ceperley and Alder.

16,027 citations

Journal ArticleDOI
TL;DR: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature.
Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

10,260 citations