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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.


Papers
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TL;DR: In this article, the authors trace the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion-energy oxide TIs that can be stabilized by pressure; and c) search for covalent oxides where TI-removing atomic displacements can be effectively screened out.
Abstract: Stable oxide topological insulators (TIs) have been sought for years, but none have been found; whereas heavier (selenides, tellurides) chalcogenides can be TIs. The basic contradiction between topological insulation and thermodynamic stability is pointed out, offering a narrow window of opportunity. The electronic motif is first identified and can achieve topological band inversion in ABO3 as a lone-pair, electron-rich B atom (e.g., Te, I, Bi) at the octahedral site. Then, twelve ABO3 compounds are designed in the assumed cubic perovskite structure, which satisfy this electronic motif and are indeed found by density function theory calculations to be TIs. Next, it is illustrated that poorly screened ionic oxides with large inversion energies undergo energy-lowering atomic distortions that destabilize the cubic TI phase and remove band inversion. The coexistence windows of topological band inversion and structure stability can nevertheless be expanded under moderate pressures (15 and 35 GPa, respectively, for BaTeO3 and RbIO3). This study traces the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion-energy oxide TIs that can be stabilized by pressure; and c) a search for covalent oxides where TI-removing atomic displacements can be effectively screened out.

28 citations

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TL;DR: In this paper, a generalized Ising-like approach is proposed for phase diagrams of binary A-B systems, which is based on various approximate solutions to the same broad class of physical Hamiltonians.

28 citations

Journal ArticleDOI
TL;DR: This work studies the ground-state orbital and spin configurations of up to six electrons or holes loaded into self-assembled quantum dots using a general phase-diagram approach constructed from single-particle pseudopotential and many-particles configuration interaction methods to find that while the charging of electrons follows both Hund's rule and the Aufbau principle, thecharging of holes follows a nontrivial charging pattern which violates both the Aau principle and Hund’s rule.
Abstract: We study the ground-state orbital and spin configurations of up to six electrons or holes loaded into self-assembled $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots. We use a general phase-diagram approach constructed from single-particle pseudopotential and many-particle configuration interaction methods. The predicted hole charging energies agree with recent charging experiments, but offer a different interpretation: we find that while the charging of electrons follows both Hund's rule and the Aufbau principle, the charging of holes follows a nontrivial charging pattern which violates both the Aufbau principle and Hund's rule.

28 citations

Journal ArticleDOI
TL;DR: In this paper, the spontaneous long-range ordering of pseudobinary A 0.5B0.5C isovalent semiconductor alloys into the (AC)1(BC)1 superlattice structure gives rise to characteristic changes in the optical and photoemission spectra.
Abstract: It is shown how the recently predicted and subsequently observed spontaneous long‐range ordering of pseudobinary A0.5B0.5C isovalent semiconductor alloys into the (AC)1(BC)1 superlattice structure (a CuAuI‐type crystal) gives rise to characteristic changes in the optical and photoemission spectra. We predict new direct transitions and substantial splittings of transitions absent in the disordered alloy.

28 citations

Journal ArticleDOI
TL;DR: In this paper, a static density functional approach that does not restrict positional or spin degrees of freedom (symmetry-broken structures) is proposed to remove the need for symmetry-restricted structures.
Abstract: In some $d$-electron oxides, the measured effective mass ${m}_{\mathrm{exptl}}^{*}$ has long been known to be significantly larger than the model effective mass ${m}_{\mathrm{model}}^{*}$ deduced from mean-field band theory, ie, ${m}_{\mathrm{exptl}}^{*}=\ensuremath{\beta}{m}_{\mathrm{model}}^{*}$, where $\ensuremath{\beta}g1$ is the ``mass-enhancement'' or ``mass-renormalization'' factor Previous applications of density functional theory (DFT), based on a symmetry-restricted structure with the smallest number of possible magnetic, orbital, and structural degrees of freedom, missed such mass enhancement This fact has been taken as evidence of strong electronic correlation, often described via the symmetry-restricted dynamic mean-field approach of the many-body theory, being the exclusive enabling physics This paper uses instead a static density functional approach that does not restrict positional or spin degrees of freedom (symmetry-broken structures) This approach analyzes the contributions of different symmetry-broken modalities to mass enhancement for a few nominally highly correlated $d$-electron perovskites as well as the nominally uncorrelated, closed-shell $s\text{\ensuremath{-}}p$ bonding perovskites It shows that the energy-lowering symmetry-broken spin effects (eg, nonzero local moment in the paramagnetic phase) and structural effects (eg, atomic displacement) as described in mean-field DFT already manifest mass enhancement for both electrons and holes in a range of $d$-electron perovskites $\mathrm{SrV}{\mathrm{O}}_{3}, \mathrm{SrTi}{\mathrm{O}}_{3}, \mathrm{BaTi}{\mathrm{O}}_{3}$, and $\mathrm{LaMn}{\mathrm{O}}_{3}$, as well as $p$-electron perovskites $\mathrm{CsPb}{\mathrm{I}}_{3}$ and $\mathrm{SrBi}{\mathrm{O}}_{3}$, including both metals ($\mathrm{SrV}{\mathrm{O}}_{3}$) and insulators (the rest) This is revealed only when enlarged unit cells of the same parent global symmetry, which are large enough to allow for symmetry-breaking distortions and concomitant variations in spin order, are explored for their ability to lower the total energy Positional symmetry breaking that leads to mass enhancement includes octahedral rotation in halide perovskites such as $\mathrm{CsPb}{\mathrm{I}}_{3}$, Jahn-Teller-like ${Q}_{2}^{+}$ distortion in $\mathrm{LaMn}{\mathrm{O}}_{3}$, and bond disproportionation in $\mathrm{SrBi}{\mathrm{O}}_{3}$, while magnetic symmetry breaking resulting in mass enhancement includes the formation of a distribution of local moments in $\mathrm{SrV}{\mathrm{O}}_{3}$ that averages to zero in the paramagnetic phase Not all symmetry breaking leads to significant mass enhancement, eg, the rather small octahedral rotations in the nearly perfectly cubic $\mathrm{SrTi}{\mathrm{O}}_{3}$ cause negligible mass enhancement, as do the paraelectric displacements in $\mathrm{BaTi}{\mathrm{O}}_{3}$ In principle, by ergodicity, the two descriptions, ie, the symmetry-restricted dynamic approach with a single time-fluctuating magnetic moment and the symmetry-broken mean-field approach with a static distribution of spatially fluctuated local moments, are not mutually exclusive but are a choice of representation and consequently, a choice of computational efficiency In approximate implementations, the symmetry-broken mean-field approach appears to remove much of what was strong correlation in dynamically correlated symmetry-restricted solutions, leaving smaller (``weak'') residual correlation with respect to the exact solution

28 citations


Cited by
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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

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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