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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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Journal ArticleDOI
TL;DR: A simple model for {Delta}{ital H} which includes the short-range nonoctet-bond energies and the long-range electro-static interactions between donor and acceptor bonds is shown to capture the chemical trends of all calculated {Delta} H's.
Abstract: First-principles pseudopotential theory is used to calculate the formation enthalphies \ensuremath{\Delta}H and valence-band offsets for the heterovalent (${\mathrm{Si}}_{2}$${)}_{\mathit{n}}$/(GaP${)}_{\mathit{n}}$, (${\mathrm{Ge}}_{2}$${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{n}}$, and (${\mathrm{Si}}_{2}$${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{n}}$ superlattices with repeat periods n\ensuremath{\le}4 and growth directions G?[001], [110], and [111]. All of these superlattices are found to be unstable with respect to phase separation; the [111] system is the least unstable. The unreconstructed [001] and [111] polar superlattices have large internal electric fields, causing the [001] superlattices to reconstruct for n\ensuremath{\ge}2; [111] are predicted to reconstruct for n\ensuremath{\ge}6. Different types of reconstruction lead to different band offsets. A simple model for \ensuremath{\Delta}H which includes the short-range nonoctet-bond energies and the long-range electro-static interactions between donor and acceptor bonds is shown to capture the chemical trends of all calculated \ensuremath{\Delta}H's.

49 citations

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TL;DR: It is shown here that using a genetic-algorithm search with a pseudopotential "Inverse-band-structure (IBS) approach it is possible to identify those configurations that are naturally lattice matching and have a specific band gap at more than one composition.
Abstract: Quaternary systems illustrated by (Ga,In)(As,Sb) manifest a huge configurational space, offering in principle the possibility of designing structures that are lattice matched to a given substrate and have given electronic properties (e.g., band gap) at more than one composition. Such specific configurations were however, hitherto, unidentified. We show here that using a genetic-algorithm search with a pseudopotential "Inverse-band-structure (IBS) approach it is possible to identify those configurations that are naturally lattice matching (to GaSb) and have a specific band gap (310 meV) at more than one composition. This is done by deviating from randomness, allowing the IBS to find a partial atomic ordering. This illustrates multitarget design of the electronic structure of multinary systems.

49 citations

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TL;DR: The Ising Hamiltonian is defined, which allows us to accurately calculate structural energies of relaxed substitutional systems containing thousands of transition-metal atoms, and extends the applicability of the local-density method to finite temperatures and to huge substitutional supercells.
Abstract: Total-energy local-density calculations on approximately 20 periodic crystal structures of a given AB compound are used to define a long-range Ising Hamiltonian which correctly represents atomic relaxations. This allows us to accurately calculate structural energies of relaxed substitutional ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$ systems containing thousands of transition-metal atoms, simply by adding up spin products in the Ising Hamiltonian. The computational cost is thus size independent. We then apply Monte Carlo and simulated-annealing techniques to this Ising Hamiltonian, finding (i) the T=0 ground-state structures, (ii) the order-disorder transition temperatures ${\mathit{T}}_{\mathit{c}}$, and (iii) the Tg${\mathit{T}}_{\mathit{c}}$ short-range-order parameters. The method is illustrated for a transition-metal alloy (${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pd}}_{\mathit{x}}$) and a semiconductor alloy (${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$P). It extends the applicability of the local-density method to finite temperatures and to huge substitutional supercells. We find for ${\mathrm{Cu}}_{0.75}$${\mathrm{Pd}}_{0.25}$ a characteristic fourfold splitting of the diffuse scattering intensity due to short-range order as observed experimentally.

48 citations

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TL;DR: In this paper, it was shown that the spin splitting in R-2 is enforced by specific symmetries, such as the non-symmorphic in the present example, which ensures that the pertinent spin wave functions segregate spatially on just one of the two inversion-partner sectors and thus avoid compensation.
Abstract: Hidden Rashba and Dresselhaus spin-splittings in centrosymmetric crystals with subunits (sectors) having non-centrosymmetric symmetries (the R-2 and D-2 effects) have been predicted and observed experimentally, but the microscopic mechanism remains unclear. Here we demonstrate that the spin-splitting in R-2 is enforced by specific symmetries (such as the non-symmorphic in the present example) which ensures that the pertinent spin wavefunctions segregate spatially on just one of the two inversion-partner sectors and thus avoid compensation. This finding establishes a common fundamental source for the conventional Rashba (R-1) effect and the R-2 effect, both originating from the local sector symmetries, rather than from the global crystal asymmetry alone for R-1 per se. We further show that the effective Hamiltonian for the R-1 effect is also applicable for the R-2 effect, but applying a symmetry-breaking electric field to an R-2 compound produces different spin-splitting pattern than applying a field to a trivial (non-R-2) centrosymmetric compound.

48 citations

Journal ArticleDOI
TL;DR: In this paper, an empirical pseudopotential approach, fitted to bulk and interfacial reference systems, provides a unified description of the electronic structure of random alloys (bulk and epitaxial), superlattices and related complex systems.
Abstract: We show how an empirical pseudopotential approach, fitted to bulk and interfacial reference systems, provides a unified description of the electronic structure of random alloys (bulk and epitaxial), superlattices, and related complex systems. We predict the composition and superlattice-period dependence of the band offsets and interband transitions of InAs/GaAs systems on InP and GaAs substrates.

48 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

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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

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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