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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: The ability of generalized Ising-like cluster expansions to describe the energetics and thermodynamics associated with large atomic displacements in alloys and combination of the expansion with Monte Carlo simulations is shown to provide an efficient means for calculating thermodynamic properties.
Abstract: We demonstrate the ability of generalized Ising-like cluster expansions to describe the energetics and thermodynamics associated with large atomic displacements in alloys. Although the expansion is constructed only from the energies of a few (small-unit-cell) ordered structures, it provides accurate predictions of the atomically relaxed energies of random, ordered, or partially ordered alloys, as compared with direct, large scale energy-minimizing simulations. Relaxed energies are obtained without having to compute relaxed geometries. Combination of the expansion with Monte Carlo simulations is shown to provide an efficient means for calculating thermodynamic properties.

68 citations

Journal ArticleDOI
TL;DR: In this article, the phonon structure of GaP quantum dots is studied using an atomistic potential model and the dot eigenmodes are obtained from a direct diagonalization of the dynamical matrix and classified using an efficient dual space analysis method.
Abstract: The phonon structure of GaP quantum dots is studied using an atomistic potential model. The dot eigenmodes are obtained from a direct diagonalization of the dynamical matrix and classified using an efficient dual-space analysis method. Our calculations provide a theoretical explanantion for several experimental observations. (1) Depending on the spatial localization, the phonon modes of dots are either dot-interior (bulklike) or surfacelike. (2) The frequencies of the dot-interior modes can be qualitatively described by the {open_quotes}truncated crystal method{close_quotes} using a single branch and a single wave vector of the bulk-phonon dispersion. In contrast, the surface modes cannot be described by this model. (3) The dot-interior modes have a dominant bulk parentage from a specific part of the Brillouin zone, while the surface modes do not. (4) The frequencies of the bulklike {Gamma}-derived longitudinal optical (LO) and transverse optical (TO) phonon modes are found to decrease with decreasing dot size. This decrease reflects the downward dispersion of the bulk optical-phonon branches away from the {Gamma} point. (5) The surface modes located between the bulk TO- and LO-phonon bands have a significant bulk {Gamma} character, and are thus Raman detectable. (6) The dot-interior modes exhibit only a slight LO/TO mode mixing, whilemore » the surfacelike modes show a strong mode mixing. {copyright} {ital 1999} {ital The American Physical Society}« less

67 citations

Journal ArticleDOI
TL;DR: In this paper, order-disorder transformations in pseudobinary semiconductor alloys AxB1−xC are shown to belong to a broader class of such transformations in AnB4−nC4 semiconducting compounds (e.g., chalcopyrites, for n = 2).
Abstract: The recently discovered order‐disorder transformations in pseudobinary semiconductor alloys AxB1−xC are shown to belong to a broader class of such transformations in AnB4−nC4 semiconducting compounds (e.g., chalcopyrites, for n=2). Strain energy, set up by the atomic size mismatch between the A–C and B–C bonds, is shown to control the nature of the state of order in chalcopyrites and pseudobinary alloys alike. These considerations lead to a classification of all bulk tetrahedral semiconductors into four classes of order‐disorder characteristics.

67 citations

Journal ArticleDOI
TL;DR: In this paper, an iterative procedure was proposed to predict the formation enthalpy of an arbitrary fcc lattice configuration with precision comparable to that of ab initio calculations themselves.
Abstract: We describe an iterative procedure which yields an accurate cluster expansion for Au-Pd using only a limited number of ab initio formation enthalpies. Our procedure addresses two problems: (a) given the local-density-approximation (LDA) formation energies for a fixed set of structures, it finds the pair and many-body cluster interactions best able to predict the formation energies of new structures, and (b) given such pair and many-body interactions, it augments the LDA set of ``input structures'' by identifying additional structures that carry most information not yet included in the ``input.'' Neither step can be done by intuitive selection. Using methods including genetic algorithm and statistical analysis to iteratively solve these problems, we build a cluster expansion able to predict the formation enthalpy of an arbitrary fcc lattice configuration with precision comparable to that of ab initio calculations themselves. We also study possible competing non-fcc structures of Au-Pd, using the results of a ``data mining'' study. We then address the unresolved problem of bulk ordering in Au-Pd. Experimentally, the phase diagram of Au-Pd shows only a disordered solid solution. Even though the mixing enthalpy is negative, implying ordering, no ordered bulk phases have been detected. Thin film growth shows $L{1}_{2}$-ordered structures with composition ${\mathrm{Au}}_{3}\mathrm{Pd}$ and $\mathrm{Au}{\mathrm{Pd}}_{3}$ and $L{1}_{0}$ structure with composition AuPd. We find that (i) all the ground states of Au-Pd are fcc structures; (ii) the low-$T$ ordered states of bulk Au-Pd are different from those observed experimentally in thin films; specifically, the ordered bulk ${\mathrm{Au}}_{3}\mathrm{Pd}$ is stable in $D{0}_{23}$ structure and and AuPd in chalcopyritelike ${\mathrm{Au}}_{2}{\mathrm{Pd}}_{2}$ (201) superlattice structure, whereas thin films are seen in the $L{1}_{2}$ and $L{1}_{0}$ structures; (iii) $\mathrm{Au}{\mathrm{Pd}}_{3}$ $L{1}_{2}$ is stable and does not phase separate, contrary to the suggestions of an earlier investigation; (iv) at compositions around ${\mathrm{Au}}_{3}\mathrm{Pd}$, we find several long-period superstructures (LPS's) to be stable, specifically, the one-dimensional LPS $D{0}_{23}$ at composition ${\mathrm{Au}}_{3}\mathrm{Pd}$ and two two-dimensional LPS's at compositions ${\mathrm{Au}}_{13}{\mathrm{Pd}}_{4}$ and ${\mathrm{Au}}_{11}{\mathrm{Pd}}_{4}$; (v) Au-Pd has a number of unsuspected ground states, including the structure ${\mathrm{Au}}_{7}{\mathrm{Pd}}_{5}$ with the lowest formation enthalpy and the (301) ``adaptive structures'' in the Au-rich composition range, all of which could not be predicted by other theoretical methods.

66 citations

01 Mar 2004
TL;DR: In this article, the authors apply the semi-empirical nonlocal pseudopotential method to the investigation of prospects for direct carrier multiplication (DCM) in neutral and negatively charged CdSe nanocrystals.
Abstract: We apply the semiempirical nonlocal pseudopotential method to the investigation of prospects for direct carrier multiplication (DCM) in neutral and negatively charged CdSe nanocrystals. In this process, known in the bulk as impact ionization, a highly excited carrier transfers, upon relaxation to the band edge, its excess energy Δ to a valence electron, promoting it across the band gap and thus creating two excitons from one. For excess energies just a few meV above the energy gap Eg (the DCM threshold), we find the following: (i) DCM is much more efficient in quantum dots than in bulk materials, with rates of the order of 1010 s-1. In conventional bulk solids, comparable rates are obtained only for excess energies about 1 eV above Eg. (ii) Unlike the case in the bulk, in both neutral and charged nanocrystals the DCM rate is not an increasing function of the excess energy but oscillates as Δ moves in and out of resonance with the energy of the discrete spectrum of these 0D systems, (iii) The main contribution to the DCM rates is found to come from the dot surface, as in the case of Auger multiexciton recombination rates, (iv) Direct radiative recombination of excited electron-hole pairs and phonon-assisted decay are slower than DCM, but (v) the rate of Auger cooling (where the relaxation energy of an excited electron is used to excite a hole into deeper levels) can be of the same order of magnitude as that of the DCM process. Furthermore, for excess energies well above the DCM threshold, the presence of an energy gap within the hole manifold considerably slows DCM compared to Auger cooling (AC), which is not affected by it. Achieving competitive DCM processes will, therefore, require the suppression of Auger cooling, for example, by removing the hole from the dot or by trapping it at the surface.

66 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