Author
Alex Zunger
Other affiliations: Tel Aviv University, University of Wisconsin-Madison, Braunschweig University of Technology ...read more
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 published on a yearly basis
Papers
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TL;DR: In this paper, an inverse design approach is formulated to accelerate the design and discovery of novel functional materials, such as p-type transparent conducting oxides, by articulating the target properties and selecting an initial pool of candidates based on design principles.
Abstract: To accelerate the design and discovery of novel functional materials, here, p-type transparent conducting oxides, an inverse design approach is formulated, integrating three steps: i) articulating the target properties and selecting an initial pool of candidates based on "design principles", ii) screening this initial pool by calculating the "selection metrics" for each member, and iii) laboratory realization and more-detailed theoretical validation of the remaining "best-of-class" materials. Following a design principle that suggests using d55 cations for good p-type conductivity in oxides, the Inverse Design approach is applied to the class of ternary Mn(II) oxides, which are usually considered to be insulating materials. As a result, Cr2MnO4 is identified as an oxide closely following "selection metrics" of thermodynamic stability, wide-gap, p-type dopability, and band-conduction mechanism for holes (no hole self-trapping). Lacking an intrinsic hole-producing acceptor defect, Li is further identified as a suitable dopant. Bulk synthesis of Li-doped Cr 2MnO4 exhibits at least five orders of magnitude enhancement of the hole conductivity compared to undoped samples. This novel approach of stating functionality first, then theoretically searching for candidates that merits synthesis and characterization, promises to replace the more traditional non-systematic approach for the discovery of advanced functional materials. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
63 citations
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TL;DR: In this article, the effects of charge and relaxational fluctuations on the electronic structure of ordered L{1}-2}-type compounds were studied, where the sites of a periodic supercell are occupied by A and B atoms so that the first few radial correlation functions closely reproduce the average correlation functions in an infinite substitutional random network.
Abstract: In ordered L${1}_{2}$-type ${\mathit{A}}_{3}$B compounds, each A atom is coordinated by 8A+4B atoms, while each B atom is coordinated by 12A atoms. By symmetry, all A-A, A-B, and B-B bond lengths are equal. When this structure disorders to form the substitutionally random ${\mathit{A}}_{0.75}$${\mathit{B}}_{0.25}$ alloy, each atom acquires a distribution of different types of coordination shells. Concomitantly with this reduction in site symmetries, (i) topologically different A atoms (and separately, different B atoms) can have unequal charges, and (ii) the various bonds need not be of equal average lengths 〈R〉 (i.e., 〈${\mathit{R}}_{\mathit{A}\mathrm{\ensuremath{-}}\mathit{A}}$〉\ensuremath{
e}〈${\mathit{R}}_{\mathit{A}\mathrm{\ensuremath{-}}\mathit{B}}$〉\ensuremath{
e} 〈${\mathit{R}}_{\mathit{B}\mathrm{\ensuremath{-}}\mathit{B}}$〉). Furthermore, (iii) there can be a distribution of bond-length values around 〈${\mathit{R}}_{\mathit{i}\mathit{j}}$〉 for each of the three chemical bond types. In this work we study the effects of such charge fluctuations (i) and relaxational fluctuations [(ii) and (iii)] on the electronic structure of ${\mathrm{Cu}}_{3}$Au and ${\mathrm{Cu}}_{3}$Pd. The random alloys are modeled by the special quasirandom structure (SQS), whereby the sites of a periodic supercell are occupied by A and B atoms so that the first few radial correlation functions closely reproduce the average correlation functions in an infinite substitutional random network. Instead of requiring that each atom ``see'' an identical, average medium, as is the case in the homogeneous site-coherent-potential approximation (SCPA), we thus create a distribution of distinct local environments whose average corresponds to the random alloy.Application of a first-principles local-density method (linearized augmented-plane-wave method) to the SQS then provides the energy-minimizing equilibrium relaxations, charge density, density of states, and formation enthalpy. We find that charge and relaxational fluctuations neglected in the SCPA lead to a significant stabilization of the alloy (\ensuremath{\sim}30% lowering in mixing enthalpy) and to substantial (\ensuremath{\sim}1 eV) nonrigid shifts in the electronic energy levels.
63 citations
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TL;DR: Over a broad range of scattering strengths, the dominant spectral features predicted by the first two techniques are well reproduced by calculations for an SQS with 16 atoms/unit-cell ( SQS-8'', suggesting that the SQS approach might also be useful in cases where the other methods are difficult to apply.
Abstract: The spectral properties of an ${\mathit{sp}}^{3}$${\mathit{s}}^{\mathrm{*}}$ tight-binding Hamiltonian for a random, unrelaxed ${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As alloy are calculated using three different techniques: the coherent-potential approximation, the recursion method (as applied to a g2000 atom supercell), and the recently introduced ``special-quasirandom-structures'' (SQS) approach. Over a broad range of scattering strengths, the dominant spectral features predicted by the first two techniques are well reproduced by calculations for an SQS with 16 atoms/unit-cell (``SQS-8''). This suggests that the SQS approach might also be useful in cases where the other methods are difficult to apply, e.g., in first-principles calculations for structurally relaxed alloys.
62 citations
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TL;DR: In this paper, it was shown that at large interdot separations, the exciton states are built from heteronuclear single-particle states and homonuclear electron states.
Abstract: The ability of a quantum dot to confine photogenerated electron-hole pairs created interest in the behavior of such an exciton in a ``dot molecule,'' being a possible register in quantum computing. When two quantum dots are brought close together, the quantum state of the exciton may extend across both dots. The exciton wave function in such a dot molecule may exhibit entanglement. Atomistic pseudopotential calculations of the wave function for an electron-hole pair in a dot molecule made of two identical ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ dots reveal that the common assumption of single-particle wave functions forming bonding and antibonding states is erroneous. The true behavior of single-particle electrons and holes leads to symmetry-broken excitonic two-particle wave functions, dramatically suppressing entanglement. We find that at large interdot separations, the exciton states are built from heteronuclear single-particle states while at small interdot separations the exciton is derived from heteronuclear hole states and homonuclear electron states. We calculate the entanglement of the excitons and find a maximum value of 80% at an interdot separation of $8.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and very small values for larger and smaller distances.
62 citations
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TL;DR: In this paper, the authors apply the first principles theory of doping to a few prototype compounds in the half-Heusler filled tetrahedral structures (FTSs) and describe the key ingredients controlling the materials' propensity for both intrinsic and extrinsic doping: spontaneous deviations from 1:1:1 stoichiometry reflect predictable thermodynamic stability of specific competing phases.
Abstract: The half-Heusler filled tetrahedral structures (FTSs) are zinc-blende-like compounds, where an additional atom is filling its previously empty interstitial site. The FTSs having 18 valence electrons per formula unit are an emerging family of functional materials, whose intrinsic doping trends underlying a wide range of electronic functionalities are yet to be understood. Interestingly, even pristine compounds without any attempt at impurity/chemical doping exhibit intriguing trends in the free carriers they exhibit. Applying the first principles theory of doping to a few prototype compounds in the ${A}^{\mathrm{IV}}{B}^{\mathrm{X}}{C}^{\mathrm{IV}}$ and ${A}^{\mathrm{IV}}{B}^{\mathrm{IX}}{C}^{\mathrm{V}}$ groups, we describe the key ingredients controlling the materials' propensity for both intrinsic and extrinsic doping: (a) The spontaneous deviations from 1:1:1 stoichiometry reflect predictable thermodynamic stability of specific competing phases. (b) Bulk ABC compounds containing $3d$ elements in the $B$ position (ZrNiSn and ZrCoSb) are predicted to be naturally $3d$ rich. The $B=3d$ interstitials are the prevailing shallow donors, whereas the potential acceptors (e.g., Zr vacancy and Sn-on-Zr antisite) are ineffective electron killers, resulting in an overall uncompensated $n$-type character, even without any chemical doping. In these materials, the band edges are ``natural impurity bands'' due to non-Daltonian off-stoichiometry, such as $B$ interstitials, not intrinsic bulk controlled states as in a perfect crystal. (c) Bulk ABC compounds containing $5d$ elements in the $B$ position (ZrPtSn, ZrIrSb, and TaIrGe) are predicted to be naturally $C$ rich and $A$ poor. This promotes the hole-producing $C$-on-$A$ antisite defects rather than $B$-interstitial donors. The resultant $p$-type character (without chemical doping) therein is ``latent'' for $C=\mathrm{Sn}$ and Sb; however, as the $C$-on-$A$ hole-producing acceptors are rather deep and $p$ typeness is manifest only at high temperature or via impurity doping. In contrast, in TaIrGe $(B=\mathrm{Ir},\phantom{\rule{0.16em}{0ex}}5d)$, the prevailing hole-producing Ge-on-Ta antisite ($C$-on-$A$) is shallow, making it a real $p$-type compound. This general physical picture establishes the basic trends of carriers in this group of materials.
62 citations
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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