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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 effect of different degrees of mixing between valence bands on the optical properties of cylindrical quantum wires was analyzed by switching on and off the spin-orbit interaction, and the fundamental transition in these QWRs exhibit a large anisotropy, with emission polarized prevalently along the wire axis.
Abstract: The electronic and optical properties of [001]-oriented free-standing InAs cylindrical quantum wires (QWRs) with diameters 10--100 \AA{} are calculated using an atomistic, empirical pseudopotential plane-wave method. We analyze the effect of different degrees of mixing between valence bands on the optical properties of these nanostructures, by switching on and off the spin-orbit interaction. The fundamental transition in these QWRs exhibit a large anisotropy, with emission polarized prevalently along the wire axis $z$. The magnitude of such an anisotropy is found to depend on both degree of valence band mixing and wire size. In higher energy interband transitions, we find anisotropies close to 100% with emission polarized perpendicular to the wire axis. Furthermore, in large wires, transitions involving highly excited valence states show in-plane polarization anisotropies between the [110] and $[1\overline{1}0]$ directions. InAs wires can therefore switch between $z$-polarized to $xy$-polarized emission/absorption for different excitation energies. This makes them ideally suited for application in orientation-sensitive devices.

29 citations

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TL;DR: In this article, the authors used first-principles total energy and stability calculations to confirm that the previous assignment of the $P4$/nmm structure to $\mathrm{LaOBi}{S}}_{2}$ is incorrect and that the unstable structure is replaced by a family of energetically closely spaced modifications differing by the layer sequences and orientations.
Abstract: $\mathrm{LaOBi}{\mathrm{S}}_{2}$-type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each $\mathrm{LaOBi}{\mathrm{S}}_{2}$-type compound has a unique and specific crystallographic structure (with a space group $P4$/nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the $P4$/nmm structure to $\mathrm{LaOBi}{\mathrm{S}}_{2}$ is incorrect. Furthermore, we find that the unstable structure is replaced by a family of energetically closely spaced modifications (polytypes) differing by the layer sequences and orientations. We find that the local Bi-S distortion leads to three polytypes of $\mathrm{LaOBi}{\mathrm{S}}_{2}$ with different stacking patterns of the distorted $\mathrm{Bi}{\mathrm{S}}_{2}$ layers. The energy difference between the polytypes of $\mathrm{LaOBi}{\mathrm{S}}_{2}$ is merely \ensuremath{\sim}1 meV/u.c., indicating the possible coexistence of all polytypes in the real sample and that the particular distribution of polytypes may be growth induced. The in-plane distortion can be suppressed by pressure, leading to a phase transition from polytypes to the high-symmetry $P4$/nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms ($\mathrm{Bi}{\mathrm{S}}_{2}$) could also manifest different ground-state structures. One can thus tune the distortion and the ground state by pressure or by substituting covalence atoms in the $\mathrm{LaOBi}{\mathrm{S}}_{2}$ family.

29 citations

Journal ArticleDOI
TL;DR: The authors' self-consistent electronic-structure calculations for Ga(As,Sb) show very little s mixing in the VBM and an enhancement of ..delta../sub 0/. Intraband p-p coupling is shown to dominate the changes in ..d delta../ Sub 0/.
Abstract: Early measurements on many bulk III-V alloys showed that the spin-orbit splitting ${\ensuremath{\Delta}}_{0}$ at the valence-band maximum (VBM) was universally reduced relative to the average value of the end-point constituents. This led to the assumption, guiding much of the subsequent data analysis, that such alloys universally mix some conduction-band s character into the VBM (``interband coupling''), suppressing ${\ensuremath{\Delta}}_{0}$. Our self-consistent electronic-structure calculations for Ga(As,Sb) show very little s mixing in the VBM and an enhancement of ${\ensuremath{\Delta}}_{0}$. Intraband p-p coupling is shown to dominate the changes in ${\ensuremath{\Delta}}_{0}$.

29 citations

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TL;DR: Using atomistic pseudopotential and configuration-interaction many-body calculations, an excitonic ground state is predicted in the InAs/InSb quantum-dot system, which is energetically more favorable than the dot without excitons.
Abstract: Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.

29 citations

Journal Article
TL;DR: In this article, the authors provided a full-zone description of the Dresselhaus splitting in zinc blende semiconductors by using pseudopotentials, empirically corrected to rectify local density approximation errors by fitting $GW$ results.
Abstract: The spin-orbit interaction---a fundamental electroweak force---is equivalent to an effective magnetic field intrinsic to crystals, leading to band spin splitting for certain $k$ points in sufficiently low-symmetry structures. This (Dresselhaus) splitting has usually been calculated at restricted regions in the Brillouin zone via small wave vector approximations (e.g., $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$), potentially missing the ``big picture.'' We provide a full-zone description of the Dresselhaus splitting in zinc blende semiconductors by using pseudopotentials, empirically corrected to rectify local density approximation errors by fitting $GW$ results. In contrast to what was previous thought, we find that the largest spin splitting in the lowest conduction band and upper valence band (VB1) occurs surprisingly along the (210) direction, not the (110) direction, and that the splitting of the VB1 is comparable to that of the next two valence bands VB2 and VB3.

29 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

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