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.

Proposal for III‐V ordered alloys with infrared band gaps

Abstract: It is shown theoretically that the recently observed spontaneous ordering of III‐V alloys that yields alternate monolayer (111) superlattices provides the opportunity for achieving infrared band gaps in systems such as (InAs)1(InSb)1 and (GaSb)1(InSb)1. A substantial reduction in the direct band gap is predicted to result from the L‐point folding that repel the Γ band‐edge states.

Bond disproportionation, charge self-regulation, and ligand holes in s -p and in d -electron A B X 3 perovskites by density functional theory

Abstract: Some $AB{X}_{3}$ perovskites exhibit different local environments (DLE) for the same $B$ atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from common perovskites that have single local environments (SLE). The basic phenomenology associated with such disproportionation involves the absence of $B$-atom charge ordering, the creation of different B-X bond length (``bond alternation'') for different local environments, the appearance of metal (in SLE) to insulator (in DLE) transitions, and the formation of ligand holes. We point out that this phenomenology is common to a broad range of chemical bonding patterns in $AB{X}_{3}$ compounds, either with s-p electron $B$-metal cations (${\mathrm{BaBiO}}_{3}$, ${\mathrm{CsTlF}}_{3}$) or with noble-metal cations (${\mathrm{CsAuCl}}_{3}$), as well as with $d$-electron cations (${\mathrm{SmNiO}}_{3}$, ${\mathrm{CaFeO}}_{3}$). We show that underlying much of this phenomenology is the ``self-regulating response,'' whereby in strongly bonded metal-ligand systems with high-lying ligand orbitals, the system protects itself from creating highly charged cations by transferring ligand electrons to the metal, thus preserving a nearly constant metal charge in different local environments, while creating $B$-ligand bond alternation and ligand-like conduction band (``ligand hole'' states). We are asking what are the minimal theory ingredients needed to explain the main features of this SLE-to-DLE phenomenology, such as its energetic driving force, bond length changes, possible modifications in charge density, and density of state changes. Using as a guide the lowering of the total energy in DLE relative to SLE, we show that density functional calculations describe this phenomenology across the whole chemical bonding range without resort to special strong correlation effects, beyond what DFT naturally contains. In particular, lower total energy configurations (DLE) naturally develop bond alternation, gapping of the metallic SLE state, and absence of charge ordering with ligand hole formation.

Atomic-scale structure of disordered Ga1-xInxP alloys.

Abstract: Extended x-ray-absorption fine-structure experiments have previously demonstrated that for each composition x, the sample average of all nearest-neighbor A-C distances in an ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$C semiconductor alloy is closer to the values in the pure (x\ensuremath{\rightarrow}0) AC compound than to the composition-weighted (virtual) lattice average. Such experiments do not reveal, however, the distribution of atomic positions in an alloy, so the principle displacement directions and the degrees of correlation among such atomic displacements remain unknown. Here we calculate both structural and thermodynamic properties of ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$P alloys using an explicit occupation- and position-dependent energy functional. The latter is taken as a modified valence force field, carefully fit to structural energies determined by first-principles local-density calculations. Configurational and vibrational degrees of freedom are then treated via the continuous-space Monte Carlo approach. We find good agreement between the calculated and measured mixing enthalpy of the random alloy, nearest-neighbor bond lengths, and temperature-composition phase diagram. In addition, we predict yet unmeasured quantities such as (a) distributions, fluctuations, and moments of first- and second-neighbor bond lengths as well as bond angles, (b) radial distribution functions, (c) the dependence of short-range order on temperature, and (d) the effect of temperature on atomic displacements. Our calculations provide a detailed picture of how atoms are arranged in substitutionally random but positionally relaxed alloys, and offer an explanation for the effects of site correlations, static atomic relaxations, and dynamic vibrations on the phase-diagram and displacement maps. We find that even in a chemically random alloy (where sites are occupied by Ga or In according to a coin toss), there exists a highly correlated static position distribution whereby the P atoms are displaced deterministically in certain high-symmetry directions.

Ordering tendencies in Pd-Pt, Rh-Pt, and Ag-Au alloys

Abstract: First-principles quantum-mechanical calculations indicate that the mixing enthalpies for Pd-Pt and Rh-Pt solid solutions are negative, in agreement with experiment. Calculations of the diffuse-scattering intensity due to short-range order also exhibits ordering tendencies. Further, the directly calculated enthalpies of formation of ordered intermetallic compounds are negative. These ordering tendencies are in direct conflict with a 1959 prediction of Raub that Pd-Pt and Rh-Pt will phase-separate below ~760 °C (hence their mixing energy will be positive), a position that has been adopted by all binary alloy phase diagram compilations. The present authors predict that Pd1-xPtx will order in the L12, L10, and L12 structures ([001] superstructures) at compositionsx = 1/4, 1/2, and 3/4, respectively, while the ordered structures of Rh1-xPtx are predicted to be superlattices stacked along the [012] directions. While the calculated ordering temperatures for these intermetallic compounds are too low to enable direct growth into the ordered phase, diffuse-scattering experiments at higher temperatures should reveal ordering rather than phase-separation characteristics (i.e., off-F peaks). The situation is very similar to the case of Ag-Au, where an ordering tendency is manifested both by a diffuse scattering intensity and by a negative enthalpy of mixing. An experimental reexamination of PdPt and Rh-Pt is needed.

First-principles theoretical study on the electronic properties of the B 32 intermetallic compound LiAl

Abstract: The electronic properties of the ordered LiAl crystal are studied within the self-consistent (non-muffin-tin) numerical-basis-set approach to the local-density formalism. The material appears to be electronically a semimetal with an electron pocket near $X$ (along $\ensuremath{\Delta}$) and a hole pocket at $\ensuremath{\Gamma}$. The band structure and density of states have characteristics similar to that of the tetrahedrally bonded IV-IV semiconductors (LiAl has a ${T}_{d}$ site symmetry); however, the indirect ${\ensuremath{\Gamma}}_{{25}^{\ensuremath{'}}}\ensuremath{-}{X}_{1}$ band gap (which decreases progressively as one goes along the diamond, Si, Ge, and $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ series) becomes negative in LiAl. A study of charge redistribution effects indicates that while the Li-Al bond is an ionically polarized covalent bond, the Al-Al bonds are metalliclike and the Li-Li bonds are essentially nonbonding. Wave-function population analysis indicates that the bottom of the occupied valence band is of predominantly $\mathrm{Li} 2s$ character (hybridized with $\mathrm{Al} 3s$), while at higher energies the $\mathrm{Li} 2s$ character is reduced in favor of the Li and $\mathrm{Al} p$ character, which are dominant around the Fermi energy. The main intrasite-charge-redistribution effects involve pronounced $\mathrm{Li} 2s$ to $\mathrm{Li} 2p$ promotion (with a smaller $s\ensuremath{-}p$ promotion on the Al site) while the intersite (ionic) redistribution effects are found to be small. The observed trends in the measured Knight shifts (relative to the pure constituent metals) as well as the small paramagnetism and its dependence on the Li concentration are discussed in terms of these bonding effects. The abrupt changes in the differential electrical resistivity at \ensuremath{\sim} 100\ifmmode^\circ\else\textdegree\fi{}K is tentatively assigned to a structural instability induced by electron-hole interaction effects.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

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.

Self-interaction correction to density-functional approximations for many-electron systems

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.

A comprehensive review of zno materials and devices

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. ...