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.

Spin-dependent correlated atomic pseudopotentials

Abstract: The previously developed first-principles density-functional (nonlocal) atomic pseudopotentials are extended to include explicit spin effects as well as electronic correlation effects beyond the local-spin-density (LSD) formalism. Such angular-momentum-and spin-dependent pseudopotentials enable the extension of pseudopotential applications to study magnetic problems (e.g., transition-metal and other open-shell impurities in solids, ferromagnetic surfaces, etc.). As the spurious electronic self-interaction terms characterizing the LSD energy functional are self-consistently removed, these pseudopotentials can also be used to calculate reliably localized electronic states (e.g., deep defect levels, surface and interface states, narrow-band states in solids, etc.). Applications to atoms show that this pseudopotential method removes many of the anomalies of the LSD approach, including the systematically high total energy, the failure to predict the stability of negative ions, the lack of correlation between orbital energies and observed ionization potentials, and the erroneous ordering of $s$ and $d$ levels of the $3d$ transition elements Sc to Fe in their ${d}^{n\ensuremath{-}1}{s}^{1}$ configuration.

Invertible and Non-invertible Alloy Ising Models

Abstract: Physical properties of alloys are compared as computed from ``direct'' and ``inverse'' procedures. The direct procedure involves Monte Carlo simulations of a set of local density approximation (LDA)-derived pair and multibody interactions { u_f}, generating short-range order (SRO), ground states, order- disorder transition temperatures, and structural energy differences. The inverse procedure involves ``inverting'' the SRO generated from { u_f} via inverse-Monte-Carlo to obtain a set of pair only interactions {\tilde{ u}_f}. The physical properties generated from {\tilde{ u}_f} are then compared with those from { u_f}. We find that (i) inversion of the SRO is possible (even when { u_f} contains multibody interactions but {\tilde{ u}_f} does not) but, (ii) the resulting interactions {\tilde{ u}_f} agree with the input interactions { u_f} only when the problem is dominated by pair interactions. Otherwise, {\tilde{ u}_f} are very different from { u_f}. (iii) The same SRO pattern can be produced by drastically different sets { u_f}. Thus, the effective interactions deduced from inverting SRO are not unique. (iv) Inverting SRO always misses configuration-independent (but composition- dependent) energies such as the volume deformation energy G(x); consequently, the ensuing {\tilde{ u}_f} cannot be used to describe formation enthalpies or two-phase regions of the phase diagram, which depend on G(x).

Strain effects on the spectra of spontaneously ordered GaxIn1−xP

Abstract: Spontaneous (111) CuPt‐like ordering has been widely observed in GaxIn1−xP lattice matched (x=x0) to a GaAs(001) substrate. This leads to a band‐gap reduction ΔEg and to a valence‐band splitting ΔE12. We explore here the consequence of the coexistence of (001) epitaxial strain (produced by selecting x≠x0) and (111) chemical ordering. This leads to distinct changes in ΔEg and ΔE12 which could serve as new fingerprints of ordering.

Comparison of experimental and theoretical electronic charge distribution in γ-TiAl

Abstract: Using critical voltage electron diffraction, Fox has recently determined the lowest seven X-ray structure factors of [gamma]-TiAl (L1[sub 0] structure). The authors present here a comparison of these accurately measured (0.15%) structure factors with first-principles local density calculations, finding an agreement within 0.7% and an r.m.s. error of 0.013 e/atom. While such measurements are limited to the first few structure factors [rho](G) (where G is the crystal momentum), theory is able to obtain [rho](G) for arbitrarily high G. If they construct charge density deformation maps by Fourier summations up to the lowest measured G, the calculated and experimental density deformation maps agree very closely. However, if they include in the theoretical density deformation map high G values (outside the range accessible to experiment), qualitatively different bonding patterns appear, in particular between Ti atoms. Systematic study of the total, valence, and deformation charge densities as well as comparison with result for NiAl in the hypothetical L1[sub 0] structure elucidate the bonding patterns in these transition metal aluminides.

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. ...