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.

Relativity-Induced Ordering and Phase Separation in Intermetallic Compounds

Abstract: The formation enthalpies of ordered compounds and the mixing enthalpies of random alloys were calculated for Ni-Au, Ni-Pt, and Au-Pt using an Ising-like cluster expansion based on the local-density formalism. We show that relativity i) induces long-range order in Ni-Pt due to a reduction in packing strain and enhancement of s-d coupling, but ii) it leads to phase separation in Au-Pt due to suppression of the Au(s,p) → Pt(d) charge transfer.

Transforming Common III–V and II–VI Semiconductor Compounds into Topological Heterostructures: The Case of CdTe/InSb Superlattices

Abstract: Currently, known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. It is shown via first-principle calculations that a heterovalent superlattice made of common semiconductor building blocks can transform its non-TI components into a topological nanostructure, illustrated by III–V/II–VI superlattice InSb/CdTe. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. The relationship between the interfacial stability and the topological transition is revealed, finding a “window of opportunity” where both conditions can be optimized. Once a critical InSb layer thickness above ≈1.5 nm is reached, both [111] and [100] superlattices have a relative energy of 1.7–9.5 meV A–2, higher than that of the atomically exchanged interface and an excitation gap up to ≈150 meV, affording room-temperature quantum spin Hall effect in semiconductor superlattices. The understanding gained from this study could broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next-generation superstructured functional materials.

Optical anisotropy and spin polarization in ordered GaInP

Abstract: Spontaneous CuPt‐like ordering of GaxIn1−xP causes a splitting at the valence band maximum (VBM) and induces an anisotropy in the intensities of the transitions between these split VBM components and the conduction band minimum. We calculate these intensities as function of ordering parameter η, and show that the transition intensities depend strongly on the light polarization e and the degree of long‐range order η in the sample. Furthermore, for sufficiently ordered single‐subvariant sample, 100% spin polarization of emitted photoelectrons is predicted.

Large Insulating Gap in Topological Insulators Induced by Negative Spin-Orbit Splitting

Abstract: In a cubic topological insulator (TI), there is a band inversion whereby the $s$-like ${\ensuremath{\Gamma}}_{6\mathrm{c}}$ conduction band is below the $p$-like ${\ensuremath{\Gamma}}_{7\mathrm{v}}+{\ensuremath{\Gamma}}_{8\mathrm{v}}$ valence bands by the ``inversion energy'' ${\ensuremath{\Delta}}_{i}l0$. In TIs based on the zinc-blende structure such as HgTe, the Fermi energy intersects the degenerate ${\ensuremath{\Gamma}}_{8\mathrm{v}}$ state so the insulating gap E${}_{g}$ between occupied and unoccupied bands vanishes. To achieve an insulating gap E${}_{g}g0$ critical for TI applications, one often needs to resort to structural manipulations such as structural symmetry lowering (e.g., Bi${}_{2}$Se${}_{3}$), strain, or quantum confinement. However, these methods have thus far opened an insulating gap of only $l$0.1 eV. Here we point out that there is an electronic rather than structural way to affect an insulating gap in a TI: if one can invert the spin-orbit levels and place ${\ensuremath{\Gamma}}_{8\mathrm{v}}$ below ${\ensuremath{\Gamma}}_{7\mathrm{v}}$ (``negative spin-orbit splitting''), one can realize band inversion (${\ensuremath{\Delta}}_{i}l0$) with a large insulating gap (E${}_{g}$ up to 0.5 eV). We outline design principles to create negative spin-orbit splitting: hybridization of $d$ orbitals into $p$-like states. This general principle is illustrated in the ``filled tetrahedral structures'' (FTS) demonstrating via GW and density functional theory (DFT) calculations E${}_{g}g0$ with ${\ensuremath{\Delta}}_{i}l0$, albeit in a metastable form of FTS.

Self-consistent LCAO calculation of the electronic properties of graphite. II. Point vacancy in the two-dimensional crystal

Abstract: The electronic properties of a point vacancy in the two-dimensional graphite crystal are investigated within the small-periodic-cluster approach using a self-consistent all-valence-electron LCAO (linear combination of atomic orbitals) scheme previously employed for the calculations of the band structure and optical spectra of the regular lattice (part I). Eight crystal bands, 54-96 $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$ points in the Brillouin zone, selected according to the "mean value theorem" and ${2}^{2}$-${5}^{2}$ primitive unit cells around the defect site are allowed to interact. A doubly degenerate singly occupied $\ensuremath{\sigma}$ defect level is shown to appear in the $\ensuremath{\sigma}\ensuremath{-}{\ensuremath{\sigma}}^{*}$ band gap, 3.5 eV above the $\ensuremath{\sigma}$ band edge, with a wave function that is about 80% localized on the three nearest-neighbor atoms. The density of electronic states, charge distribution and Poisson electrostatic potential of the defect structure are computed and used to discuss the characteristic feature of the defect in connection with Coulson's "defect molecule" model and with current models of electron trapping mechanisms used to interpret the experimental data on Hall coefficient, resistivity and diamagnetic susceptibility of damaged graphite. Both symmetric and Jahn-Teller lattice distortions are introduced around the defect site, the results being used to interpret the experimentally observed decrease in lattice constant, the observed optical absorption and the vibronic parameters of the Jahn-Teller effect. Symmetric lattice relaxations are shown to have a moderate effect on the lattice energy and on the position of the defect level, these changes being mainly due to the response of the $\ensuremath{\pi}$ subsystem to accumulation of excess $\ensuremath{\pi}$ charge on the surrounding bonds, while Jahn-Teller distortions are shown to have a small effect on the system due to the relative rigidity of the $\ensuremath{\sigma}$ skeleton. The energy of vacancy formation as well as the energy of atom displacement and vacancy migration are directly computed from the change in total lattice energy, the results being in good agreement with experiment. The importance of introducing charge self-consistency in treating the charge redistribution in the system as well as the significance of allowing more distant atoms to interact with the vacancy electrons, is emphasized.

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. ...