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.

Asymmetric cation nonstoichiometry in spinels: Site occupancy in Co 2 ZnO 4 and Rh 2 ZnO 4

Abstract: Two cations $A$ and $B$ in ${A}_{2}B{\text{O}}_{4}$ spinels appear in precise 2:1 Daltonian ratio (``line compounds'') only at very low temperature. More typically, at finite temperature, they tend to become either $A$ rich or $B$ rich. Here we survey the experimentally observed stoichiometry asymmetries and describe the first-principles framework for calculating these. Defect calculations based on first principles are used to calculate the enthalpies of substitution of $A$ atom $\ensuremath{\Delta}H({A}_{{\mathrm{T}}_{\mathrm{d}}}$) and $B$ atom $\ensuremath{\Delta}H({B}_{{\mathrm{O}}_{\mathrm{h}}}$) and determine their site occupancies leading to (non)-stoichiometry. In Co${}_{2}$ZnO${}_{4}$, the result of the calculation for site occupancy compares well with that measured via anomalous x-ray diffraction. Further, the calculated phase boundary also compares well with that measured via Rietveld refinement of x-ray diffraction data on bulk ceramic sintered samples of Co${}_{2}$ZnO${}_{4}$ and Rh${}_{2}$ZnO${}_{4}$. These results show that Co${}_{2}$ZnO${}_{4}$ is heavily Co nonstoichiometric above 500 ${}^{\ensuremath{\circ}}$C, whereas Rh${}_{2}$ZnO${}_{4}$ is slightly Zn nonstoichiometric. We found that, in general, the calculated $\ensuremath{\Delta}H({A}_{{\mathrm{T}}_{\mathrm{d}}}$) is smaller than $\ensuremath{\Delta}H({B}_{{\mathrm{O}}_{\mathrm{h}}}$), if the $A$-rich competing phase is isostructural with the ${A}_{2}B$O${}_{4}$ host, for example, ${A}_{2}A{\text{O}}_{4}$, whereas $B$-rich competing phase is not, for example, $B$O. This observation is used to qualitatively explain nonstoichiometry and solid solutions observed in other spinels.

Why are the 3 d -5 d compounds CuAu and NiPt stable, whereas the 3 d -4 d compounds CuAg and NiPd are not

Abstract: We show that the existence of stable, ordered $3d\ensuremath{-}5d$ intermetallics CuAu and NiPt, as opposed to the unstable $3d\ensuremath{-}4d$ isovalent analogs CuAg and NiPd, results from relativity. First, in shrinking the equilibrium volume of the $5d$ element, relativity reduces the atomic size mismatch with respect to the $3d$ element, thus lowering the elastic packing strain. Second, in lowering the energy of the bonding $6s,p$ bands and raising the energy of the $5d$ band, relativity enhances (diminishes) the occupation of the bonding (antibonding) bands. The raising of the energy of the $5d$ band also brings it closer to the energy of the $3d$ band, improving the $3d\ensuremath{-}5d$ bonding.

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. We show via first-principle calculations how 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. We reveal the relationship between the interfacial stability and the topological transition, 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 5-14 meV/A2 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 significantly broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next-generation super-structured functional materials.

Phenomenology of solid solubilities and ion-implantation sites: An orbital-radii approach

Abstract: The crossing points of the first-principles nonlocal screened atomic pseudopotentials of the elements were shown previously to constitute a sensitive anisotropic atomic-size scale This scale allows systematization of the crystal structure of as many as 565 binary compounds A Zunger, [Phys Rev B 22, 5839 (1980)] In this paper we apply the same coordinates for systematizing the trends in the solid solubilities in the divalent solvents Be, Mg, Zn, Cd, Hg, and in the semiconductor solvents Si and Ge (192 data points), as well as the location of the ion-implantation sites in Be and Si (60 data points) We find that these nonempirical and atomic coordinates produce a systematization of the data that overall is equal to or better than that produced by the empirical coordinates of Miedema and Darken-Gurry which are derived from properties of the condensed phases Furthermore, it is found that the orbital-radii coordinates which incorporate directly the effects of only the $s$ and $p$ atomic orbitals are capable of predicting the solubility trends and ion-implantation sites even for the (nonmagnetic) transition atom impurities

Electronic structure and stability of II--VI semiconductors and their alloys: The role of metal d bands

Abstract: It has been traditionally accepted in various theoretical approaches to II–VI semiconductors (e.g., tight binding, pseudopotentials) to neglect the effects of the cation d bands, hoping that in some sense they are ‘‘deep,’’ ‘‘localized,’’ and hence, unresponsive to many perturbations of chemical interest. There are, however, two qualitative reasons to think that this is not so: first, d bands in II–VI’s are only 7–11 eV below the valence‐band maximum (VBM) (i.e., inside the main valence band), and second, in tetrahedral (but not octahedral) symmetry, cation d orbitals have the same representation (Γ15) as the anion p orbitals, hence the two can interact. We have considered the effects of the cation d bands in II–VI’s on both the electronic and structural properties of the binary and ternary compounds, treating all electrons on the same footing, in a self‐consistent first‐principles manner. We find that the d orbitals: (i) reverse the direction of charge transfer in the alloy (e.g., relative to s–p tight b...

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. ...