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.

Practical rules for orbital-controlled ferromagnetism of 3d impurities in semiconductors

Abstract: We distill from first-principles spin-polarized total-energy calculations some practical rules for predicting the magnetic state (ferromagnetic/antiferromagnetic/paramagnetic) of substitutional transition-metal impurity with different charge state in various host crystal groups IV, III-V, II-VI, I-III-VI2, and II-IV-V2 semiconductors. The basic mechanism is the stabilization of a ferromagnetic bond between two transition metals if the interacting orbitals are partially-occupied. These rules are then subjected to quantitative tests, which substantiate the mechanism of ferromagnetism in these systems. We discuss cases where current electronic structure calculations agree with these rules, and identify a few cases where conflicts exist. The effect of doping on transition-metal magnetic properties is also covered by these rules by considering the oxidation state changes due to doping. In addition, we systematically apply these rules to ideal substitutional impurities, contrasting our predictions with experime...

Ferroelectric properties of Cd1−xZnxTe solid solutions

Abstract: Measurements of the spontaneous polarization P, x‐ray diffraction, birefringence, dielectric constant at different frequencies, and specific heat Cp of the Cd0.9Zn0.1Te alloy are presented. The results demonstrate that this system exhibits a diffuse, second‐order ferroelectric transition. The transition is of order‐disorder type as deduced from the dielectric measurements. It is found that: (a) The birefringence is proportional to P, as expected from a system with a piezoelectric paraelectric phase; (b) the heat capacity is given by Cp=(Tc/C)‖(PdP/dT)‖, where C is the Curie constant. One of the main phenomena observed in these solid solutions is the instability of the ferroelectric phase: Once the neighborhood of the transition temperature is reached, the transition disappears upon subsequent cooling. A hypothesis for this instability is offered in terms of a two‐state configuration‐coordinate diagram.

Origins versus fingerprints of the Jahn-Teller effect in d -electron A B X 3 perovskites

Abstract: The authors identify the modalities enabling an electronically induced distortion, which is identical across the board of 3d elements showing electronic degenerate states in the high symmetry cubic cell. This constitutes the fingerprint of a Jahn-Teller effect. Materials without electronic instabilities such as LaMnO3 display an alternate lattice distortion simply resulting from lattice mode couplings with the sterically induced distortions.

Calculation of conduction-to-conduction and valence-to-valence transitions between bound states in (In,Ga)As/GaAs quantum dots

Abstract: We have calculated the conduction-to-conduction and valence-to-valence absorption spectrum of bound states in $(\mathrm{In},\mathrm{Ga})\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots charged with up to three electrons or holes. Several features emerge: (i) In pure (nonalloyed) $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ dots, the $1S\text{\ensuremath{-}}1{P}_{1}$ and $1S\text{\ensuremath{-}}1{P}_{2}$ conduction intraband transitions are fully in-plane polarized along $[1\overline{1}0]$ and [110], respectively, while valence transitions are weakly polarized because the hole $P$ states do not show any in-plane preferential orientation. (ii) In alloyed ${\mathrm{In}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ dots the [110] and $[1\overline{1}0]$ polarization of the corresponding $1S\text{\ensuremath{-}}1P$ conduction intraband transitions is weakened since the two $1P$ states are mixed by alloy fluctuations. The polarization of valence intraband transitions is insensitive to changes in alloy fluctuations. (iii) For light polarized along [001], we find a strong valence-to-valence transition that involves a weakly confined hole state with predominant light-hole character. (iv) When charging the dots with a few electrons, the conduction intraband transitions display spectroscopic shifts of $\ensuremath{\sim}1\char21{}2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. These shifts are a result of correlation effects (captured by configuration interaction) and not well described within the Hartree-Fock approximation. (v) When charging the dots with holes, valence intraband spectra are more complex than the conduction intraband spectra as hole states are strongly affected by spin-orbit coupling, and configuration mixing is more pronounced. Spectroscopic shifts can no longer be identified unambiguously. These predictions could be tested in single-dot spectroscopy of $n$-doped and $p$-doped quantum dots.

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. ...