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.

Piezoelectricity in Nominally Centrosymmetric Phases

Abstract: Compound phases often display properties that are symmetry-forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, imperfections) are not apparent. Here, we investigate macroscopic inversion symmetry breaking in nominally centrosymmetric materials and measure Resonant Piezoelectric Spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e. NaCl,cfused silica, and CaF2. We exclude the flexoelectric effect as a source of the observed piezoelectricity yetcobserve piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally non-piezoelectric phases, finding that the "symmetry-forbidden" piezoelectric effect ranges from 1 pm/V to 10E-5 pm/V. The values for unpoled ferroelectric phases are only slightly higher than those in the paraelectric phase of the same material. The lowest coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as an ultra-highly sensitive method to measure piezoelectricity. We suggestt hat symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is ubiquitous.

Quantum architecture of novel solids

Abstract: The current status of our understanding of Quantum Mechanics is that if one specifies the chemical formula of a compound (e.g., CuAu, or GaAs, or NiPt) it is still impossible to predict if this material is a superconductor or not, but it is now possible to predict its crystal structure. This is a nontrivial accomplishment for there are as many as 2N possible structures for a binary compound. This article reviews this classic question of structural chemistry and condensed matter physics: How can one figure out which of the astronomic number of possible crystal structures is selected by Nature?

M ay 2 00 4 Metal-nonmetal transition and excitonic ground state in InAs / InSb quantum dots

Abstract: Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict a metal-nonmetal transition and an excitonic ground state in the InAs/InSb quantum dot (QD) system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the single-particle gap Eg becomes very small. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wavefunctions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of fully discrete 0D confined energy levels. These correlation effects overcome Eg, leading to the formation of a bi-excitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by ∼ 15 meV) than the state without excitons. We discuss the excitonic phase transition on QD arrays in the low dot density limit.

Structural instability in zinc-blende semiconductors

Abstract: The conditions for the occurrence of off-center atomic displacements in pure zinc-blende semiconductors have been studied. A coupling of a chemically active valence d band with an s-like conduction band is predicted to lead to such a metastability. Total energy calculations confirm that this is the case in CuCl and CuBr. The unusual experimental manifestations of this metastability are outlined.

Prediction and Synthesis of Strain Tolerant RbCuTe Crystals Based on Rotation of One-Dimensional Nano Ribbons within a Three-Dimensional Inorganic Network.

Abstract: A unique possibility for a simple strain tolerant inorganic solid is envisioned whereby a set of isolated, one-dimensional (1D) nano objects are embedded in an elastically soft three-dimensional (3D) atomic matrix thus forming an interdimensional hybrid structure (IDHS). We predict theoretically that the concerted rotation of 1D nano objects could allow such IDHSs to tolerate large strain values with impunity. Searching theoretically among the 1:1:1 ABX compounds of I-I-VI composition, we identified, via first-principles thermodynamic theory, RbCuTe, which is a previously unreported but now predicted-to-be-stable compound in the MgSrSi-type structure, in space group Pnma. The predicted structure of RbCuTe consists of ribbons of copper and telluride atoms placed antipolar to one another throughout the lattice with rubidium atoms acting as a matrix. A novel synthetic adaptation utilizing liquid rubidium and vacuum annealing of the mixed elemental reagents in fused silica tubes as well as in situ (performed at the Advanced Photon Source) and ex situ structure determination confirmed the stability and predicted structure of RbCuTe. First-principles calculations then showed that the application of up to ∼30% uniaxial strain on the ground-state structure result in a buildup of internal stress not exceeding 0.5 GPa. The increase in total energy is 15-fold smaller than what is obtained for the same RbCuTe material but in structures having a contiguous set of 3D chemical bonds spanning the entire crystal. Furthermore, electronic structure calculations revealed that the HOMO is a 1D energy band localized on the CuTe ribbons and that the 1D insulating band structure is also resilient to such large strains. This combined theory and experiment study reveals a new type of strain tolerant inorganic material.

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. ...