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.

Instability of diatomic deuterium in fcc palladium

Abstract: To clarify some of the solid-state aspects of cold fusion in deuterated transition metal electrodes, we have carried out first-principles self-consistent total energy calculations for various configurations of atomic and diatomic deuterium inside fcc palladium. We find that the stability of the Pd+D system is controlled by the relative position of the deuterium-inducedantibonding level with respect to the Fermi energy. The equilibrium D-D distance in dense PdD α up to α=3 is found to be much larger than the free space value. The calculated Born-Oppenheimer energy surface of diatomic D2 in crystalline palladiuim is shown to have but metastable local minima whose internuclear separation is at least 0.2 Alarger than that of the isolated D2 molecule. We conclude that D2 incrystalline Pd will have a substantially lower tunneling probability than hitherto thought and that explanation for fusion mechanisms should be sought elsewhere.

Ferri-chiral compounds with potentially switchable Dresselhaus spin splitting

Abstract: Spin splitting of energy bands can be induced by relativistic spin-orbit interactions in materials without inversion symmetry Whereas polar space-group symmetries permit Rashba (R-1) spin splitting with helical spin textures in momentum space, which could be reversed upon switching a ferroelectric polarization via applied electric fields, the ordinary Dresselhaus effect $(\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}})$ is active in materials exhibiting nonpolar noncentrosymmetric crystal classes with atoms occupying exclusively nonpolar lattice sites Consequently, the spin-momentum locking induced by $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}}$ is not electric field switchable Here we find a type of ferri-chiral materials with an alternative type of Dresselhaus symmetry, referred to as $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{B}}$, exhibiting crystal class constraints similar to $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}}$ (all dipoles add up to zero), but unlike $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}}$, at least one polar site is occupied The spin splitting is associated with the crystalline chirality, which in principle could be reversed upon a change in chirality Focusing on alkali metal chalcogenides, we identify ${\mathrm{NaCu}}_{5}{\mathrm{S}}_{3}$ in the nonenantiomorphic ferri-chiral structure, which exhibits ${\mathrm{CuS}}_{3}$ chiral units differing in the magnitude of their Cu displacements We then synthesize ${\mathrm{NaCu}}_{5}{\mathrm{S}}_{3}$ (space group $P{6}_{3}22$) and confirm its ferri-chiral structure with powder x-ray diffraction Our electronic structure calculations demonstrate it exhibits $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{B}}$ spin splitting as well as a ferri-chiral phase transition, revealing spin splitting interdependent on chirality Our electronic structure calculations show that a few percent biaxial tensile strain can reduce (or nearly quench) the switching barrier separating the monodomain ferri-chiral $P{6}_{3}22$ states We compute the circular dichroism absorption spectrum of the two ferri-chiral orientations and discuss what type of external stimuli might switch the chirality so as to reverse the (nonhelical) Dresselhaus $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{B}}$ spin texture Our study suggests the design of ferri-chiral crystals as potential spintronic and optoelectronic materials

FeSe as a Polymorphous Network

Abstract: The observed electronic structure of FeSe has lower apparent symmetry than the one suggested by its macroscopic crystallographic structure. It has been argued that such nematicity must be electronic symmetry lowering, driven by strong correlations, rather than a local structural symmetry lowering, the latter being judged on the observation of a too small global structural symmetry lowering. Standard structure predictions use small unit cells that cannot accommodate structural symmetry lowering. Using a predictive first principles minimization of the internal total energy without restricting the system to a small unit cell reveals that the lowest energy configuration whose average macroscopic symmetry is tetragonal consists of a distribution of different local low-symmetries. This polymorphous network explains the observed Pair Distribution Function (PDF) pattern in both the local and long-range regions without a fit. When used as input to electronic structure calculations, the predicted polymorphous structure reveals electronic symmetry breaking that is unique to this unusual compound.

Prediction of low-Z collinear and noncollinear antiferromagnetic compounds having momentum-dependent spin splitting even without spin-orbit coupling

Abstract: Recent study (Yuan et al, Phys Rev B 102, 014422 (2020)) revealed a SOC-independent spin splitting and spin polarization effect induced by antiferromagnetic ordering which do not necessarily require breaking of inversion symmetry or the presence of SOC, hence can exist even in centrosymmetric, low-Z light element compounds, considerably broadening the material base for spin polarization In the present work we develop the magnetic symmetry conditions enabling such effect, dividing the 1651 magnetic space groups into 7 different spin splitting prototypes (SST-1 to SST-7) We use the 'Inverse Design' approach of first formulating the target property (here, spin splitting in low-Z compounds not restricted to low symmetry structures), then derive the enabling physical design principles to search realizable compounds that satisfy these a priori design principles This process uncovers 422 magnetic space groups (160 centrosymmetric and 262 non-centrosymmetric) that could hold AFM-induced, SOC-independent spin splitting and spin polarization We then search for stable compounds following such enabling symmetries We investigate the electronic and spin structures of some selected prototype compounds by density functional theory (DFT) and find spin textures that are different than the traditional Rashba-Dresselhaus patterns We provide the DFT results for all antiferromagnetic spin splitting prototypes (SST-1 to SST-4) and concentrate on revealing of the AFM-induced spin splitting prototype (SST-4) The symmetry design principles along with their transformation into an Inverse Design material search approach and DFT verification could open the way to their experimental examinationM) The symmetry design principles along with their transformation into an Inverse Design material search approach and DFT verification could open the way to their experimental examination

Rules of Peak Multiplicity and Peak Alignment in Multiexcitonic Spectra of (In,Ga)As Quantum Dots

Abstract: A simple model---the single-configuration perturbation theory---has traditionally been used to explain the main features of the multiexcitonic spectra of quantum dots, where an electron and a hole recombine in the presence of other ${N}_{e}\ensuremath{-}1$ electrons and ${N}_{h}\ensuremath{-}1$ holes. The model predicts the $({N}_{h},{N}_{e})$ values for which such spectra consist of a single line or multiple lines and whether singlet lines of different $({N}_{h},{N}_{e})$ values are energetically aligned. Here we use a nonperturbative, correlated approach that shows when such simple rules work and when they fail, thereby establishing a basis for the appropriate use of such rules.

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. ...