Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

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.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

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.

Self-interaction correction to density-functional approximations for many-electron systems

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

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. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

Whereas low-temperature ferroelectrics have a well understood ordered spatial dipole arrangement, the fate of these dipoles in paraelectric phases remains poorly understood. This is studied here as an energy minimization problem using both static and molecular dynamic (MD) density functional theory (DFT). We find that considering the non-thermal internal energy already reveals the formation of a distribution of static local displacements that (i) mimic the symmetries of the low temperature phases, while (ii) being the precursors of what high temperature DFT MD finds as thermal motifs.

Intrinsic local symmetry-breaking in nominally cubic paraelectric BaTiO3

The total energy of ordered and disordered phases of AlAs-GaAs systems is expanded in a series of multiatom interaction energies determined from first-principles linear muffin-tin orbital and linear augmented-plane-wave calculations of simple superstructures. These interaction energies are used to discuss the stability of different superlattices and that of the random alloy.

First-principles calculation of the formation energies of ordered and disordered phases of AlAs-GaAs

Comment on ``Anomalous Temperature Dependence of the X-Ray Diffuse Scattering Intensity of C u 3 Au”

We consider (AlAs)n/(GaAs)n superlattices with random thickness fluctuations Δn around the nominal period n. Using three-dimensional pseudopotential plane-wave band theory, we show that i) any amount Δn/n of thickness fluctuations leads to band edge wave function localization, ii) for small Δn/n the SL band gap is pinned at the gap level produced by a single layer with "wrong" thickness n + Δn, iii) the bound states due to monolayer thickness fluctuations lead to significant band gap reductions, e.g., in n = 2, 4, 6, and 10 monolayer SLs the reductions are 166, 67, 29, and 14 meV for 111 SLs, and 133, 64, 36, and 27 meV for 001 SLs, iv) 001 AlAs/GaAs SLs with monolayer thickness fluctuations have a direct band gap, while the ideal 001 SLs are indirect for n < 4.

https://iopscience.iop.org/article/10.1209/0295-5075/31/2/008/pdf

Localization and Band Gap Pinning in Semiconductor Superlattices with Layer Thickness Fluctuations

The complexity of current {ital ab} {ital initio} quantum-mechanical calculations of the total energy of given distributions of atoms on a periodic lattice often limits explorations to just a few configurations. We show how such a small number of calculations can be used instead to compute the interaction energies of a generalized Ising model, which then readily provides predicted energies of many more interesting configurations. This is illustrated for AlAs/GaAs systems.

Alex Zunger

Papers

Intrinsic local symmetry-breaking in nominally cubic paraelectric BaTiO3

First-principles calculation of the formation energies of ordered and disordered phases of AlAs-GaAs

Comment on ``Anomalous Temperature Dependence of the X-Ray Diffuse Scattering Intensity of C u 3 Au”

Localization and Band Gap Pinning in Semiconductor Superlattices with Layer Thickness Fluctuations

Predicting structural energies of atomic lattices.