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. ...

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A comprehensive review of zno materials and devices

(where each anion C is coordinated by two A-type cations and two 8-type cations) At the same time, the advantage of the EPM approach lies in its flexibility to emulate the experimentally known optical gaps into the band-structure calculation through small adjustments of the parameters, usually producing a perfect agreement with the available data The present authors have previously described a series of all-electron, nonempirical self-consistent studies of the electronic structure of I-III-VI2 chalcopyrites (eg, CuInSe2) where the presence of chemically active Cu 3d orbitals at the vicinity of the top of the valence band makes the EPM particularly problematic In the present paper we extend these calculations to five of the II-IV-Vz chalcopyrite-structure ternary pnictides While existing EMP calculations for many of these compounds are adequate and generally produce fundamental band gaps in

Electronic structure of the ternary pnictide semiconductors ZnSiP 2 , ZnGeP 2 , ZnSnP 2 , ZnSiAs 2 , and MgSiP 2

To predict the ground-state structures and finite-temperature properties of an alloy, the total energies of many different atomic configurations $\mathbf{\ensuremath{\sigma}}\ensuremath{\equiv}{{\ensuremath{\sigma}}_{i};i=1,\dots{},N}$, with $N$ sites $i$ occupied by atom A $({\ensuremath{\sigma}}_{i}=\ensuremath{-}1)$, or B $({\ensuremath{\sigma}}_{i}=+1)$, must be calculated accurately and rapidly. Direct local-density approximation (LDA) calculations provide the required accuracy, but are not practical because they are limited to small cells and only a few of the ${2}^{N}$ possible configurations. The ``mixed-basis cluster expansion'' (MBCE) method allows to parametrize LDA configurational energetics ${E}_{\mathrm{LDA}}[{\ensuremath{\sigma}}_{i};i=1,\dots{},N]$ by an analytic functional ${E}_{\mathrm{MBCE}}[{\ensuremath{\sigma}}_{i};i=1,\dots{},N]$. We extend the method to bcc alloys, describing how to select ${N}_{\ensuremath{\sigma}}$ ordered structures (for which LDA total energies are calculated explicitly) and ${N}_{F}$ pair and multibody interactions, which are fit to the ${N}_{\ensuremath{\sigma}}$ energies to obtain a deterministic MBCE mapping of LDA. We apply the method to bcc Mo-Ta. This system reveals an unexpectedly rich ground-state line, pitting Mo-rich (100) superlattices against Ta-rich complex structures. Predicted finite-$T$ properties such as order-disorder temperatures, solid-solution short-range order and the random alloy enthalpy of mixing are consistent with experiment.

Mixed-basis cluster expansion for thermodynamics of bcc alloys

${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P alloys order spontaneously during growth into a (111) monolayer superlattice despite the fact that this is not the lowest-energy structure of the three-dimensional bulk compound. Using first-principles total-energy calculations, we show that a novel electronically driven surface reconstruction provides a driving force for such ordering.

Surface-induced ordering in GaInP.

Many common crystal structures can be described by a single (or very few) repeated structural motif (``monomorphous structures'') such as octahedron in cubic halide perovskites. Interestingly, recent accumulated evidence suggests that electronic structure calculations based on such macroscopically averaged monomorphous cubic ($Pm\text{\ensuremath{-}}3m$) halide perovskites obtained from x-ray diffraction, show intriguing deviations from experiment. These include systematically too small band gaps, dielectric constants dominated by the electronic, negative mixing enthalpy of alloys, and significant deviations from the measured pair distribution function. We show here that a minimization of the systems $T=0$ internal energy via density functional theory reveals a distribution of different low-symmetry local motifs, including tilting, rotations, and B-atom displacements (``polymorphous networks''). This is found only if one allows for larger-than-minimal cell size that does not geometrically exclude low symmetry motifs. As the (super) cell size increases, the energy is lowered relative to the monomorphous cell, and stabilizes after \ensuremath{\sim}32 formula units (\ensuremath{\ge}160 atoms) are included. Being a result of nonthermal energy minimization in the internal energy without entropy, this correlated set of displacements must represent the intrinsic geometry preferred by the underlying chemical bonding (lone pair bonding), and as such has a different origin than the normal, dynamic thermal disorder modeled by molecular dynamics. Indeed, the polymorphous network, not the monomorphous ansatz, is the kernel structure from which high temperature thermal agitation develops. The emerging physical picture is that the polymorphous network has an average structure with high symmetry, yet the local structural motifs have low symmetries. We find that, compared with monomorphous counterparts, the polymorphous networks have significantly lower predicted total energies, larger band gaps, and ionic dominated dielectric constants, and agree much more closely with the observed pair distribution functions. An analogous polymorphous situation is found in the paraelectric phase of a few cubic oxide perovskites where local polarization takes the role of local displacements in halide perovskites, and in the paramagnetic phases of a few $3d$ oxides where the local spin configuration takes that role.

Polymorphous nature of cubic halide perovskites

Materials that are both electrically conducting and transparent to light are vital for optoelectronic devices, but are rare. Here, the authors perform a quantum mechanical search for such materials and identify the compound TaIrGe as an unexpected possibility, which they then synthesize.

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Alex Zunger

Papers

Electronic structure of the ternary pnictide semiconductors ZnSiP 2 , ZnGeP 2 , ZnSnP 2 , ZnSiAs 2 , and MgSiP 2

Mixed-basis cluster expansion for thermodynamics of bcc alloys

Surface-induced ordering in GaInP.

Polymorphous nature of cubic halide perovskites

Design and discovery of a novel half-Heusler transparent hole conductor made of all-metallic heavy elements