Point-charge electrostatics in disordered alloys

A simple analytic model of point-ion electrostatics has been previously proposed in which the magnitude of the net charge q_i on each atom in an ordered or random alloy depends linearly on the number N_i^(1) of unlike neighbors in its first coordination shell. Point charges extracted from recent large supercell (256-432 atom) local density approximation (LDA) calculations of Cu-Zn random alloys now enable an assessment of the physical validity and accuracy of the simple model. We find that this model accurately describes (i) the trends in q_i vs. N_i^(1), particularly for fcc alloys, (ii) the magnitudes of total electrostatic energies in random alloys, (iii) the relationships between constant-occupation-averaged charges and Coulomb shifts (i.e., the average over all sites occupied by either $A$ or $B$ atoms) in the random alloy, and (iv) the linear relation between the site charge q_i and the constant- charge-averaged Coulomb shift (i.e., the average over all sites with the same charge) for fcc alloys. However, for bcc alloys the fluctuations predicted by the model in the q_i vs. V_i relation exceed those found in the LDA supercell calculations. We find that (a) the fluctuations present in the model have a vanishing contribution to the electrostatic energy. (b) Generalizing the model to include a dependence of the charge on the atoms in the first three (two) shells in bcc (fcc) - rather than the first shell only - removes the fluctuations, in complete agreement with the LDA data. We also demonstrate an efficient way to extract charge transfer parameters of the generalized model from LDA calculations on small unit cells.Comment: 15 pages, ReVTeX galley format, 7 eps figures embedded using psfig, to be published in Phys. Rev.

Spin-polarization-induced structural selectivity in Pd3X_3X and Pt3X_3X (X=3dX=3d) compounds

Spin-polarization is known to lead to important {\it magnetic} and {\it optical} effects in open-shell atoms and elemental solids, but has rarely been implicated in controlling {\it structural} selectivity in compounds and alloys. Here we show that spin-polarized electronic structure calculations are crucial for predicting the correct $T=0$ crystal structures for Pd$_3X$ and Pt$_3X$ compounds. Spin-polarization leads to (i) stabilization of the $L1_2$ structure over the $DO_{22}$ structure in Pt$_3$Cr, Pd$_3$Cr, and Pd$_3$Mn, (ii) to the stabilization of the $DO_{22}$ structure over the $L1_2$ structure in Pd$_3$Co and to (iii) ordering (rather than phase-separation) in Pt$_3$Co and Pd$_3$Cr. The results are analyzed in terms of first-principles local spin density calculations.Comment: 4 pages, REVTEX, 3 eps figures, to appear in PR

Electric fields and valence band offsets at strained [111] heterojunctions

[111] ordered common atom strained layer superlattices (in particular the common anion GaSb/InSb system and the common cation InAs/InSb system) are investigated using the ab initio full potential linearized augmented plane wave (FLAPW) method. We have focused our attention on the potential line-up at the two sides of the homopolar isovalent heterojunctions considered, and in particular on its dependence on the strain conditions and on the strain induced electric fields. We propose a procedure to locate the interface plane where the band alignment could be evaluated; furthermore, we suggest that the polarization charges, due to piezoelectric effects, are approximately confined to a narrow region close to the interface and do not affect the potential discontinuity. We find that the interface contribution to the valence band offset is substantially unaffected by strain conditions, whereas the total band line-up is highly tunable, as a function of the strain conditions. Finally, we compare our results with those obtained for [001] heterojunctions.Comment: 18 pages, Latex-file, to appear in Phys.Rev.

Effects of macroscopic polarization in III-V nitride multi-quantum-wells

Huge built-in electric fields have been predicted to exist in wurtzite III-V nitrides thin films and multilayers. Such fields originate from heterointerface discontinuities of the macroscopic bulk polarization of the nitrides. Here we discuss the background theory, the role of spontaneous polarization in this context, and the practical implications of built-in polarization fields in nitride nanostructures. To support our arguments, we present detailed self-consistent tight-binding simulations of typical nitride QW structures in which polarization effects are dominant.Comment: 11 pages, 9 figures, uses revtex/epsf. submitted to PR

Importance of Correlation Effects on Magnetic Anisotropy in Fe and Ni

We calculate magnetic anisotropy energy of Fe and Ni by taking into account the effects of strong electronic correlations, spin-orbit coupling, and non-collinearity of intra-atomic magnetization. The LDA+U method is used and its equivalence to dynamical mean-field theory in the static limit is emphasized. Both experimental magnitude of MAE and direction of magnetization are predicted correctly near U=4 eV for Ni and U=3.5 eV for Fe. Correlations modify one-electron spectra which are now in better agreement with experiments.Comment: 4 pages, 2 figure

First-principles calculations of the self-trapped exciton in crystalline NaCl

The atomic and electronic structure of the lowest triplet state of the off-center (C2v symmetry) self-trapped exciton (STE) in crystalline NaCl is calculated using the local-spin-density (LSDA) approximation. In addition, the Franck-Condon broadening of the luminescence peak and the a1g -> b3u absorption peak are calculated and compared to experiment. LSDA accurately predicts transition energies if the initial and final states are both localized or delocalized, but 1 eV discrepancies with experiment occur if one state is localized and the other is delocalized.Comment: 4 pages with 4 embeddded figure

Electronic states and optical properties of GaAs/AlAs and GaAs/vacuum superlattices by the linear combination of bulk bands method

The linear combination of bulk bands method recently introduced by Wang, Franceschetti and Zunger [Phys. Rev. Lett.78, 2819 (1997)] is applied to a calculation of energy bands and optical constants of (GaAs)$_n$/(AlAs)$_n$ and (GaAs)$_n$/(vacuum)$_n$ (001) superlattices with n ranging from 4 to 20. Empirical pseudopotentials are used for the calculation of the bulk energy bands. Quantum-confined induced shifts of critical point energies are calculated and are found to be larger for the GaAs/vacuum system. The $E_1$ peak in the absorption spectra has a blue shift and splits into two peaks for decreasing superlattice period; the $E_2$ transition instead is found to be split for large-period GaAs/AlAs superlattices. The band contribution to linear birefringence of GaAs/AlAs superlattices is calculated and compared with recent experimental results of Sirenko et al. [Phys. Rev. B 60, 8253 (1999)]. The frequency-dependent part reproduces the observed increase with decreasing superlattice period, while the calculated zero-frequency birefringence does not account for the experimental results and points to the importance of local-field effects.Comment: 10 pages, 11 .eps figures, 1 tabl

Cation- and vacancy-ordering in Li_xCoO_2

Using a combination of first-principles total energies, a cluster expansion technique, and Monte Carlo simulations, we have studied the Li/Co ordering in LiCoO_2 and Li-vacancy/Co ordering in CoO_2. We find: (i) A ground state search of the space of substitutional cation configurations yields the (layered) CuPt structure as the lowest-energy state in the octahedral system LiCoO_2 (and CoO_2), in agreement with the experimentally observed phase. (ii) Finite temperature calculations predict that the solid-state order- disorder transitions for LiCoO_2 and CoO_2 occur at temperatures (~5100 K and ~4400 K, respectively) much higher than melting, thus making these transitions experimentally inaccessible. (iii) The energy of the reaction E(LiCoO_2) - E(CoO_2) - E(Li) gives the average battery voltage V of a Li_xCoO_2/Li cell. Searching the space of configurations for large average voltages, we find that CuPt (a monolayer superlattice) has a high voltage (V=3.78 V), but that this could be increased by cation randomization (V=3.99 V), partial disordering (V=3.86 V), or by forming a 2-layer Li_2Co_2O_4 superlattice along (V=4.90 V).Comment: 12 Pages, RevTeX galley format, 5 figures embedded using epsf Phys. Rev. B (in press, 1998

BAs and boride III-V alloys

Boron arsenide, the typically-ignored member of the III-V arsenide series BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an X_1c-like indirect band gap, and its bond charge is distributed almost equally on the two atoms in the unit cell, exhibiting nearly perfect covalency. The reasons for these are tracked down to the anomalously low atomic p orbital energy in the boron and to the unusually strong s-s repulsion in BAs relative to most other III-V compounds. We find unexpected valence band offsets of BAs with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is significantly higher than that of AlAs, despite the much smaller bond length of BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects result from the unusually strong mixing of the cation and anion states at the VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and composition-independent band gap bowing. This means that while addition of small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing enthalpies which are smaller than in GaN-GaAs alloys. The unique features of boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for publication in Phys. Rev. B. Scheduled to appear Oct. 15 200

Cross-sectional scanning tunneling microscopy of InAsSb/InAsP superlattices

Cross-sectional scanning tunneling microscopy has been used to characterize compositional structure in InAs{sub 0.87}Sb{sub 0.13}/InAs{sub 0.73}P{sub 0.27} and InAs{sub 0.83}Sb{sub 0.17}/InAs{sub 0.60}P{sub 0.40} strained-layer superlattice structures grown by metal-organic chemical vapor deposition. High-resolution STM images of the (110) cross section reveal compositional features within both the InAs{sub x}Sb{sub 1{minus}x} and InAs{sub y}P{sub 1{minus}y} alloy layers oriented along the [{bar 1}12] and [1{bar 1}2] directions--the same as those in which features would be observed for CuPt-B type ordered alloys. Typically one variant dominates in a given area, although occasionally the coexistence of both variants is observed. Furthermore, such features in the alloy layers appear to be correlated across heterojunction interfaces in a manner that provides support for III-V alloy ordering models which suggest that compositional order can arise from strain-induced order near the surface of an epitaxially growing crystal. Finally, atomically resolved (1{bar 1}0) images obtained from the InAs{sub 0.87}Sb{sub 0.13}/InAs{sub 0.73}P{sub 0.27} sample reveal compositional features in the [112] and [{bar 1}{bar 1}2] directions, i.e., those in which features would be observed for CuPt-A type ordering