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

Investigation of the phase transitions in cesium by the average atom model

Abstract: Using the average atom method (quasizone model) we show that the cesium cold curve has two minimums – the first one is at relatively small densities 0.3 g/cm3and the second one is at higher densities 6.2 g/cm3. The first one leads to the usual critical point characterizing the liquid-vapor phase transition, which is confirmed by experimental data. The reason for the second minimum is connected with s-d transition of valence electron. This leads to a second phase transition with the critical point at temperature 6400 K, density 5.3 g/cm3and pressure approximately 61000 atm. This phase transition takes place in the cesium plasma state. In this degenerate nonideal plasma with mean ion charge 1.5 and density 3 g/cm3jumps in a highly ionized state with mean charge of 3.5 and density 6.5 g/cm3.Note: Research direction:Mathematical modelling in actual problems of science and technic