Silicon undergoes a phase transition from the semiconducting diamond phase to the metallic ␤-Sn phase under pressure. We use quantum Monte Carlo calculations to predict the transformation pressure and compare the results to density-functional calculations employing the local-density approximation, the generalizedgradient approximations PBE, PW91, WC, AM05, PBEsol, and the hybrid functional HSE06 for the exchangecorrelation functional. Diffusion Monte Carlo predicts a transition pressure of 14.0Ϯ 1.0 GPa slightly above the experimentally observed transition pressure range of 11.3-12.6 GPa. The HSE06 hybrid functional predicts a transition pressure of 12.4 GPa in excellent agreement with experiments. Exchange-correlation functionals using the local-density approximation and generalized-gradient approximations result in transition pressures ranging from 3.5 to 10.0 GPa, well below the experimental values. The transition pressure is sensitive to stress anisotropy. Anisotropy in the stress along any of the cubic axes of the diamond phase of silicon lowers the equilibrium transition pressure and may explain the discrepancy between the various experimental values as well as the small overestimate of the quantum Monte Carlo transition pressure.

We show that carbon-doped hexagonal boron nitride (h-BN) has extraordinary properties with many possible applications. We demonstrate that the substitution-induced impurity states, associated with carbon atoms, and their interactions dictate the electronic structure and properties of C-doped h-BN. Furthermore, we show that stacking of localized impurity states in small C clusters embedded in h-BN forms a set of discrete energy levels in the wide gap of h-BN. The electronic structures of these C clusters have a plethora of applications in optics, magneto-optics, and opto-electronics.The advent of the field of two-dimensional (2D) crystals is marked by the isolation and electronic characterization of graphene.1-3 Not long after that, other layered 2D crystals were also

We demonstrate the accuracy of the hybrid functional HSE06 for computing band offsets of semiconductor alloy heterostructures. The highlight of this study is the computation of conduction band offsets with a reliability that has eluded standard density functional theory. A high-quality special quasirandom structure models an infinite random pseudobinary alloy for constructing heterostructures along the (001) growth direction. Our excellent results for a variety of heterostructures establish HSE06's relevance to band engineering of high-performance electrical and optoelectronic devices.PACS numbers: 78.55.Cr Heterostructures are ubiquitous in semiconductor technology.For instance, AlInAs/InGaAs is used for quantum cascade lasers and infrared photodetectors; InGaP/AlGaAs for high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs), and phototransistors; AlInAs/InP for HEMTs; In-GaP/GaAs for HBTs; and InGaAs/InP for single-photon avalanche photodiodes and HBTs. Among the most important properties that determine the feasibility and performance of heterostructure devices are the band offsets. These are the discontinuities between the valence band maxima (VBM) or conduction bands minima (CBM) of each semiconductor at their common interface, and act as barriers to electrical transport across the interface. Band engineering of novel devices with desired properties, particularly quantum cascade lasers and quantum dot-based devices, critically require a precise knowledge of band offsets. However, reliable measurements and predictions of band offsets continue to be challenging despite extensive theoretical and experimental efforts. [1][2][3] Density functional theory (DFT) is an efficient method for calculating electronic structure. The accuracy of DFT calculations is controlled by the choice of exchangecorrelation (XC) functional. Local and semi-local functionals such as LDA (local density approximation) and PBE (Perdew-Burke-Ernzerhof) 4 fail to produce accurate bandgaps, and in extreme cases predict small gap semiconductors as metals. Hybrid XC functionals, that include a fraction of Hartree-Fock (HF) exchange, provide a promising alternative. In this letter, we determine the suitability of a hybrid functional HSE06 (Heyd-Scuseria-Ernzerhof) 5 to compute band offsets of several III-V compounds and pseudobinary alloy heterostructures. HSE06 includes a fraction, α, of screened, shortrange HF exchange to improve the derivative discontinuity of the Kohn-Sham potential for integer electron numbers (default HSE06 uses α=0.25). This functional was recently used to predict the band alignments throughout a) 1.52 (ind) 0.21 1.12 0.18 mod−HSE06 Experiment PBE 2.10 (ind) 0.27 0.31 1.52 2.19 (ind) 0.26 0.42 1.52 FIG. 1. Band alignments for Al0.5Ga0.5As/GaAs heterostructure computed with mod-HSE06 (α = 0.30, see text) and PBE, in comparison with experiment. 2 The direct and indirect bandgaps are shown for GaAs and Al0.5Ga0.5As, respectively. The hybrid functional shows significant improvement over PBE ...

High field nuclear magnetic resonance in transition metal substituted BaFe2As2Are trinuclear superhalogens promising candidates for building blocks of novel magnetic materials? A theoretical prospect from combined broken-symmetry density functional theory and ab initio study

ABSTRACT:Energy eigenvalues, probability densities, and ͗x 2r ͘ values (r ϭ 1 to 6) of one-dimensional self-interacting nonlinear oscillators have been obtained by evolving the time-dependent Schrö dinger equation in imaginary time, coupled with the minimization of energy expectation values. For excited states, the orthogonality constraint with lower states is maintained. Probability density plots for the ground and first three excited states are presented. A comparison of energy eigenvalues and probability densities plots is also made between oscillators with and without self-interaction.