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 ...

We report the compositional dependence of the electronic band structure for a range of III-V alloys. Density functional theory with the PBE functional is insufficient to mimic the electronic gap energies at different symmetry points of the Brillouin zone. The HSE hybrid functional with screened exchange accurately reproduces the experimental band gaps and, more importantly, the alloy concentration of the direct-indirect gap crossovers for the III-V alloys studied here: AlGaAs, InAlAs, AlInP, InGaP, and GaAsP.PACS numbers: 71.20. Nr, 71.55.Eq Knowing the alloy concentrations where the semiconductor is direct or indirect is essential for optoelectronic device design. Consequently a correct quantitative assessment of the electronic structure across different symmetry points of the Brillouin zone for the alloy is vital. Most studies focused on density functional theory (DFT) calculations using the local density approximation (LDA) or the generalized gradient approximation (GGA) which do poorly on excited states. Only recently have hybrid functionals been used in predicting accurate excited states in individual semiconducting alloys. 1,2The Heyd-Scuseria-Ernzerhof (HSE) hybrid functional 3-7 , which combines the screened exchange with the Perdew-Burke-Ernzerhof (PBE) GGA functional, 8 out performs previous DFT methods in reproducing bulk band gaps. 6,9,10 We report HSE reproduces not only the band gaps across the entire composition range of each alloy studied here but also the direct-indirect band gap crossovers seen experimentally.Figure 1(a) demonstrates this significant improvement of HSE over PBE in predicting the direct-indirect (Γ-X) crossover (denoted by vertical arrows) for the AlGaAs alloy compared with experiment. 11 HSE reproduces the direct-indirect crossover within 5% Al concentration from the most recent experimental value published by Yi et al. 12 The PBE functional which doesn't take into account screened exchange overestimates this crossover by 23% Al concentration.The disordered zinc-blende (cubic) alloys are best modeled by special quasirandom structures (SQS), 13 ordered structures designed to reproduce the most important pair-correlation functions of a random alloy. The best possible 32-atom SQS's we produce for concentrations of x = 0.25 and x = 0.50 match the pair-correlation functions of a random alloy up to 3rd and 7th nearest neighbors, respectively. 14 The SQS with concentration x = 0.25 can be used interchangeably with that of x = 0.75. Table I gives the lattice vectors for each SQS used to describe the optical transitions as seen in the zinc-blende primitive cell through folding relations in the Brillouin zone. The SQS with concentration x = 0.50 has a basecentered orthorhombic space group symmetry with the

Hybrid screened density functional theory better describes the electronic structure of HgTe, CdTe, and HgCdTe systems in comparison with standard density functional theory. The newer hybrid functional reproduces the band inversion in the popular HgCdTe alloy justifying it as a better method than standard density functional theory in the search for new topological insulators. In addition, the 0.53 eV valence band offset obtained using the hybrid functional supports the recently observed higher band offset in the HgTe/CdTe heterostructure.

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

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Density-functional theory forces, stresses and energies comprise a database from which the optimal parameters of a spline-based empirical potential combining Stillinger-Weber and modified embeddedatom forms are determined. Accuracy of the potential is demonstrated by calculations of ideal shear, stacking fault, vacancy migration, elastic constants and phonons all between 0 and 100 GPa. Consistency with existing models and experiments is demonstrated by predictions of screw dislocation core structure and deformation twinning in a tungsten nanorod. Lastly, the potential is used to study the stabilization of fcc tungsten at high pressure.

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Open OnDemand supports Interactive HPC web applications enabling the interactive and distributed environments for Jupyter and RStudio running on an HPC cluster. These web applications provide a simple user-interface for building and submitting the batch job responsible for launching the interactive environment as well as proxying the connection between the user's browser and the web server running on the cluster. Support for distributive computing through a Jupyter notebook and RStudio session is provided by an Apache Spark cluster launched concurrently in standalone mode on the allocated nodes within the batch job. Alternatively, users can directly use the corresponding MPI bindings for either R or Python.This paper describes the design of Interactive HPC web applications on an Open OnDemand deployment for launching and connecting to Jupyter notebooks and RStudio sessions as well as the architecture and software required for supporting Jupyter, RStudio, and Apache Spark on the corresponding HPC cluster. Singularity can be leveraged for packaging and portability of this architecture across HPC clusters. This paper also discusses the challenges encountered in providing interactive access to HPC resources that are in need of general solutions.