[en] The electronic and atomic structure of substitutional nitrogen pairs, triplets, and clusters in GaP and GaAs is studied using the multiband empirical pseudopotential method with atomistically relaxed supercells. A single nitrogen impurity creates a localized a₁(N) gap state in GaP, but in GaAs, the state is resonant above the conduction-band minimum. We show how the interaction of multiple a₁ impurity levels, for more than one nitrogen, results in a nonmonotonic relationship between energy level and impurity separation. We assign the lowest (NN1) line in GaP to a [2,2,0] oriented pair, the second (NN2) line to a triplet of nitrogen atoms, and identify the origin of a deeper observed level as an [1,1,0] oriented triplet. We also demonstrate that small nitrogen clusters readily create very deep levels in both GaP and GaAs

[en] Using the empirical pseudopotential method and large atomistically relaxed supercells, we have systematically studied the evolution of the electronic structure of GaP_1-xN_x and GaAs_1-xN_x, from the dilute nitrogen impurity regime to the nascent nitride alloy. We show how substitutional nitrogen forms perturbed host states (PHS) inside the conduction band, whereas small nitrogen aggregates form localized cluster states (CS) in the band gap. By following the evolution of these states and the ''perturbed host states'' with increasing nitrogen composition, we propose a new model for low-nitrogen-content GaAs_1-xN_x and GaP_1-xN_x alloys: As the nitrogen composition increases, the energy of the CS is pinned while the energy of the PHS plunges down as the nitrogen composition increases. The impurity limit (PHS above CS) is characterized by strongly localized wave functions, low pressure coefficients, and sharp emission lines from the CS. The amalgamation limit (PHS overtake the CS) is characterized by a coexistence of localized states (leading to high effective mass, exciton localization, Stokes shift in emission versus absorption) overlapping delocalized PHS (leading to asymmetrically broadened states, low temperature coefficeint, delocalized E₊ band at higher energies). The alloy limit (PHS well below CS) may not have been reached experimentally, but is predicted to be characterized by conventional extended states. Our theory shows that these alloy systems require a polymorphous description, permitting the coexistence of many different local environments, rather than an isomorphous model that focuses on few impurity-host motifs

[en] Addition of nitrogen to III-V semiconductor alloys radically changes their electronic properties. We report large-scale electronic structure calculations of GaAsN and GaPN using an approach that allows arbitrary states to emerge, couple, and evolve with composition. We find a novel mechanism of alloy formation where localized cluster states within the gap are gradually overtaken by a downwards moving conduction band edge, composed of both localized and delocalized states. This localized to delocalized transition explains many of the hitherto puzzling experimentally observed anomalies in III-V nitride alloys

[en] The electronic structure and optical properties of cubic (nonpiezoelectric) InGaN are investigated using large scale atomistic empirical pseudopotential calculations. We find that (i) strong hole localization exists even in the homogeneous random alloy, with a preferential localization along the [1,1,0] In--N--In--N--In chains, (ii) even modest sized (<50 {angstrom}) indium rich quantum dots provide substantial quantum confinement and readily reduce emission energies relative to the random alloy by 200--300 meV, depending on size and composition, consistent with current photoluminescence, microscopy, and Raman data. The dual effects of alloy hole localization and localization of electrons and hole at intrinsic quantum dots are responsible for the emission characteristics of current grown cubic InGaN alloys

[en] The equations, valid in the weak non-linear approximation, are derived to describe the interaction of two waves of different frequency in a bounded, uniform, cold, collisional magnetoplasma. The boundaries are the walls of a cyclindrical cavity resonator, the magnetization is axial, and ion motion is neglected. A matrix formalism is used to obtain a general solution satisfying boundary condition for a Fourier component in time, azimuthal, and axial coordinates. The solution is applied to the excitation of the TM-011 mode by the harmonic of the TM-010, when both are resonant. The computed results show a decrease of harmonic amplitude at the cavity wall as the magnetization is increased from zero and an abrupt increase of several orders of magnitude when the harmonic is close to the upper hybrid resonance. An equivalent circuit is proposed to account for the results at low magnetization, but it fails for cyclotron frequencies greater than the fundamental frequency. For all magnetic fields, the harmonic amplitude is inversely proportional to the collision frequency. A second application is to the excitation of a resonant Trivelpiece mode by the interaction of two resonant high frequency modes. In this case infinite magnetization is assumed. The results show Trivelpiece mode excitation to be less than the harmonic generation by at least one order of magnitude. Since calculations apply to one set of cavity dimensions, the requirement of simultaneous resonance at two or more frequencies limits the variation of plasma frequency. No conclusions concerning the effects of this parameter can be drawn. (author)

[en] Plane wave density functional calculations have traditionally been able to use the largest available supercomputing resources. We analyze the scalability of modern projector-augmented wave implementations to identify the challenges in performing molecular dynamics calculations of large systems containing many thousands of electrons. Benchmark calculations on the Cray XT4 demonstrate that global linear-algebra operations are the primary reason for limited parallel scalability. Plane-wave related operations can be made sufficiently scalable. Improving parallel linear-algebra performance is an essential step to reaching longer timescales in future large-scale molecular dynamics calculations

[en] We present an efficient low-rank updating algorithm for updating the trial wave functions used in quantum Monte Carlo (QMC) simulations. The algorithm is based on low-rank updating of the Slater determinants. In particular, the computational complexity of the algorithm is O(kN) during the kth step compared to traditional algorithms that require O(N²) computations, where N is the system size. For single determinant trial wave functions the new algorithm is faster than the traditional O(N²) Sherman-Morrison algorithm for up to O(N) updates. For multideterminant configuration-interaction-type trial wave functions of M+1 determinants, the new algorithm is significantly more efficient, saving both O(MN²) work and O(MN²) storage. The algorithm enables more accurate and significantly more efficient QMC calculations using configuration-interaction-type wave functions.

[en] The selection of a salt species for use in a molten salt reactor MSR is a key part of any MSR design. However, many salts have sparse or no thermophysical property data sets, especially those with higher melting points. One such salt is the eutectic mixture of (NaF)0.345(KF)0.59(MgF₂)0.065, or FMgNaK, and is explored here through ab initio molecular dynamics (AIMD) simulation (1023-1273K) as well as experimental measurement of the liquid density using the Archimedean method (973-1223K). Predicted densities from AIMD are similar to experimentally-measured densities, though with more noise, suggesting that the small scale necessitated by AIMD simulations may be problematic for simulating FMgNaK in particular. The coefficient of thermal expansion is predicted from simulation, and salt structure is characterized. Mg-F-Mg chaining is observed in the salt network, though the low concentration of Mg inhibits chaining on the scale that is observed in (LiF)_0.67(BeF₂)_0.33, or FLiBe.

[en] The Energy Sciences Network (ESnet) is the primary provider of network connectivity for the US Department of Energy Office of Science (SC), the single largest supporter of basic research in the physical sciences in the United States. In support of the Office of Science programs, ESnet regularly updates and refreshes its understanding of the networking requirements of the instruments, facilities, scientists, and science programs that it serves. This focus has helped ESnet to be a highly successful enabler of scientific discovery for over 20 years.

[en] Resonances in the ¹²C(¹⁶O,γ)²⁸Si radiative capture process at energies around the Coulomb barrier have been probed using the very selective 0 deg. Dragon spectrometer at Triumf and its associated BGO γ-array. For the first time the full level scheme involved in this process has been measured and shows previously unobserved γ-decay to doorway states around 11 MeV in ²⁸Si.