2,824 research outputs found
Two-electron bond-orbital model, 2
The two-electron bond-orbital model of tetrahedrally-coordinated solids is generalized and its application extended. All intrabond matrix elements entering the formalism are explicitly retained, including the direct overlap S between the anion and cation sp3 hybrid wavefunctions. Complete analytic results are obtained for the six two-electron eigenvalues and eigenstates of the anion-cation bond in terms of S, one-electron parameters V2 and V3, and two-electron correlation parameters V4, V5 and V6. Refined formulas for the dielectric constant and the nuclear exchange and pseudodipolar coefficients, as well as new expressions for the valence electron density, polarity of the bond and the cohesive energy, are then derived. The theory gives a good account of the experimentally observed trends in all properties considered and approximate quantitative agreement is achieved for the pseudodipolar coefficient
Transition-metal interactions in aluminum-rich intermetallics
The extension of the first-principles generalized pseudopotential theory
(GPT) to transition-metal (TM) aluminides produces pair and many-body
interactions that allow efficient calculations of total energies. In
aluminum-rich systems treated at the pair-potential level, one practical
limitation is a transition-metal over-binding that creates an unrealistic TM-TM
attraction at short separations in the absence of balancing many-body
contributions. Even with this limitation, the GPT pair potentials have been
used effectively in total-energy calculations for Al-TM systems with TM atoms
at separations greater than 4 AA. An additional potential term may be added for
systems with shorter TM atom separations, formally folding repulsive
contributions of the three- and higher-body interactions into the pair
potentials, resulting in structure-dependent TM-TM potentials. Towards this
end, we have performed numerical ab-initio total-energy calculations using VASP
(Vienna Ab Initio Simulation Package) for an Al-Co-Ni compound in a particular
quasicrystalline approximant structure. The results allow us to fit a
short-ranged, many-body correction of the form a(r_0/r)^{b} to the GPT pair
potentials for Co-Co, Co-Ni, and Ni-Ni interactions.Comment: 18 pages, 5 figures, submitted to PR
Silicon superlattices: Theory and application to semiconductor devices
Silicon superlattices and their applicability to improved semiconductor devices were studied. The device application potential of the atomic like dimension of III-V semiconductor superlattices fabricated in the form of ultrathin periodically layered heterostructures was examined. Whether this leads to quantum size effects and creates the possibility to alter familiar transport and optical properties over broad physical ranges was studied. Applications to improved semiconductor lasers and electrondevices were achieved. Possible application of silicon sperlattices to faster high speed computing devices was examined. It was found that the silicon lattices show features of smaller fundamental energyband gaps and reduced effective masses. The effects correlate strongly with both the chemical and geometrical nature of the superlattice
Silicon superlattices. 2: Si-Ge heterostructures and MOS systems
Five main areas were examined: (1) the valence-and conduction-band-edge electronic structure of the thin layer ( 11 A) silicon-superlattice systems; (2) extension of thin-layer calculations to layers of thickness 11 A, where most potential experimental interest lies; (3) the electronic structure of thicker-layer (11 to 110 A) silicon superlattices; (4) preliminary calculations of impurity-scattering-limited electron mobility in the thicker-layer superlattices; and (5) production of the fine metal lines that would be required to produce on MOS superlattice
Procuring load curtailment from local customers under uncertainty
J.M. was supported by EPSRC grant no. EP/K00557X/2, A.M. was partially supported by EPSRC grant EP/P003818/1 and J.V. by a President’s PhD Scholarship from Imperial College London
A Metropolis-class sampler for targets with non-convex support
We aim to improve upon the exploration of the general-purpose random walk Metropolis algorithm when the target has non-convex support A⊂Rd, by reusing proposals in Ac which would otherwise be rejected. The algorithm is Metropolis-class and under standard conditions the chain satisfies a strong law of large numbers and central limit theorem. Theoretical and numerical evidence of improved performance relative to random walk Metropolis are provided. Issues of implementation are discussed and numerical examples, including applications to global optimisation and rare event sampling, are presented
Reflected Brownian motion in a wedge: sum-of-exponential stationary densities
We give necessary and sufficient conditions for the stationary density of
semimartingale reflected Brownian motion in a wedge to be written as a finite
sum of terms of exponential product form. Relying on geometric ideas
reminiscent of the reflection principle, we give an explicit formula for the
density in such cases
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