Bayesian Updating of Material Balances Covariance Matrices Using Training Data

The main quantitative measure of nuclear safeguards effectiveness is nuclear material accounting (NMA), which consists of sequences of measured material balances that should be close to zero if there is no loss of special nuclear material such as Pu. NMA is essentially “accounting with measurement errors,” which arise from good, but imperfect, measurements. The covariance matrix MB of a sequence of material balances is the key quantity that determines the probability to detect loss. There is a recent push to include process monitoring (PM) data along with material balances from NMA in new schemes to monitor for material loss. PM data includes near-real-time measurements by the operator to monitor and control process operations. One concern regarding PM data is the need to estimate normal behavior of PM residuals, which requires a training period prior to ongoing testing for material loss. Assuming that a training period is used for PM data prior to its use in statistical testing for loss, that same training period could also be used for improving the estimate of MB that is used in NMA. We consider updating MB using training data with a Bayesian approach. A simulation study assesses the improvement gained with larger amounts of training data

Effect of Semicore Orbitals on the Electronic Band Gaps of Si, Ge, and GaAs within the GW Approximation

We study the effect of semicore states on the self-energy corrections and electronic energy gaps of silicon, germanium and GaAs. Self-energy effects are computed within the GW approach, and electronic states are expanded in a plane-wave basis. For these materials, we generate {\it ab initio} pseudopotentials treating as valence states the outermost two shells of atomic orbitals, rather than only the outermost valence shell as in traditional pseudopotential calculations. The resulting direct and indirect energy gaps are compared with experimental measurements and with previous calculations based on pseudopotential and ``all-electron'' approaches. Our results show that, contrary to recent claims, self-energy effects due to semicore states on the band gaps can be well accounted for in the standard valence-only pseudopotential formalism.Comment: 6 pages, 3 figures, submitted to Phys. Rev.

Exo-hydrogenated Single Wall Carbon Nanotubes

An extensive first-principles study of fully exo-hydrogenated zigzag (n,0) and armchair (n,n) single wall carbon nanotubes (C$_n$H$_n$), polyhedral molecules including cubane, dodecahedrane, and C$_{60}$H$_{60}$ points to crucial differences in the electronic and atomic structures relevant to hydrogen storage and device applications. C$_n$H$_n$'s are estimated to be stable up to the radius of a (8,8) nanotube, with binding energies proportional to 1/R. Attaching a single hydrogen to any nanotube is always exothermic. Hydrogenation of zigzag nanotubes is found to be more likely than armchair nanotubes with similar radius. Our findings may have important implications for selective functionalization and finding a way of separating similar radius nanotubes from each other.Comment: 5 pages, 4 postscript figures, Revtex file, To be appear in Physical Review

All-electron GW calculation based on the LAPW method: application to wurtzite ZnO

We present a new, all-electron implementation of the GW approximation and apply it to wurtzite ZnO. Eigenfunctions computed in the local-density approximation (LDA) by the full-potential linearized augmented-plane-wave (LAPW) or the linearized muffin-tin-orbital (LMTO) method supply the input for generating the Green function G and the screened Coulomb interaction W. A mixed basis is used for the expansion of W, consisting of plane waves in the interstitial region and augmented-wavefunction products in the augmentation-sphere regions. The frequency-dependence of the dielectric function is computed within the random-phase approximation (RPA), without a plasmon-pole approximation. The Zn 3d orbitals are treated as valence states within the LDA; both core and valence states are included in the self-energy calculation. The calculated bandgap is smaller than experiment by about 1eV, in contrast to previously reported GW results. Self-energy corrections are orbital-dependent, and push down the deep O 2s and Zn 3d levels by about 1eV relative to the LDA. The d level shifts closer to experiment but the size of shift is underestimated, suggesting that the RPA overscreens localized states.Comment: 10 pages, 3 figures, submitted to Phys. Rev.

Electronic Structure of Carbon Nanotube Ropes

We present a tight binding theory to analyze the motion of electrons between carbon nanotubes bundled into a carbon nanotube rope. The theory is developed starting from a description of the propagating Bloch waves on ideal tubes, and the effects of intertube motion are treated perturbatively in this basis. Expressions for the interwall tunneling amplitudes between states on neighboring tubes are derived which show the dependence on chiral angles and intratube crystal momenta. We find that conservation of crystal momentum along the tube direction suppresses interwall coherence in a carbon nanorope containing tubes with random chiralities. Numerical calculations are presented which indicate that electronic states in a rope are localized in the transverse direction with a coherence length corresponding to a tube diameter.Comment: 15 pages, 10 eps figure

Allowed Gamow-Teller Excitations from the Ground State of 14N

Motivated by the proposed experiment $^{14}N(d,{^2He})^{14}C$, we study the final states which can be reached via the allowed Gamow-Teller mechanism. Much emphasis has been given in the past to the fact that the transition matrix element from the $J^{\pi}=1^+ T=0$ ground state of $^{14}N$ to the $J^{\pi}=0^+ T=1$ ground state of $^{14}C$ is very close to zero, despite the fact that all the quantum numbers are right for an allowed transition. We discuss this problem, but, in particular, focus on the excitations to final states with angular momenta $1^+$ and $2^+$. We note that the summed strength to the $J^{\pi}=2^+ T=1$ states, calculated with a wide variety of interactions, is significantly larger than that to the $J^{\pi}=1^+ T=1$ final states.Comment: Submitted to Phys. Rev.

Backward diode composed of a metallic and semiconducting nanotube

The conditions necessary for a nanotube junction connecting a metallic and semiconducting nanotube to rectify the current are theoretically investigated. A tight binding model is used for the analysis, which includes the Hartree-Fock approximation and the Green's function method. It is found that the junction has a behavior similar to the backward diode if the gate electrode is located nearby and the Fermi level of the semiconducting tube is near the gap. Such a junction would be advantageous since the required length for the rectification could be reduced.Comment: 4 pages, RevTeX, uses epsf.st

Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes

Using first-principles total-energy pseudopotential calculations, we have studied the properties of chains of potassium and aluminum in nanotubes. For BN tubes, there is little interaction between the metal chains and the tubes, and the conductivity of these tubes is through carriers located at the inner part of the tube. In contrast, for small radius carbon nanotubes, there are two types of interactions: charge-transfer (dominant for alkali atoms) leading to strong ionic cohesion, and hybridization (for multivalent metal atoms) resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show that both effects contribute. New electronic properties related to these confined atomic chains of metal are analyzed.Comment: 12 pages + 3 figure

Reversible Band Gap Engineering in Carbon Nanotubes by Radial Deformation

We present a systematic analysis of the effect of radial deformation on the atomic and electronic structure of zigzag and armchair single wall carbon nanotubes using the first principle plane wave method. The nanotubes were deformed by applying a radial strain, which distorts the circular cross section to an elliptical one. The atomic structure of the nanotubes under this strain are fully optimized, and the electronic structure is calculated self-consistently to determine the response of individual bands to the radial deformation. The band gap of the insulating tube is closed and eventually an insulator-metal transition sets in by the radial strain which is in the elastic range. Using this property a multiple quantum well structure with tunable and reversible electronic structure is formed on an individual nanotube and its band-lineup is determined from first-principles. The elastic energy due to the radial deformation and elastic constants are calculated and compared with classical theories.Comment: To be appear in Phys. Rev. B, Apr 15, 200

Magnetic polarons in weakly doped high-Tc superconductors

We consider a spin Hamiltonian describing $d$-$d$ exchange interactions between localized spins $d$ of a finite antiferromagnet as well as $p$-$d$ interactions between a conducting hole ($p$) and localized spins. The spin Hamiltonian is solved numerically with use of Lanczos method of diagonalization. We conclude that $p$-$d$ exchange interaction leads to localization of magnetic polarons. Quantum fluctuations of the antiferromagnet strengthen this effect and make the formation of polarons localized in one site possible even for weak $p$-$d$ coupling. Total energy calculations, including the kinetic energy, do not change essentially the phase diagram of magnetic polarons formation. For parameters reasonable for high-$T_c$ superconductors either a polaron localized on one lattice cell or a small ferron can form. For reasonable values of the dielectric function and $p$-$d$ coupling, the contributions of magnetic and phonon terms in the formation of a polaron in weakly doped high-$T_c$ materials are comparable.Comment: revised, revtex-4, 12 pages 8 eps figure