1,630 research outputs found
Oxygen impurities in NiAl: Relaxation effects
We have used a full-potential linear muffin-tin orbital method to calculate
the effects of oxygen impurities on the electronic structure of NiAl. Using the
supercell method with a 16-atom supercell we have investigated the cases where
an oxygen atom is substitutionally placed at either a nickel or an aluminum
site. Full relaxation of the atoms within the supercell was allowed. We found
that oxygen prefers to occupy a nickel site over an aluminum site with a site
selection energy of 138 mRy (21,370 K). An oxygen atom placed at an aluminum
site is found to cause a substantial relaxation of its nickel neighbors away
from it. In contrast, this steric repulsion is hardly present when the oxygen
atom occupies the nickel site and is surrounded by aluminum neighbors. We
comment on the possible relation of this effect to the pesting degradation
phenomenon (essentially spontaneous disintegration in air) in nickel
aluminides.Comment: To appear in Phys. Rev. B (Aug. 15, 2001
Size-dependent decoherence of excitonic states in semiconductor microcrystallites
The size-dependent decoherence of the exciton states resulting from the
spontaneous emission is investigated in a semiconductor spherical
microcrystallite under condition . In general, the
larger size of the microcrystallite corresponds to the shorter coherence time.
If the initial state is a superposition of two different excitonic coherent
states, the coherence time depends on both the overlap of two excitonic
coherent states and the size of the microcrystallite. When the system with
fixed size is initially in the even or odd coherent states, the larger average
number of the excitons corresponds to the faster decoherence. When the average
number of the excitons is given, the bigger size of the microcrystallite
corresponds to the faster decoherence. The decoherence of the exciton states
for the materials GaAs and CdS is numerically studied by our theoretical
analysis.Comment: 4 pages, two figure
Predictive Ability of QCD Sum Rules for Excited Baryons
The masses of octet baryons are calculated by the method of QCD sum rules.
Using generalized interpolating fields, three independent sets of QCD sum rules
are derived which allow the extraction of low-lying N* states with spin-parity
1/2+, 1/2- and 3/2- in both the non-strange and strange channels. The
predictive ability of the sum rules is examined by a Monte-Carlo based analysis
procedure in which the three phenomenological parameters (mass, coupling,
threshold) are treated as free parameters simultaneously. Realistic
uncertainties in these parameters are obtained by simultaneously exploring all
uncertainties in the QCD input parameters. Those sum rules with good predictive
power are identified and their predictions are compared with experiment where
available.Comment: 33 pages, 2 figure
Diffusion Enhances Spontaneous Electroweak Baryogenesis
We include the effects of diffusion in the electroweak spontaneous
baryogenesis scenario and show that it can greatly enhance the resultant baryon
density, by as much as a factor of over previous
estimates. Furthermore, the baryon density produced is rather insensitive to
parameters characterizing the first order weak phase transition, such as the
width and propagation velocity of the phase boundary.Comment: 15 pages, uses harvmac and epsf macro
A Lattice Study of the Magnetic Moment and the Spin Structure of the Nucleon
Using an approach free from momentum extrapolation, we calculate the nucleon
magnetic moment and the fraction of the nucleon spin carried by the quark
angular momentum in the quenched lattice QCD approximation. Quarks with three
values of lattice masses, 210, 124 and 80 MeV, are formulated on the lattice
using the standard Wilson approach. At every mass, 100 gluon configurations on
16^3 x 32 lattice with \beta=6.0 are used for statistical averaging. The
results are compared with the previous calculations with momentum
extrapolation. The contribution of the disconnected diagrams is studied at the
largest quark mass using noise theory technique.Comment: 14 pages, 3 figures, Talk given at Lattice2001, Berlin, German
Parity-violating neutron spin rotation in hydrogen and deuterium
We calculate the (parity-violating) spin rotation angle of a polarized
neutron beam through hydrogen and deuterium targets, using pionless effective
field theory up to next-to-leading order. Our result is part of a program to
obtain the five leading independent low-energy parameters that characterize
hadronic parity-violation from few-body observables in one systematic and
consistent framework. The two spin-rotation angles provide independent
constraints on these parameters. Using naive dimensional analysis to estimate
the typical size of the couplings, we expect the signal for standard target
densities to be 10^-7 to 10^-6 rad/m for both hydrogen and deuterium targets.
We find no indication that the nd observable is enhanced compared to the np
one. All results are properly renormalized. An estimate of the numerical and
systematic uncertainties of our calculations indicates excellent convergence.
An appendix contains the relevant partial-wave projectors of the three-nucleon
system.Comment: 44 pages, 17 figures; minor corrections; to be published in EPJ
Parity violation in nuclear systems
Parity violation in nuclear systems is reviewed. A few ingredients relevant
to the description of the parity-violating nucleon-nucleon force in terms of
meson exchanges are reminded. Effects in nuclear systems are then considered.
They involve pp scattering, some complex nuclei and the deuteron system.Comment: 4 pages, to be published in the proceedings of the worksho
Nonperturbative renormalisation of composite operators with overlap quarks
We compute non-perturbatively the renormalisation constants of composite
operators on a lattice with lattice spacing = 0.093 fm
for the overlap fermion action by using the regularisation independent (RI)
scheme. The quenched gauge configurations are generated by tadpole improved
plaquette plus rectangle action. We test the perturbative continuum relation
and and find that they agree well above = 1.6 GeV.
We also perform a Renormalisation Group analysis at the next-to-next-to-leading
order and convert the renormalisation constants to the scheme.Comment: Talk given at LHP2003, Cairns, Australi
The Complete Flux Scheme : error analysis and application to plasma simulation
The Complete Flux scheme (CFS) (J.H.M. ten Thije Boonkkamp et al., J. Sci. Comput. 46 (2011) 47–70) is an extension of the widely used exponential di¿erence scheme for advection-di¿usion-reaction equations. In the present paper we provide a rigorous proof that the convergence order of this scheme is 2 for all grid Péclet numbers, whereas that of the exponential scheme reduces to 1 for high grid Péclet numbers in the presence of source terms. The performance of both schemes is compared in two case studies: a model system and a real-world model of a parallel-plate glow discharge. The results indicate that the usage of CFS allows a considerable reduction of the number of grid points that is required to obtain the same accuracy. The MATLAB/Octave source code that has been used in these studies has been made available
Ion-beam-driven intense electrostatic solitary waves in reconnection jet
Electrostatic solitary waves (ESWs) have been reported inside reconnection jets, but their source and role remain unclear hitherto. Here we present the first observational evidence of ESWs generation by cold ion beams inside the jet, by using high-cadence measurements from the Magnetospheric Multiscale spacecraft in the Earth's magnetotail. Inside the jet, intense ESWs with amplitude up to 30 mV m(-1) and potential up to similar to 7% of the electron temperature are observed in association with accelerated cold ion beams. Instability analysis shows that the ion beams are unstable, providing free energy for the ESWs. The waves are observed to thermalize the beams, thus providing a new channel for ion heating inside the jet. Our study suggests that electrostatic turbulence can play an important role in the jet dynamics.Peer reviewe
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