2,340 research outputs found
Gallium self-interstitial relaxation in Gallium Arsenide: an {ab initio} characterization
Ga interstitials in GaAs () are studied using the local-orbital
{ab-initio} code SIESTA in a supercell of {216+1} atoms. Starting from eight
different initial configurations, we find five metastable structures: the two
tetrahedral sites in addition to the 110-split,
111-split, and 100-split. Studying
the competition between various configuration and charges of , we find
that predominant gallium interstitials in GaAs are charged +1, neutral or at
most -1 depending on doping conditions and prefer to occupy the tetrahedral
configuration where it is surrounded by Ga atoms. Our results are in excellent
agreement with recent experimental results concerning the dominant charge of
, underlining the importance of finite size effects in the calculation
of defects.Comment: v1) 18 pages, 5 figures, submitted to PRB (Latex preprint version)
v2) 9 pages, 5 figures, reviewed version resubmitted to PRB (correction to
equation 1, some changes and reformulations, minor grammatical and typo
corrections, added reference
Heavy quark effects on parton distribution functions in the unpolarized virtual photon up to the next-to-leading order in QCD
We investigate the heavy quark mass effects on the parton distribution
functions in the unpolarized virtual photon up to the next-to-leading order in
QCD. Our formalism is based on the QCD-improved parton model described by the
DGLAP evolution equation as well as on the operator product expansion
supplemented by the mass-independent renormalization group method. We evaluate
the various components of the parton distributions inside the virtual photon
with the massive quark effects, which are included through the initial
condition for the heavy quark distributions, or equivalently from the matrix
element of the heavy quark operators. We discuss some features of our results
for the heavy quark effects and their factorization-scheme dependence.Comment: 16 pages, 16 figures, version to appear in Phys. Rev.
Motivation for Technology Adoption and Its Impact on Abandonment: A Case Study of U.S. Cotton Farmers
We estimate a bivariate probit model with sample selection to identify factors affecting adoption and abandonment of precision farming technologies for cotton farmers, using the 2009 Southern Cotton Precision Farming Survey conducted in 12 Southern states in the United States. Farmers for whom being at the forefront of agricultural technology is not an important reason for adoption are more likely to abandon precision farming technologies. This study identified various factors associated with adoption and retention of precision farming technologies. Findings from this study offer significant information to policy‐makers for a better formulation of agri‐environmental programs that encourage farmers to adopt environmentally benign farming practices including precision farming technologies.Technology Abandonment, Technology Adoption, Bivariate Probit with Sample Selection, Multinomial Logit, Precision Farming, Agribusiness, Crop Production/Industries, Farm Management, Production Economics, Research and Development/Tech Change/Emerging Technologies, Q10, Q12, Q16,
Target Mass Corrections for the Virtual Photon Structure Functions to the Next-to-next-to-leading Order in QCD
We investigate target mass effects in the unpolarized virtual photon
structure functions and in
perturbative QCD for the kinematical region , where
is the mass squared of the probe (target) photon and is
the QCD scale parameter. We obtain the Nachtmann moments for the structure
functions and then, by inverting the moments, we get the expressions in closed
form for up to the next-to-next-to-leading order and
for up to the next-to-leading order, both of which
include the target mass corrections. Numerical analysis exhibits that target
mass effects appear at large and become sizable near , the maximal value of , as the ratio
increases.Comment: 24 pages, LaTeX, 7 eps figures, REVTeX
Has the QCD RG-Improved Parton Content of Virtual Photons been Observed?
It is demonstrated that present and DIS ep data on the structure of
the virtual photon can be understood entirely in terms of the standard `naive'
quark--parton model box approach. Thus the QCD renormalization group (RG)
improved parton distributions of virtual photons, in particular their gluonic
component, have not yet been observed. The appropriate kinematical regions for
their future observation are pointed out as well as suitable measurements which
may demonstrate their relevance.Comment: 24 pages, LaTeX, 5 figure
Virtual photon structure functions and positivity constraints
We study the three positivity constraints among the eight virtual photon
structure functions, derived from the Cauchy-Schwarz inequality and which are
hence model-independent. The photon structure functions obtained from the
simple parton model show quite different behaviors in a massive quark or a
massless quark case, but they satisfy, in both cases, the three positivity
constraints. We then discuss an inequality which holds among the unpolarized
and polarized photon structure functions , and
, in the kinematic region , where is the mass squared of the probe (target) photon, and we examine
whether this inequality is satisfied by the perturbative QCD results.Comment: 24 pages, 13 eps figure
Parton distributions in the virtual photon target up to NNLO in QCD
Parton distributions in the virtual photon target are investigated in
perturbative QCD up to the next-to-next-to-leading order (NNLO). In the case
, where () is the mass squared of the
probe (target) photon, parton distributions can be predicted completely up to
the NNLO, but they are factorisation-scheme-dependent. We analyse parton
distributions in two different factorisation schemes, namely and
schemes, and discuss their scheme dependence. We show that
the factorisation-scheme dependence is characterised by the large-
behaviours of quark distributions. Gluon distribution is predicted to be very
small in absolute value except in the small- region.Comment: 28 pages, 5 figures, version to appear in Eur. Phys. J.
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