1,109 research outputs found
Improved Bounds on the Phase Transition for the Hard-Core Model in 2-Dimensions
For the hard-core lattice gas model defined on independent sets weighted by
an activity , we study the critical activity
for the uniqueness/non-uniqueness threshold on the 2-dimensional integer
lattice . The conjectured value of the critical activity is
approximately . Until recently, the best lower bound followed from
algorithmic results of Weitz (2006). Weitz presented an FPTAS for approximating
the partition function for graphs of constant maximum degree when
where is the
infinite, regular tree of degree . His result established a certain
decay of correlations property called strong spatial mixing (SSM) on
by proving that SSM holds on its self-avoiding walk tree
where and is an ordering on the neighbors of vertex . As
a consequence he obtained that . Restrepo et al. (2011) improved Weitz's approach for
the particular case of and obtained that
. In this paper, we establish an upper bound for
this approach, by showing that, for all , SSM does not hold on
when . We also present a
refinement of the approach of Restrepo et al. which improves the lower bound to
.Comment: 19 pages, 1 figure. Polished proofs and examples compared to earlier
versio
Phoretic Motion of Spheroidal Particles Due To Self-Generated Solute Gradients
We study theoretically the phoretic motion of a spheroidal particle, which
generates solute gradients in the surrounding unbounded solvent via chemical
reactions active on its surface in a cap-like region centered at one of the
poles of the particle. We derive, within the constraints of the mapping to
classical diffusio-phoresis, an analytical expression for the phoretic velocity
of such an object. This allows us to analyze in detail the dependence of the
velocity on the aspect ratio of the polar and the equatorial diameters of the
particle and on the fraction of the particle surface contributing to the
chemical reaction. The particular cases of a sphere and of an approximation for
a needle-like particle, which are the most common shapes employed in
experimental realizations of such self-propelled objects, are obtained from the
general solution in the limits that the aspect ratio approaches one or becomes
very large, respectively.Comment: 18 pages, 5 figures, to appear in European Physical Journal
Photoproduction of mesons in nuclei at GeV energies
In a transport model that combines initial state interactions of the photon
with final state interactions of the produced particles we present a
calculation of inclusive photoproduction of mesons in nuclei in the energy
range from 1 to 7 GeV. We give predictions for the photoproduction cross
sections of pions, etas, kaons, antikaons, and invariant mass
spectra in ^{12}C and ^{208}Pb. The effects of nuclear shadowing and final
state interaction of the produced particles are discussed in detail.Comment: Text added in summary in general reliability of the method,
references updated. Phys. Rev. C (2000) in pres
Sequence effects in donor-acceptor oligomeric semiconductors comprising benzothiadiazole and phenylenevinylene monomers
To understand the influence of monomer sequence on the properties and performance of conjugated oligomers, a series of dimers, trimers, and tetramers were prepared from phenylene (P) and benzothiadiazole (B) monomers linked by vinylene groups. Optical and electrochemical studies established the influence of sequence on both the λmax and redox potentials of this series of structurally related oligomers. For tetramers with bromo end groups (PBBP, BPPB, PBPB, PPBB), the λmax ranged from 493 to 512 nm (Δ = 19 nm), the electrochemical oxidation potential from 0.65 to 0.82 (Δ = 0.17 V) and the reduction potential from-1.45 to-1.31 (Δ = 0.14 V), all of which are sequence-dependent. The effect of end groups (cyano, bromo, and alkyl) was also demonstrated to be important for the properties of these oligomers. DFT calculations of the tetramers were performed and the energy levels were correlated well with the experimentally determined spectroscopic data. Bulk heterojunction (BHJ) solar cells fabricated with selected tetramers as the donor and PC61BM as the acceptor exhibited power conversion efficiencies that varied by a factor of 3 as a function of sequence (0.47-1.85%). These results suggest that sequence control is important for tuning optoelectronic properties and photovoltaic performance of these structurally related conjugated oligomers
Color Transparency Effects in Electron Deuteron Interactions at Intermediate Q^2
High momentum transfer electrodisintegration of polarized and unpolarized
deuterium targets, is studied. We show that the importance of final
state interactions-FSI, occuring when a knocked out nucleon interacts with the
other nucleon, depends strongly on the momentum of the spectator nucleon. In
particular, these FSI occur when the essential contributions to the scattering
amplitude arise from internucleon distances . But the absorption
of the high momentum may produce a point like configuration, which
evolves with time. In this case, the final state interactions probe the point
like configuration at the early stage of its evolution. The result is that
significant color transparency effects, which can either enhance or suppress
computed cross sections, are predicted to occur for .Comment: 37 pages LaTex, 12 uuencoded PostScript Figures as separate file, to
be published in Z.Phys.
Ballistic electron transport through magnetic domain walls
Electron transport limited by the rotating exchange-potential of domain walls
is calculated in the ballistic limit for the itinerant ferromagnets Fe, Co, and
Ni. When realistic band structures are used, the domain wall magnetoresistance
is enhanced by orders of magnitude compared to the results for previously
studied two-band models. Increasing the pitch of a domain wall by confinement
in a nano-structured point contact is predicted to give rise to a strongly
enhanced magnetoresistance.Comment: 4 pages, 2 figures; to appear in PRB as a brief repor
Social Structure of Lions (Panthera leo) Is Affected by Management in Pendjari Biosphere Reserve, Benin
Lion populations have undergone a severe decline in West Africa. As baseline for conservation management, we assessed the group structure of lions in the Pendjari Biosphere Reserve in Benin. This reserve, composed of one National Park and two Hunting Zones, is part of the WAP transboundary complex of protected areas. Overall mean group size was 2.661.7 individuals (n = 296), it was significantly higher in the National Park (2.761.7, n = 168) than in the Hunting Zones (2.261.5, n = 128). Overall adult sex ratio was even, but significantly biased towards females (0.67) in the National Park and towards males (1.67) in the Hunting Zones. Our results suggest that the Pendjari lion population is affected by perturbations, such as trophy hunting.Conservation Biolog
Charmless hadronic decays and new physics effects in the general two-Higgs doublet models
Based on the low-energy effective Hamiltonian with the generalized
factorization, we calculate the new physics contributions to the branching
ratios of the two-body charmless hadronic decays of and mesons
induced by the new gluonic and electroweak charged-Higgs penguin diagrams in
the general two-Higgs doublet models (models I, II and III). Within the
considered parameter space, we find that: (a) the new physics effects from new
gluonic penguin diagrams strongly dominate over those from the new -
and - penguin diagrams; (b) in models I and II, new physics contributions
to most studied B meson decay channels are rather small in size: from -15% to
20%; (c) in model III, however, the new physics enhancements to the
penguin-dominated decay modes can be significant, , and
therefore are measurable in forthcoming high precision B experiments; (d) the
new physics enhancements to ratios {\cal B}(B \to K \etap) are significant in
model III, , and hence provide a simple and plausible new
physics interpretation for the observed unexpectedly large B \to K \etap
decay rates; (e) the theoretical predictions for and
in model III are still consistent with the data
within errors; (f) the significant new physics enhancements to the
branching ratios of and decays are helpful to improve the
agreement between the data and the theoretical predictions; (g) the theoretical
predictions of in the 2HDM's are generally
consistent with experimental measurements and upper limits ()Comment: 55 pages, Latex file, 17 PS and EPS figures. With minor corrections,
final version to be published in Phys.Rev. D. Repot-no: PKU-TH-2000-4
Evading the CKM Hierarchy: Intrinsic Charm in B Decays
We show that the presence of intrinsic charm in the hadrons' light-cone wave
functions, even at a few percent level, provides new, competitive decay
mechanisms for B decays which are nominally CKM-suppressed. For example, the
weak decays of the B-meson to two-body exclusive states consisting of strange
plus light hadrons, such as B\to\pi K, are expected to be dominated by penguin
contributions since the tree-level b\to s u\bar u decay is CKM suppressed.
However, higher Fock states in the B wave function containing charm quark pairs
can mediate the decay via a CKM-favored b\to s c\bar c tree-level transition.
Such intrinsic charm contributions can be phenomenologically significant. Since
they mimic the amplitude structure of ``charming'' penguin contributions,
charming penguins need not be penguins at all.Comment: 28 pages, 6 figures, published version. References added, minor
change
Rare Decays of \Lambda_b->\Lambda + \gamma and \Lambda_b ->\Lambda + l^{+} l^{-} in the Light-cone Sum Rules
Within the Standard Model, we investigate the weak decays of and with the light-cone
sum rules approach. The higher twist distribution amplitudes of
baryon to the leading conformal spin are included in the sum rules for
transition form factors. Our results indicate that the higher twist
distribution amplitudes almost have no influences on the transition form
factors retaining the heavy quark spin symmetry, while such corrections can
result in significant impacts on the form factors breaking the heavy quark spin
symmetry. Two phenomenological models (COZ and FZOZ) for the wave function of
baryon are also employed in the sum rules for a comparison, which can
give rise to the form factors approximately 5 times larger than that in terms
of conformal expansion. Utilizing the form factors calculated in LCSR, we then
perform a careful study on the decay rate, polarization asymmetry and
forward-backward asymmetry, with respect to the decays of , .Comment: 38 pages, 15 figures, some typos are corrected and more references
are adde
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