452 research outputs found
Latitudinal variation in arrival and breeding phenology of the pied flycatcher Ficedula hypoleuca using large-scale citizen science data
Funding – TAM thanks partial support by CEAUL (funded by FCT – Fundação para a Ciência e a Tecnologia, Portugal, through the project UID/MAT/00006/2019).Many species have advanced the timing of annual reproductive cycles in response to climatic warming, sometimes leading to asynchrony between trophic levels, with negative population consequences. Long-distance migratory birds, reliant on short seasonal food pulses for breeding, are considered particularly susceptible to such disjunction because late arrival may preclude optimal timing of egg-laying. It is unknown whether the relative timing of arrival and egg-laying is sufficiently plastic, in time and space, to enable an adaptive response when arrival times change relative to local food resources. We used citizen science data, describing pied flycatcher Ficedula hypoleuca arrival and egg-laying dates, to explore temporal (2013–2016) and spatial (across Great Britain) variation in the phenology of arrival, laying and their difference. To assess the long-term trend in arrival and laying at a single location, we used data from a long-term field study. The arrival-laying interval was consistently shorter in the north, driven by the contrast between spatial variation in arrival date and spatial invariance in laying date. To understand whether a short arrival-laying interval may have consequences for productivity, we assessed the association between this interval and clutch size. We found no statistically significant correlation between these two variables. To examine long-term changes in arrival and laying dates, we focussed on a single location in southwestern England. Both dates of first male arrival and first egg laid in a season advanced since 1986, with no evidence of interval shortening. Together, our results demonstrate spatial and annual variation in the arrival-laying interval, with no detected effect on fecundity. Thus, the interval from arrival to laying is likely dictated by spatially and temporally varying local conditions, suggesting these migrant birds may have the ability to adapt this interval to align with local conditions and mitigate potential mismatch impacts.PostprintPeer reviewe
A Monte Carlo Study of Correlations in Quantum Spin Ladders
We study antiferromagnetic spin--1/2 Heisenberg ladders, comprised of
chains () with ratio of inter-- to
intra--chain couplings. From measurements of the correlation function we deduce
the correlation length . For even , the static structure factor
exhibits a peak at a temperature below the corresponding spin gap. Results for
isotropically coupled ladders () are compared to those for
the single chain and the square lattice. For , the
correlation function of the two--chain ladder is in excellent agreement with
analytic results from conformal field theory, and exhibits simple
scaling behavior.Comment: 4 pages, 5 EPS figures, submitted to Phys. Rev. Let
Resolving the phylogenetic and taxonomic status of dark-spored teleomorph genera in the Botryosphaeriaceae
Species in the Botryosphaeriaceae are common plant pathogens and saprobes found on a variety of mainly woody hosts. Teleomorphs typically have hyaline, aseptate ascospores. However, some have been reported with brown ascospores and their taxonomic status is uncertain. A multi-gene approach (SSU, ITS, LSU, EF1-α and β-tubulin) was used to resolve the correct phylogenetic position of the dark-spored ‘Botryosphaeria’ teleomorphs and related asexual species. Neodeightonia and Phaeobotryon are reinstated for species with brown ascospores that are either 1-septate (Neodeightonia) or 2-septate (Phaeobotryon). Phaeobotryosphaeria is reinstated for species with brown, aseptate ascospores that bear an apiculus at either end. The status of Sphaeropsis is clarified and shown to be the anamorph of Phaeobotryosphaeria. Two new genera, namely Barriopsis for species having brown, aseptate ascospores without apiculi and Spencermartinsia for species having brown, 1-septate ascospores with an apiculus at either end are introduced. Species of Dothiorella have brown, 1-septate ascospores and differ from Spencermartinsia in the absence of apiculi. These six genera can also be distinguished from one another based on morphological characters of their anamorphs. Although previously placed in the Botryosphaeriaceae, Dothidotthia, was shown to belong in the Pleosporales, and the new family Dothidotthiaceae is introduced to accommodate it
Dynamical Properties of Two Coupled Hubbard Chains at Half-filling
Using grand canonical Quantum Monte Carlo (QMC) simulations combined with
Maximum Entropy analytic continuation, as well as analytical methods, we
examine the one- and two-particle dynamical properties of the Hubbard model on
two coupled chains at half-filling. The one-particle spectral weight function,
, undergoes a qualitative change with interchain hopping
associated with a transition from a four-band insulator to a two-band
insulator. A simple analytical model based on the propagation of exact rung
singlet states gives a good description of the features at large . For
smaller , is similar to that of the
one-dimensional model, with a coherent band of width the effective
antiferromagnetic exchange reasonably well-described by renormalized
spin-wave theory. The coherent band rides on a broad background of width
several times the parallel hopping integral , an incoherent structure
similar to that found in calculations on both the one- and two-dimensional
models. We also present QMC results for the two-particle spin and charge
excitation spectra, and relate their behavior to the rung singlet picture for
large and to the results of spin-wave theory for small .Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised
version with isotropic t_perp=t data include
The ground state of the two-leg Hubbard ladder: a density--matrix renormalization group study
We present density-matrix renormalization group results for the ground state
properties of two-leg Hubbard ladders. The half-filled Hubbard ladder is an
insulating spin-gapped system, exhibiting a crossover from a spin-liquid to a
band-insulator as a function of the interchain hopping matrix element. When the
system is doped, there is a parameter range in which the spin gap remains. In
this phase, the doped holes form singlet pairs and the pair-field and the "" density correlations associated with pair density fluctuations decay as
power laws, while the "" charge density wave correlations decay
exponentially. We discuss the behavior of the exponents of the pairing and
density correlations within this spin gapped phase. Additional one-band
Luttinger liquid phases which occur in the large interband hopping regime are
also discussed.Comment: 14 pages, 18 figures, uses Revtex with epsfig to include the figure
Ab initio calculation of the neutron-proton mass difference
The existence and stability of atoms rely on the fact that neutrons are more
massive than protons. The measured mass difference is only 0.14\% of the
average of the two masses. A slightly smaller or larger value would have led to
a dramatically different universe. Here, we show that this difference results
from the competition between electromagnetic and mass isospin breaking effects.
We performed lattice quantum-chromodynamics and quantum-electrodynamics
computations with four nondegenerate Wilson fermion flavors and computed the
neutron-proton mass-splitting with an accuracy of kilo-electron volts,
which is greater than by standard deviations. We also determine the
splittings in the , , and isospin multiplets,
exceeding in some cases the precision of experimental measurements.Comment: 57 pages, 15 figures, 6 tables, revised versio
Inelastic Neutron Scattering from the Spin Ladder Compound (VO)2P2O7
We present results from an inelastic neutron scattering experiment on the
candidate Heisenberg spin ladder vanadyl pyrophosphate, (VO)2P2O7. We find
evidence for a spin-wave excitation gap of meV, at a
band minimum near . This is consistent with expectations for
triplet spin waves in (VO)2P2O7 in the spin-ladder model, and is to our
knowledge the first confirmation in nature of a Heisenberg antiferromagnetic
spin ladder.Comment: 11 pages and 2 figures (available as hard copy or postscript files
from the authors, send request to [email protected] or
[email protected]), TEX using jnl, reforder and eqnorder, ORNL-CCIP-94-05
/ RAL-94-04
Porosity Reduction in the High-Speed Processing of Glass-Fiber Composites by Resin Transfer Molding (RTM)
High-speed processing is essential to achieve lower production cost in the fabrication of fiber-reinforced composites with the current liquid molding practices. A major consequence of increasing the resin injection velocity is the formation of defects such as voids and dry regions that decrease the load-bearing capability of the composite. Void formation mechanisms and analytical predictions of the detrimental effect of porosity on the structural integrity of molded parts have been studied extensively. In contrast, knowledge of void removal strategies is very limited. In this investigation, various postfill pressure levels were applied to disk-shaped random-mat glass/epoxy parts molded at high volumetric flow rates as a method to reduce their voidage content. Quantitative image analysis over cross-sections cut from these composites revealed that significant changes in porosity concentration take place with the postfill pressure. For instance, overall void content dropped more than 70% with the application of a postfill pressure as low as 300 kPa. Other important void morphometry characteristics such as void shape, size, and spatial distribution could also be manipulated by this method. As the packing pressure increases, large voids gradually disappear, and at the same time, the small circular voids are mobilized towards radial locations near the vents. In addition to this spatial voidage gradient in the radial direction, voidage gradient also exists through the specimen thickness. It seems that higher front velocities promote the appearance of secondary flow phenomena inside the mold cavity (e.g. microfountain flow), which may explain why more voids tend to concentrate at the surface of the specimen irrespective of the postfill pressure level reached inside the mold.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Thermodynamic Properties of the Dimerised and Frustrated S=1/2 Chain
By high temperature series expansion, exact diagonalisation and temperature
density-matrix renormalisation the magnetic susceptibility and the
specific heat of dimerised and frustrated chains are computed.
All three methods yield reliable results, in particular for not too small
temperatures or not too small gaps. The series expansion results are provided
in the form of polynomials allowing very fast and convenient fits in data
analysis using algebraic programmes. We discuss the difficulty to extract more
than two coupling constants from the temperature dependence of .Comment: 14 pages, 13 figures, 4 table
Enhancement of long-range magnetic order by magnetic field in superconducting La2CuO(4+y)
We report a detailed study, using neutron scattering, transport and
magnetization measurements, of the interplay between superconducting (SC) and
spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below
the same critical temperature. However, the SDW order grows with applied
magnetic field, whereas SC order is suppressed. Most importantly, the field
dependence of the SDW Bragg peak intensity has a cusp at zero field, as
predicted by a recent theory of competing SDW and SC order. This leads us to
conclude that there is a repulsive coupling between the two order parameters.
The question of whether the two kinds of order coexist or microscopically phase
separate is discussed.Comment: Version accepted for publication in Phys. Rev. B. Improved discussion
in connection with the muSR result
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