4,425 research outputs found
Genetic diversity of the rain tree (Albizia saman) in Colombian seasonally dry tropical forest for informing conservation and restoration interventions
Albizia saman is a multipurpose tree species of seasonally dry tropical forests (SDTFs) of Mesoamerica and northern South America typically cultivated in silvopastoral and other agroforestry systems around the world, a trend that is bound to increase in light of multimillion hectare commitments for forest and landscape restoration. The effective conservation and sustainable use of A. saman requires detailed knowledge of its genetic diversity across its native distribution range of which surprisingly little is known to date. We assessed the genetic diversity and structure of A.saman across twelve representative locations of SDTF in Colombia, and how they may have been shaped by past climatic changes and human influence. We found four different genetic groups which may be the result of differentiation due to isolation of populations in preglacial times. The current distribution and mixture of genetic groups across STDF fragments we observed might be the result of range expansion of SDTFs during the last glacial period followed by range contraction during the Holocene and humanâinfluenced movement of germplasm associated with cattle ranching. Despite the fragmented state of the presumed natural A. saman stands we sampled, we did not find any signs of inbreeding, suggesting that gene flow is not jeopardized in humanized landscapes. However, further research is needed to assess potential deleterious effects of fragmentation on progeny. Climate change is not expected to seriously threaten the in situ persistence of A. saman populations and might present opportunities for future range expansion. However, the sourcing of germplasm for tree planting activities needs to be aligned with the genetic affinity of reference populations across the distribution of Colombian SDTFs. We identify priority source populations for in situ conservation based on their high genetic diversity, lack or limited signs of admixture, and/or genetic uniqueness
Monte Carlo study of the magnetic critical properties of the two-dimensional Ising fluid
A two-dimensional fluid of hard spheres each having a spin and
interacting via short-range Ising-like interaction is studied near the second
order phase transition from the paramagnetic gas to the ferromagnetic gas
phase. Monte Carlo simulation technique and the multiple histogram data
analysis were used. By measuring the finite-size behaviour of several different
thermodynamic quantities,we were able to locate the transition and estimate
values of various static critical exponents. The values of exponents
and are close to the ones for the two-dimensional
lattice Ising model. However, our result for the exponent is very
different from the one for the Ising universality class.Comment: 6 pages, 8 figures. To appear in Phys. Rev.
Wigner's little group and Berry's phase for massless particles
The ``little group'' for massless particles (namely, the Lorentz
transformations that leave a null vector invariant) is isomorphic to
the Euclidean group E2: translations and rotations in a plane. We show how to
obtain explicitly the rotation angle of E2 as a function of and we
relate that angle to Berry's topological phase. Some particles admit both signs
of helicity, and it is then possible to define a reduced density matrix for
their polarization. However, that density matrix is physically meaningless,
because it has no transformation law under the Lorentz group, even under
ordinary rotations.Comment: 4 pages revte
Analytic Solution for the Critical State in Superconducting Elliptic Films
A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.Comment: Revtex file and 13 postscript figures, gives 10 pages of text with
figures built i
Anti-de Sitter curvature radius constrained by quasars in brane-world scenarios
This paper is intended to investigate the luminosity due to accretion of gas
in supermassive black holes (SMBHs) in the center of quasars, using a
brane-world scenario naturally endowed with extra dimensions, whereon theories
formulated introduce corrections in the field equations at high energies. SMBHs
possess the necessary highly energetic environment for the introduction of
these corrections, which are shown to produce small deviations in all SMBH
properties and, consequentely, corrections in the accretion theory that
supports quasars radiative processes. The radiative flux observed from quasars
indicates these deviations, from which the magnitude of the AdS bulk
curvature radius, and consequently the extra dimension compactification radius
is estimated.Comment: 11 pages, RevTeX, Eq.(2) and (3) expanded, and comments thereon
update
Lateralization of Simulated Sources and Echoes Differing in Frequency Based on Interaural Temporal Differences
This study examined listenersâ ability to process interaural temporal differences (ITDs) in one of two sequential sounds when the two differed in spectral content. A correlational analysis assessed weights given to ITDs of simulated source and echo pulses for echo delays of 8â128ms for conditions in which responses were based on the source or echo, a 3000-Hz Gaussian (target) pulse. The other (distractor) pulse was spectrally centered at 1500, 2000, 3000, 4000, or 5000 Hz. Also measured were proportion correct and proportion of responses predicted from the weights. Regardless of whether the echo or source pulse served as the target, target weight, and proportion correct increased with increasing distractor frequency, consistent with low-frequency dominance [Divenyi, J. Acoust. Soc. Am. 91, 1078â1084 (1992)]. Effects of distractor frequency were observed at echo delays out to 128 ms when the source served as the target, but only out to 64 ms when the echo served as the target. At echo delays beyond 8 ms, recency effects were exhibited with higher proportions correct obtained for judgments based on the echo pulse than the source pulse
Kinetic Inductance and Penetration Depth of Thin Superconducting Films Measured by THz Pulse Spectroscopy
We measure the transmission of THz pulses through thin films of YBCO at
temperatures between 10K and 300K. The pulses possess a useable bandwidth
extending from 0.1 -- 1.5 THz (3.3 cm^-1 -- 50 cm^-1). Below T_c we observe
pulse reshaping caused by the kinetic inductance of the superconducting charge
carriers. From transmission data, we extract values of the London penetration
depth as a function of temperature, and find that it agrees well with a
functional form (\lambda(0)/\lambda(T))^2 = 1 - (T/T_c)^{\alpha}, where
\lambda(0) = 148 nm, and \alpha = 2. *****Figures available upon request*****Comment: 7 Pages, LaTe
Nuclear transparencies for nucleons, knocked-out under various semi-inclusive conditions
Using hadron dynamics we calculate nuclear transparencies for protons,
knocked-out in high-, semi-inclusive reactions. Predicted transparencies
are, roughly half a standard deviation above the NE18 data. The latter contain
the effects of binned proton missing momenta and mass, and of finite detector
acceptances. In order to test sensitivity we compare computed transparencies
without restrictions and the same with maximal cuts for missing momenta and the
electron energy loss. We find hardly any variation, enabling a meaningful
comparison with data and predictions based on hadron dynamics. Should
discrepancies persist in high-statistics data, the above may with greater
confidence be attributed to exotic components in the description of the
outgoing proton.Comment: 13 pages + 3 figsin appended PS file, report # WIS-94/43/Oct-P
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