1,090 research outputs found
A genome-wide investigation of the worldwide invader Sargassum muticum shows high success albeit (almost) no genetic diversity
Twenty years of genetic studies of marine invaders have shown that successful invaders are often characterized by native and introduced populations displaying similar levels of genetic diversity. This pattern is presumably due to high propagule pressure and repeated introductions. The opposite pattern is reported in this study of the brown seaweed, Sargassum muticum, an emblematic species for circumglobal invasions. Albeit demonstrating polymorphism in the native range, microsatellites failed to detect any genetic variation over 1,269 individuals sampled from 46 locations over the Pacific-Atlantic introduction range. Single-nucleotide polymorphisms (SNPs) obtained from ddRAD sequencing revealed some genetic variation, but confirmed severe founder events in both the Pacific and Atlantic introduction ranges. Our study thus exemplifies the need for extreme caution in interpreting neutral genetic diversity as a proxy for invasive potential. Our results confirm a previously hypothesized transoceanic secondary introduction from NE Pacific to Europe. However, the SNP panel unexpectedly revealed two additional distinct genetic origins of introductions. Also, conversely to scenarios based on historical records, southern rather than northern NE Pacific populations could have seeded most of the European populations. Finally, the most recently introduced populations showed the lowest selfing rates, suggesting higher levels of recombination might be beneficial at the early stage of the introduction process (i.e., facilitating evolutionary novelties), whereas uniparental reproduction might be favored later in sustainably established populations (i.e., sustaining local adaptation).Agence Nationale de la Recherche - ANR-10-BTBR-04; European Regional Development Fund; Fundacao para a Ciencia e a Tecnologia - SFRH/BPD/107878/2015, UID/Multi/04326/2016, UID/Multi/04326/2019; Brittany Region;info:eu-repo/semantics/publishedVersio
Absence of long-range chemical ordering in equimolar FeCoCrNi
Equimolar FeCoCrNi alloys have been the topic of recent research as "high-entropy alloys," where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to
date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi_3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi_3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order
Dynamics of iron atoms across the pressure-induced Invar transition in Pd_3Fe
The ^(57)Fe phonon partial density of states (PDOS) in L1_2-ordered Pd_3Fe was studied at high pressures by nuclear resonant inelastic x-ray scattering (NRIXS) measurements and density functional theory (DFT) calculations. The NRIXS spectra showed that the stiffening of the ^(57)Fe PDOS with decreasing volume was slower from 12 to 24âGPa owing to the pressure-induced Invar transition in Pd_3Fe, with a change from a high-moment ferromagnetic (FM) state to a low-moment (LM) state observed by nuclear forward scattering. Force constants obtained from fitting to a Bornâvon KĂĄrmĂĄn model showed a relative softening of the first-nearest-neighbor (1NN) Fe-Pd longitudinal force constants at the magnetic transition. For the FM low-pressure state, the DFT calculations gave a PDOS and 1NN longitudinal force constants in good agreement with experiment, but discrepancies for the high-pressure LM state suggest the presence of short-range magnetic order
Observation of new quantum interference effect in solids
In order to achieve quantum interference of free electrons inside a solid, we
have modified the geometry of the solid so that de Broglie waves interfere
destructively inside the solid. Quantum interference of de Broglie waves leads
to a reduction in the density of possible quantum states of electrons inside
the solid and increases the Fermi energy level. This effect was studied
theoretically within the limit of the quantum theory of free electrons inside
the metal. It has been shown that if a metal surface is modified with patterned
indents, the Fermi energy level will increase and consequently the electron
work function will decrease. This effect was studied experimentally in both Au
and SiO2 thin films of special geometry and structure. Work function reductions
of 0.5 eV in Au films and 0.2 eV in SiO2 films were observed. Comparative
measurements of work function were made using the Kelvin Probe method based on
compensation of internal contact potential difference. Electron emission from
the same thin films was studied by two independent research groups using
Photoelectron Emission Microscopy (PEEM).Comment: 11 pages, 5 figure
Observation of cosmic ray positrons from 5 to 25 GeV
The positron data gathered in conjunction with electron data published elsewhere is reported. The basic recognition scheme was to look for low mass positive particles that cause a cascade in a 7 radiation length shower counter. The mass criteria is imposed by selecting particles that were accompanied by Cherenkov light but whose rigidity was below the proton Cherenkov threshold. Thus the proton Cherenkov threshold represents an upper limit to the range of the experiment
Spin Glass Ordering in Diluted Magnetic Semiconductors: a Monte Carlo Study
We study the temperature-dilution phase diagram of a site-diluted Heisenberg
antiferromagnet on a fcc lattice, with and without the Dzyaloshinskii-Moriya
anisotropic term, fixed to realistic microscopic parameters for (IIB=Cd, Hg, Zn). We show that the dipolar Dzyaloshinskii-Moriya anisotropy
induces a finite-temperature phase transition to a spin glass phase, at
dilutions larger than 80%. The resulting probability distribution of the order
parameter P(q) is similar to the one found in the cubic lattice
Edwards-Anderson Ising model. The critical exponents undergo large finite size
corrections, but tend to values similar to the ones of the
Edwards-Anderson-Ising model.Comment: 4 pages plus 3 postscript figure
Mass resolution optimization in a large isotopic composition experiment
A range-energy experiment was built to measure the isotopic composition of galactic cosmic rays. An enrichment of neutron rich isotopes, 22Ne and (25Mg + 26Mg) in particular, when compared to the solar composition is shown. A rich statistics measurement of these and other neutron-rich isotopes in the galactic flux yields information to the source of these particles. A computer simulation of the experiment was used to estimate the instrument resolution. The Cherenkov detector light collection efficiency, was calculated. Absorption of light in the radiator was considered to determine the optimum Cherenkov medium thickness. The experiment will determine the isotopic composition for the elements neon through argon in the energy range 300 to 800 MeV per nucleon
Temperature and Pressure Dependence of the Fe-specific Phonon Density of States in Ba(Fe(1-x)Co(x))2As2
The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single
crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high
pressures with nuclear-resonant inelastic x-ray scattering. Measurements were
conducted for two different orientations of the single crystals, yielding the
orientation-projected {57}Fe-phonon density of states (DOS) for phonon
polarizations in-plane and out-of-plane with respect to the basal plane of the
crystal structure. In the tetragonal phase at 300 K, a clear stiffening was
observed upon doping with Co. Increasing pressure to 4 GPa caused a marked
increase of phonon frequencies, with the doped material still stiffer than the
parent compound. Upon cooling, both the doped and undoped samples showed a
stiffening, and the parent compound exhibited a discontinuity across the
magnetic and structural phase transition. These findings are generally
compatible with the changes in volume of the system upon doping, increasing
pressure, or increasing temperature, but an extra softening of high-energy
modes occurs with increasing temperature. First-principles computations of the
phonon DOS were performed and showed an overall agreement with the experimental
results, but underestimate the Grueneisen parameter. This discrepancy is
explained in terms of a magnetic Grueneisen parameter, causing an extra phonon
stiffening as magnetism is suppressed under pressure
Temperature and pressure dependence of the Fe-specific phonon density of states in Ba(Fe_(1âx)Co_x)_2As_2
The ^(57)Fe-specific phonon density of states (DOS) of Ba(Fe_(1âx)Co_x)_2As_2 single crystals (x=0.0,0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected ^(57)Fe-phonon density of states for phonon polarizations in-plane and out-of-plane with respect to the basal plane of the crystal structure. In the tetragonal phase at 300 K, a clear stiffening was observed upon doping with Co. Increasing pressure to 4 GPa caused a marked increase of phonon frequencies, with the doped material still stiffer than the parent compound. Upon cooling, both the doped and undoped samples showed a stiffening and the parent compound exhibited a discontinuity across the magnetic and structural phase transitions. These findings are generally compatible with the changes in volume of the system upon doping, increasing pressure, or increasing temperature, but an extra softening of high-energy modes occurs with increasing temperature. First-principles computations of the phonon DOS were performed and showed an overall agreement with the experimental results, but underestimate the GrĂŒneisen parameter. This discrepancy is explained in terms of a magnetic GrĂŒneisen parameter, causing an extra phonon stiffening as magnetism is suppressed under pressure
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