830 research outputs found
Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}
Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure
Alloparental behaviour and long-term costs of mothers tolerating other members of the group in a plurally breeding mammal
Cooperative-breeding studies tend to focus on a few alloparental behaviours in highly cooperative species exhibiting high reproductive skew and the associated short-term, but less frequently long-term, fitness costs. We analysed a suite of alloparental behaviours (assessed via filming) in a kin-structured, high-density population of plurally breeding European badgers, Meles meles, which are not highly cooperative. Group members, other than mothers, performed alloparental behaviour; however, this was not correlated with their relatedness to within-group young. Furthermore, mothers babysat, allogroomed cubs without reciprocation, and allomarked cubs more than other members of the group (controlling for observation time). For welfare reasons, we could not individually mark cubs; however, the number observed pre-independence never exceeded that trapped. All 24 trapped cubs, in three filmed groups, were assigned both parents using 22 microsatellites. Mothers may breed cooperatively, as the time they babysat their assigned, or a larger, litter size did not differ. Furthermore, two mothers probably allonursed, as they suckled more cubs than their assigned litter size. An 18-year genetic pedigree, however, detected no short-term (litter size; maternal survival to the following year) or long-term (offspring breeding probability; offspring lifetime breeding success) fitness benefits with more within-group mothers or other members of the group. Rather, the number of other members of the group (excluding mothers) correlated negatively with long-term fitness. Mothers may tolerate other members of the group, as nonbreeders undertook more digging. Our study highlights that alloparental care varies on a continuum from that seen in this high-density badger population, where alloparenting behaviour is minimal, through to species where alloparental care is common and provides fitness benefits. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved
Female teat size is a reliable indicator of annual breeding success in European badgers: Genetic validation
Assessing which females have bred successfully is a central requirement in many ecological field studies,
providing an estimate of the effective female population size. Researchers have applied teat measurements
previously to assess whether females, in a variety of mammalian species, have bred; however, this
technique has not been validated genetically. Furthermore, several analytical techniques are available to
classify individuals, but their misclassification rates have not been compared. We used 22 microsatellite
loci to assign maternity, with 95% confidence, within a high-density population of European badgers Meles
meles, as plural and subterranean breeding means that maternity cannot be inferred from behavioural
observations. The teat lengths and diameters of 136 females, measured May–July 1994–2005, from social
groups in which all offspring were assigned a mother, were reliable indicators of recent breeding success.
A Generalised Linear Mixed Model (GLMM) classified both breeding and non-breeding females with
lower error rates than discriminant analyses and crude teat-size criteria. The GLMM model logit probability
=
−20 + 1.8 month + 1.6 mean teat length + 1.0 mean teat diameter can be applied quickly in the field
to assess the probability with which a female badger should be assigned maternity. This is a low-cost
measure which, after validation, could be used in other badger or mammalian populations to assess the
breeding success of females. This may be a particularly useful welfare tool for veterinary practitioners,
especially during badger culls
Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgCNi
The anti-perovskite superconductor MgCNi was studied using
high-resolution x-ray Compton scattering combined with electronic structure
calculations. Compton scattering measurements were used to determine
experimentally a Fermi surface that showed good agreement with that of our
supercell calculations, establishing the presence of the predicted hole and
electron Fermi surface sheets. Our calculations indicate that the Fermi surface
is smeared by the disorder due to the presence of vacancies on the C and Ni
sites, but does not drastically change shape. The 20\% reduction in the Fermi
level density-of-states would lead to a significant () suppression
of the superconducting for pair-forming electron-phonon coupling.
However, we ascribe the observed much smaller reduction at our
composition (compared to the stoichiometric compound) to the suppression of
pair-breaking spin fluctuations.Comment: 11 pages, 3 figure
Dispersive Gap Mode of Phonons in Anisotropic Superconductors
We estimate the effect of the superconducting gap anisotropy in the
dispersive gap mode of phonons, which is observed by the neutron scattering on
borocarbide superconductors. We numerically analyze the phonon spectrum
considering the electron-phonon coupling, and examine contributions coming from
the gap suppression and the sign change of the pairing function on the Fermi
surface. When the sign of the pairing function is changed by the nesting
translation, the gap mode does not appear. We also discuss the suppression of
the phonon softening of the Kohn anomaly due to the onset of superconductivity.
We demonstrate that observation of the gap dispersive mode is useful for
sorting out the underlying superconducting pairing function.Comment: 7 pages, 12 figures, to be published in J. Phys. Soc. Jp
Superconductivity Near Ferromagnetism in MgCNi3
An unusual quasi-two-dimensional heavy band mass van Hove singularity (vHs)
lies very near the Fermi energy in MgCNi3, recently reported to superconduct at
8.5 K. This compound is strongly exchange enhanced and is unstable to
ferromagnetism upon hole doping with 12% Mg --> Na or Li. The 1/4-depleted fcc
(frustrated) Ni sublattice and lack of Fermi surface nesting argues against
competing antiferromagnetic and charge density wave instabilities. We identify
an essentially infinite mass along the M-Gamma line, leading to
quasi-two-dimensionality of this vHs may promote unconventional p-wave pairing
that could coexist with superconductivity.Comment: 4 two-column pages, 4 figure
Colossal Positive Magnetoresistance in a Doped Nearly Magnetic Semiconductor
We report on a positive colossal magnetoresistance (MR) induced by
metallization of FeSb, a nearly magnetic or "Kondo" semiconductor with 3d
ions. We discuss contribution of orbital MR and quantum interference to
enhanced magnetic field response of electrical resistivity.Comment: 5 pages, 5 figure
Nesting properties and anisotropy of the Fermi surface of LuNiBC
The rare earth nickel borocarbides, with the generic formula
NiBC, have recently been shown to display a rich variety of
phenomena. Most striking has been the competition between, and even coexistence
of, antiferromagnetism and superconductivity. We have measured the Fermi
surface (FS) of LuNiBC, and shown that it possesses nesting
features capable of explaining some of the phenomena experimentally observed.
In particular, it had previously been conjectured that a particular sheet of FS
is responsible for the modulated magnetic structures manifest in some of the
series. We report the first direct experimental observation of this sheet.Comment: 4 pages, 4 PS figure
- …