537 research outputs found
Spin anisotropy of the resonance in superconducting FeSe0.5Te0.5
We have used polarized-neutron inelastic scattering to resolve the spin
fluctuations in superconducting FeSe0.5Te0.5 into components parallel and
perpendicular to the layers. A spin resonance at an energy of 6.5 meV is
observed to develop below T_c in both fluctuation components. The resonance
peak is anisotropic, with the in-plane component slightly larger than the
out-of-plane component. Away from the resonance peak the magnetic fluctuations
are isotropic in the energy range studied. The results are consistent with a
dominant singlet pairing state with s^{\pm} symmetry, with a possible minority
component of different symmetry.Comment: 5 pages, 4 figure
Identification of the Moving Junction Complex of Toxoplasma gondii: A Collaboration between Distinct Secretory Organelles
Apicomplexan parasites, including Toxoplasma gondii and Plasmodium sp., are obligate intracellular protozoa. They enter into a host cell by attaching to and then creating an invagination in the host cell plasma membrane. Contact between parasite and host plasma membranes occurs in the form of a ring-shaped moving junction that begins at the anterior end of the parasite and then migrates posteriorly. The resulting invagination of host plasma membrane creates a parasitophorous vacuole that completely envelops the now intracellular parasite. At the start of this process, apical membrane antigen 1 (AMA1) is released onto the parasite surface from specialized secretory organelles called micronemes. The T. gondii version of this protein, TgAMA1, has been shown to be essential for invasion but its exact role has not previously been determined. We identify here a trio of proteins that associate with TgAMA1, at least one of which associates with TgAMA1 at the moving junction. Surprisingly, these new proteins derive not from micronemes, but from the anterior secretory organelles known as rhoptries and specifically, for at least two, from the neck portion of these club-shaped structures. Homologues for these AMA1-associated proteins are found throughout the Apicomplexa strongly suggesting that this moving junction apparatus is a conserved feature of this important class of parasites. Differences between the contributing proteins in different species may, in part, be the result of selective pressure from the different niches occupied by these parasites
Magnetic excitations of Fe_{1+y}Se_xTe_{1-x} in magnetic and superconductive phases
We have used inelastic neutron scattering and muon-spin rotation to compare
the low energy magnetic excitations in single crystals of superconducting
Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the
existence of a spin resonance in the superconducting phase of
Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The
non-superconducting sample exhibits two incommensurate magnetic excitations at
(1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is
observed at low energies. A strongly dispersive low-energy magnetic excitation
is also observed in Fe1.10Se0.25Te0.75 close to the commensurate
antiferromagnetic ordering wavevector (1/2-\delta,0,1/2) where \delta \approx
0.03. The magnetic correlations in both samples are found to be quasi-two
dimensional in character and persist well above the magnetic
(Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition
temperatures.Comment: 10 pages, 4 figure
Evidence for magnetic quasiparticle phase separation in a quasi-one-dimensional quantum magnet
Magnetic systems composed of weakly coupled spin-1/2 chains are fertile
ground for hosting the fractional magnetic excitations that are intrinsic to
interacting fermions in one-dimension (1D). However, the exotic physics arising
from the quantum many-body interactions beyond 1D are poorly understood in
materials of this class. Spinons and psinons are two mutually exclusive
low-energy magnetic quasiparticles; the excitation seen depends on the ground
state of the spin chain. Here, we present inelastic neutron scattering and
neutron diffraction evidence for their coexistence in SrCoVO
at milli-Kelvin temperatures in part of the N\'eel phase (2.4 T
H 3.9 T) and possibly also the field-induced spin density
wave phase up to the highest field probed (H 3.9 T,
H = 5.5 T). These results unveil a
novel spatial phase inhomogeneity for the weakly coupled spin chains in this
compound. This quantum dynamical phase separation is a new phenomenon in
quasi-1D quantum magnets, highlighting the non-trivial consequences of
inter-chain coupling.Comment: 8 pages, 6 figure
First-generation black-hole-forming supernovae and the metal abundance pattern of a very iron-poor star
It has been proposed theoretically that the first generation of stars in the
Universe (population III) would be as massive as 100 solar masses (100Mo),
because of inefficient cooling of the precursor gas clouds. Recently, the most
iron-deficient (but still carbon-rich) low-mass star -- HE0107-5240 -- was
discovered. If this is a population III that gained its metals (elements
heavier than helium) after its formation, it would challenge the theoretical
picture of the formation of the first stars. Here we report that the patterns
of elemental abundance in HE0107-5240 (and other extremely metal-poor stars)
are in good accord with the nucleosynthesis that occurs in stars with masses of
20-130Mo when they become supernovae if, during the explosions, the ejecta
undergo substantial mixing and fall-back to form massive black holes. Such
supernovae have been observed. The abundance patterns are not, however,
consistent with enrichment by supernovae from stars in the range 130-300 Mo. We
accordingly infer that the first-generation supernovae came mostly from
explosions of ~ 20-130Mo stars; some of these produced iron-poor but carbon-
and oxygen-rich ejecta. Low-mass second-generation stars, like HE0107-5240,
could form because the carbon and oxygen provided pathways for gas to cool.Comment: To appear in NATURE 422 (2003), 871-873 (issue 24 April 2003); Title
and the first paragraph have been changed and other minor corrections have
been mad
Near-Infrared photometry and spectroscopy of NGC 6539 and UKS 1: two intermediate metallicity Bulge Globular Clusters
Using the SofI imager at ESO/NTT and NIRSPEC spectrograph at KeckII, we have
obtained J,K images and echelle spectra covering the range 1.5 - 1.8 micron for
the intermediate metallicity Bulge globular clusters NGC6539 and UKS1. We find
[Fe/H]=-0.76 and -0.78, respectively, and an average alpha-enhancement of +0.44
dex and +0.31 dex, consistent with previous measurements of metal rich Bulge
clusters, and favoring the scenario of rapid chemical enrichment. We also
measure very low 12C/13C=4.5 +/-1 isotopic ratios in both clusters, suggesting
that extra-mixing mechanisms due to cool bottom processing are at work during
the evolution along the Red Giant Branch. Finally, we measure accurate radial
velocities of =+31 +/-4Km/s and =+57 +/-6Km/s and velocity dispersion
of about 8 Km/s and 11 Km/s for NGC6539 and UKS1, respectively.Comment: 10 pages, 7 figures, accepted for publication at MNRA
Role of defects in determining the magnetic ground state of ytterbium titanate.
Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state
Master Equation Study of Hydrogen Relaxation Using Complete Sets of State-to-state Transition Rates
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97096/1/AIAA2012-362.pd
X-ray Resonant Scattering Study of the Order Parameters in Multiferroic TbMnO
We report on an extensive investigation of the multiferroic compound
TbMnO. Non-resonant x-ray magnetic scattering (NRXMS) revealed a dominant
-type domain. The temperature dependence of the intensity and wavevector
associated with the incommensurate magnetic order was found to be in good
agreement with neutron scattering data. XRS experiments were performed in the
vicinity of the Mn and Tb edges in the high-temperature collinear
phase, the intermediate temperature cycloidal/ferroelectric phase, and the
low-temperature phase. In the collinear phase resonant satellites were
found at the Mn edge associated with -type but also -type peaks. The
azimuthal dependence of the -type satellites (and their absence in the NRXMS
experiments) indicates that they are most likely non-magnetic in origin. We
suggest instead that they may be associated with an induced charge multipole.
At the Tb edge resonant - and -type satellites () were
observed in the collinear phase. These we attribute to a polarisation of the Tb
5 states by the ordering of the Mn sublattice. In the
cycloidal/ferroelectric phase a new set of resonant satellites appear
corresponding to -type order. These appear at the Tb edge only. In
addition to a dominant component in the channel, a
weaker component is found in the pre-edge with
polarization. Calculations of the XRS were performed using the code
showing that the unrotated component of the Tb
-type peaks appearing in the ferroelectric phase contains a contribution
from a multipole that is odd with respect to both space and time, known in
various contexts as the anapole.Comment: Phys. Rev. B (In press
CLES, Code Liegeois d'Evolution Stellaire
Cles is an evolution code recently developed to produce stellar models
meeting the specific requirements of studies in asteroseismology. It offers the
users a lot of choices in the input physics they want in their models and its
versatility allows them to tailor the code to their needs and implement easily
new features. We describe the features implemented in the current version of
the code and the techniques used to solve the equations of stellar structure
and evolution. A brief account is given of the use of the program and of a
solar calibration realized with it.Comment: Comments: 8 pages, Astrophys. Space Sci. CoRoT-ESTA Volume, in the
pres
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