740 research outputs found
Magnetic structure and charge ordering in Fe3BO5 ludwigite
The crystal and magnetic structures of the three-leg ladder compound Fe3BO5
have been investigated by single crystal x-ray diffraction and neutron powder
diffraction. Fe3BO5 contains two types of three-leg spin ladders. It shows a
charge ordering transition at 283 K, an antiferromagnetic transition at 112 K,
ferromagnetism below 70 K and a weak ferromagnetic behavior below 40K. The
x-ray data reveal a smooth charge ordering and an incomplete charge
localization down to 110K. Below the first magnetic transition, the first type
of ladders orders as ferromagnetically coupled antiferromagnetic chains, while
below 70K the second type of ladders orders as antiferromagnetically coupled
ferromagnetic chains
Ground State of the Easy-Axis Rare-Earth Kagom\'e Langasite PrGaSiO
We report muon spin relaxation (SR) and Ga nuclear quadrupolar
resonance (NQR) local-probe investigations of the kagom\'e compound
PrGaSiO. Small quasi-static random internal fields develop below
40 K and persist down to our base temperature of 21 mK. They originate from
hyperfine-enhanced Pr nuclear magnetism which requires a non-magnetic
Pr crystal-field (CF) ground state. Besides, we observe a broad maximum
of the relaxation rate at K which we attribute to the population of
the first excited magnetic CF level. Our results yield a Van-Vleck paramagnet
picture, at variance with the formerly proposed spin-liquid ground state.Comment: minor change
Magnetic excitations in a new anisotropic Kagom\'{e} antiferromagnet
The Nd-langasite compound contains planes of magnetic Nd3+ ions on a lattice
topologically equivalent to a kagom\'{e} net. The magnetic susceptibility does
not reveal any signature of long-range ordering down to 2 K but rather a
correlated paramagnetism with significant antiferromagnetic interactions
between the Nd and a single-ion anisotropy due to crystal field effect.
Inelastic neutron scattering on Nd-langasite powder and single-crystal allowed
to probe its very peculiar low temperature dynamical magnetic correlations.
They present unusual dispersive features and are broadly localized in
wave-vector Q revealing a structure factor associated to characteristics short
range-correlations between the magnetic atoms. From comparison with theoretical
calculations, these results are interpreted as a possible experimental
observation of a spin liquid state in an anisotropic kagom\'{e}
antiferromagnet.Comment: to appear in Physica
Single domain magnetic helicity and triangular chirality in structurally enantiopure Ba3NbFe3Si2O14
A novel doubly chiral magnetic order is found out in the structurally chiral
langasite compound BaNbFeSiO. The magnetic moments are
distributed over planar frustrated triangular lattices of triangle units. On
each of these they form the same triangular configuration. This ferro-chiral
arrangement is helically modulated from plane to plane. Unpolarized neutron
scattering on a single crystal associated with spherical neutron polarimetry
proved that a single triangular chirality together with a single helicity is
stabilized in an enantiopure crystal. A mean field analysis allows discerning
the relevance on this selection of a twist in the plane to plane
supersuperexchange paths
Modulation of angiogenesis by inflammatory markers and the role of matrix metalloproteinases in an endothelial cell/fibroblast co-culture system.
Increased levels of inflammatory markers such as tumour necrosis factor-α (TNFα) and interleukin- 6 (IL-6) have been associated with formation of new blood vessels, or angiogenesis, and linked to chronic inflammation in obesity. This study aimed to establish and use a versatile co-culture cell system to further investigate the role of TNFα and IL-6 in modulating (i) tubule formation and (ii) cell-cell interactions via matrix metalloproteinase (MMP) enzyme activity and secretion of vascular endothelial growth factor (VEGF), E-selectin and prostaglandin E2 (PGE2). Co-cultures of human endothelial cells and fibroblasts were incubated with TNFα (10 ng/mL) or IL-6 (10 ng/mL) added 2 and/or 7 days after co-culture establishment. Cell viability by enzymatic conversion was determined by MTT assay; tubule formation was detected by immunostaining; VEGF, E-selectin and PGE2 expression by ELISA analysis and MMP enzyme activity by gel zymography. Treatmentspecific and time dependent differences in tubule formation were observed: IL-6 significantly increased tubule formation, whilst TNFα significantly inhibited tubule formation. Treatment-specific differences in levels of MMP activities which correlate to tubule formation were also observed. This study showed inflammatory markers, typically associated with obese status, affect tubule formation differently in a heterogeneous cell environment similar to that observed in vivo
Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMnMnO
By means of neutron powder diffraction, we investigated the effect of the
polar Bi ion on the magnetic ordering of the Mn ions in
BiMnMnO, the counterpart with \textit{quadruple} perovskite
structure of the \textit{simple} perovskite BiMnO. The data are consistent
with a \textit{noncentrosymmetric} spacegroup which contrasts the
\textit{centrosymmetric} one previously reported for the isovalent and
isomorphic compound LaMnMnO, which gives evidence of a
Bi-induced polarization of the lattice. At low temperature, the two
Mn sublattices of the and sites order antiferromagnetically
(AFM) in an independent manner at 25 and 55 K, similarly to the case of
LaMnMnO. However, both magnetic structures of
BiMnMnO radically differ from those of LaMnMnO.
In BiMnMnO the moments of the sites form
an anti-body AFM structure, whilst the moments \textbf{M} of the
sites result from a large and \textit{uniform} modulation along the b-axis of the moments \textbf{M} in the
-plane. The modulation is strikingly correlated with the displacements of
the Mn ions induced by the Bi ions. Our analysis unveils a strong
magnetoelastic coupling between the internal strain created by the Bi
ions and the moment of the Mn ions in the sites. This is ascribed to
the high symmetry of the oxygen sites and to the absence of oxygen defects, two
characteristics of quadruple perovskites not found in simple ones, which
prevent the release of the Bi-induced strain through distortions or
disorder. This demonstrates the possibility of a large magnetoelectric coupling
in proper ferroelectrics and suggests a novel concept of internal strain
engineering for multiferroics design.Comment: 9 pages, 7 figures, 5 table
Inhomogeneous magnetism in the doped kagome lattice of LaCuO2.66
The hole-doped kagome lattice of Cu2+ ions in LaCuO2.66 was investigated by
nuclear quadrupole resonance (NQR), electron spin resonance (ESR), electrical
resistivity, bulk magnetization and specific heat measurements. For
temperatures above ~180 K, the spin and charge properties show an activated
behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the
results indicate an insulating ground state which may or may not be charge
ordered. While the frustrated spins in remaining patches of the original kagome
lattice might not be directly detected here, the observation of coexisting
non-magnetic sites, free spins and frozen moments reveals an intrinsically
inhomogeneous magnetism. Numerical simulations of a 1/3-diluted kagome lattice
rationalize this magnetic state in terms of a heterogeneous distribution of
cluster sizes and morphologies near the site-percolation threshold
Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part2): bond valence sum approach of the hole distribution
The effects of Sr substitution on superconductivity, and more particulary the
changes induced in the hole doping mechanism, were investigated in
Hg2(Ba1-ySry)2YCu2O8-d by a "bond valence sum" analysis with Sr content from y
= 0.0 to y = 1.0. A comparison with CuBa2YCu2O7-d and Cu2Ba2YCu2O8 systems
suggests a possible explanation of the Tc enhancement from 0 K for y = 0.0 to
42 K for y = 1.0. The charge distribution among atoms of the unit cell was
determined from the refined structure, for y = 0.0 to 1.0. It shows a charge
transfer to the superconducting CuO2 plane via two doping channels pi(1) and
pi(2), i.e. through O2(apical)-Cu and Ba/Sr-O1 bonds respectively.Comment: 13 pages, 5 figures, accepted for publication in Journal of Physics:
Condensed Matte
Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry
We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a
monotonous decrease of the lattice parameters and of the c/a ratio, but no
structural transition down to parameters smaller than those of AlB2. The
transition superconducting temperature also decreases with temperature in a
sample dependent way. The results are explained by an increase of the filling
of the 2D pxy bands with pressure, the Mg stoichiometry determining the
starting position of the Fermi level. Our measurements indicate that these hole
bands are the relevant ones for superconductivity.Comment: submitted March 9th 2001, PRB accepte
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