72 research outputs found
Crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_{4} : role of the orbital degree of freedom
The crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_4 (0<x<0.7) has
been studied by diffraction techniques and high resolution capacitance
dilatometry. There is no evidence for a structural phase transition like those
found in isostructural cuprates or nickelates, but there are significant
structural changes induced by the variation of temperature and doping which we
attribute to a rearrangement of the orbital occupation.Comment: 8 pages, 6 figures, submitted to PR
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
Influence of low energy argon plasma treatment on the moisture barrier performance of hot wire-CVD grown SiNx multilayers
The reliability and stability are key issues for the commercial utilization of organic photovoltaic devices based on flexible polymer substrates. To increase the shelf-lifetime of these devices, transparent moisture barriers of silicon nitride (SiNx) films are deposited at low temperature by hot wire CVD (HW-CVD) process. Instead of the conventional route based on organic/inorganic hybrid structures, this work defines a new route consisting in depositing multilayer stacks of SiNx thin films, each single layer being treated by argon plasma. The plasma treatment allows creating smoother surface and surface atom rearrangement. We define a critical thickness of the single layer film and focus our attention on the effect of increasing the number of SiNx single-layers on the barrier properties. A water vapor transmission rate (WVTR) of 2 x 10-4 g/(m2 day) is reported for SiNx multilayer stack and a physical interpretation of the plasma treatment effect is given.Direction des Relations Extérieures, Ecole PolytechniquePICS (French–Portuguese No. 5336) projec
Flexible organic–inorganic hybrid layer encapsulation for organic opto-electronic devices
In this work we produce and study the flexible organic–inorganic hybrid moisture barrier layers for the protection of air sensitive organic opto-electronic devices. The inorganic amorphous silicon nitride layer (SiNx:H) and the organic PMMA [poly (methyl methacrylate)] layer are deposited alternatingly by using hot wire chemical vapor deposition (HW-CVD) and spin-coating techniques, respectively. The effect of organic–inorganic hybrid interfaces is analyzed for increasing number of interfaces. We produce highly transparent (∼80% in the visible region) hybrid structures. The morphological properties are analysed providing a good basis for understanding the variation of the water vapor transmission rate (WVTR) values. A minimum WVTR of 4.5 × 10−5g/m2day is reported at the ambient atmospheric conditions for 7 organic/inorganic interfaces. The hybrid barriers show superb mechanical flexibility which confirms their high potential for flexible applications.The authors would like to thank Dr. J.C. Vanel for help in electrical characterizations used in this study. The first author (S.M) acknowledges the financial support from Direction des Relations Extérieures, Ecole Polytechnique during his thesis
Deposition of silicon nitride thin films by hot-wire CVD at 100ºC and 250ºC
Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1-3 Ǻ/s) and at low substrate temperature. Films were deposited using NH3/SiH4 flow rate ratios between 1 and 70 and substrate temperatures of 100º C and 250ºC. For NH3/SiH4 ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56 - 2.74 g/cm3) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 Ǻ/s and 10 MV cm−1.Fundação para a Ciência e Tecnologia (FCT) - FCT/CNRS programa com o contracto no. 20798, bolsa de investigaçao e projecto PTDC-CTM-66558-200
The effect of argon plasma treatment on the permeation barrier properties of silicon nitride layers
In this work we produce and study silicon nitride (SiNx) thin films deposited by Hot Wire Chemical Vapor Depo- sition (HW-CVD) to be used as encapsulation barriers for flexible organic photovoltaic cells fabricated on poly- ethylene terephthalate (PET) substrates in order to increase their shelf lifetime. We report on the results of SiNx double-layers and on the equivalent double-layer stack where an Ar-plasma surface treatment was performed on the first SiNx layer. The Ar-plasma treatment may under certain conditions influences the structure of the interface between the two subsequent layers and thus the barrier properties of the whole system. We focus our attention on the effect of plasma treatment time on the final barrier properties. We assess the encapsulation barrier properties of these layers, using the calcium degradation test where changes in the electrical conductance of encapsulated Ca sensors are monitored with time. The water vapor transmission rate (WVTR) is found to be ~3 × 10−3 g/m2·day for stacked SiNx double-layer with 8 min Ar plasma surface treatment.FCT - CNRS PICS (French–Portuguese no: 5336) projectDirection des Relations Extérieures, Ecole Polytechniqu
Effect of argon ion energy on the performance of silicon nitridemultilayer permeation barriers grown by hot-wire CVD on polymers
One of the authors (S.M.) acknowledges Direction des Relations Extérieures of Ecole Polytechnique for financial support.Permeation barriers for organic electronic devices on polymer flexible substrates were realized by combining stacked silicon nitride (SiNx) single layers (50 nm thick) deposited by hot-wire chemical vapor deposition process at low-temperature (~100°C) with a specific argon plasma treatment between two successive layers.
Several plasma parameters (RF power density, pressure, treatment duration) as well as the number of single layers have been explored in order to improve the quality of permeation barriers deposited on polyethylene terephthalate. In this work, maximumion energy was highlighted as the crucial parameter making it possible to minimize water vapor transmission rate (WVTR), as determined by the electrical calcium test method, all the other parameters being kept fixed. Thus fixing the plasma treatment duration at 8 min for a stack of two SiNx single layers, a minimum WVTR of 5 × 10−4 g/(m2 day), measured at room temperature, was found for a maximum ion energy of ~30 eV. This minimum WVTR value was reduced to 7 × 10−5 g/(m2 day) for a stack of five SiNx single layers. The reduction in the permeability is interpreted as due to the rearrangement of atoms at the interfaces when average transferred ion energy to target atoms exceeds threshold displacement energy.The authors are grateful to Dr. R. Cortes (PMC, Ecole Polytechnique) for XRR analysis, to Dr. P. Chapon (HORIBA Jobin Yvon) for GD-OES analysis and Dr. J. Leroy (CEA Saclay) for XPS analysis. This work was partly supported by the PICS (FrenchPortuguese) project No. 5336. One of the authors (S.M.) acknowledges Direction des Relations Extérieures of Ecole Polytechnique for financial support
Structural and Magnetic Instabilities of LaSrCaCuO
A neutron scattering study of nonsuperconducting
LaSrCaCuO (x=0 and 0.2), a bilayer copper oxide without CuO
chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a
doping dependent transition temperature. The predominant structural
modification below the transition is an in-plane shift of the apical oxygen. In
the doped sample, the orthorhombic superstructure is strongly disordered, and a
glassy state involving both magnetic and structural degrees of freedom develops
at low temperature. The spin correlations are commensurate.Comment: published versio
Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12
The present work reports synthesis, as well as a detailed and careful
characterization of structural, magnetic, and dielectric properties of
differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this
purpose, neutron and x-ray powder diffraction, SQUID measurements, and
dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO
ceramics were investigated in great detail to document the influence of
low-level doping with 3d metals on the antiferromagnetic structure and
dielectric properties. In the light of possible magnetoelectric coupling in
these doped ceramics, the dielectric measurements were also carried out in
external magnetic fields up to 7 T, showing a minor but significant dependence
of the dielectric constant on the applied magnetic field. Undoped CCTO is
well-known for its colossal dielectric constant in a broad frequency and
temperature range. With the present extended characterization of doped as well
as undoped CCTO, we want to address the question why doping with only 1% Mn or
0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor
of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni
doping changes the dielectric properties only slightly. In addition,
diffraction experiments and magnetic investigations were undertaken to check
for possible correlations of the magnitude of the colossal dielectric constants
with structural details or with magnetic properties like the magnetic ordering,
the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that
while the magnetic ordering temperature and the effective moment of all
investigated CCTO ceramics are rather similar, there is a dramatic influence of
doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure
Impact of Zostavax Vaccination on T-Cell Accumulation and Cutaneous Gene Expression in the Skin of Older Humans After Varicella Zoster Virus Antigen-Specific Challenge
Background
The live attenuated vaccine Zostavax was developed to prevent varicella zoster virus (VZV) reactivation that causes herpes zoster (shingles) in older humans. However, the impact of vaccination on the cutaneous response to VZV is not known.
Methods
We investigated the response to intradermal VZV antigen challenge before and after Zostavax vaccination in participants >70 years of age by immunohistological and transcriptomic analyses of skin biopsy specimens collected from the challenge site.
Results
Vaccination increased the proportion of VZV-specific CD4+ T cells in the blood and promoted the accumulation of both CD4+ and CD8+ T cells in the skin after VZV antigen challenge. However, Zostavax did not alter the proportion of resident memory T cells (CD4+ and CD8+) or CD4+Foxp3+ regulatory T cells in unchallenged skin. After vaccination, there was increased cutaneous T-cell proliferation at the challenge site and also increased recruitment of T cells from the blood, as indicated by an elevated T-cell migratory gene signature. CD8+ T-cell–associated functional genes were also highly induced in the skin after vaccination.
Conclusion
Zostavax vaccination does not alter the abundance of cutaneous resident memory T cells but instead increases the recruitment of VZV-specific T cells from the blood and enhances T-cell activation, particularly cells of the CD8+ subset, in the skin after VZV antigen challenge
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