Ref HAL: hal-01802768_v1
DOI: 10.1016/j.spmi.2018.05.033
WoS: 000445713700016
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Lead halide perovskites, used to produce photovoltaic devices, have been thesubject of a huge research effort these last years. This is due to the spectac-ular improvement of the conversion effciency results within a short amount oftime. However, some issues have been identified, which include stability, leaduse and cost of some precursors. It has recently been shown that low purity, lowcost PbI2 could be succesfully used in the synthesis of the MAPbI3 perovskite,provided that HCl is added during the synthesis to prevent solubility problems.Thus, it is of high interest to provide information pertaining to the materialquality, in relation with the HCl additions performed during synthesis. In thiswork, we have grown three sets of samples with different HCl to (PbI 2 + MAI)molar ratios (R HCl ), where PbI2 and methylamine iodide (MAI) are the precur-sors of the perovskite. We used RHCl = 0 (no HCl), 0.5 and 1.2 . We performedx-ray diffraction, transmittance and 77K photoluminescence experiments in or-der to assess the material structural and optoelectronic properties and foundthat an optimum HCl concentration must be used. HCl introduction clearlyhas a beneficial effect both on domain sizes and photoluminescence intensityat RHCl = 0.5, but lead to subsequent degradation of the perovskite quality athigher RHCl .

Ref HAL: hal-01784269_v1
DOI: 10.1088/2053-1591/aab8c5
WoS: 000429835000001
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6 Citations
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We report on the growth of GaSe films by molecular beam epitaxy on both (111)B GaAs and sapphire substrates. X-ray diffraction reveals the perfect crystallinity of GaSe with the c-axis normal to the substrate surface. The samples grown under Ga rich conditions possess an additional gallium film on top of the monochalcogenide layer. This metallic film shows two normal-to-superconducting transitions which are detected at T c ≈ 1.1 K and 6.0 K. They correspond likely to the β and α-phases of gallium in the form of bulk and droplets respectively. Our results demonstrate that van der Waals epitaxy can lead to future high quality hybrid superconductor/monochalcogenide heterostructures.

Ref HAL: hal-01772009_v1
Ref Arxiv: 1804.01061
DOI: 10.1126/science.aar8438
WoS: 000430396600037
Ref. & Cit.: NASA ADS
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103 Citations
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The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing and nanoscale lasers. While plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the lengthscale of one atom. This is achieved by far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric h-BN spacer between graphene and metal rods. A theoretical model which takes into account the non-local optical response of both graphene and metal is used to describe the results. These ultra-confined plasmonic modes, addressed with far-field light excitation, enables a route to new regimes of ultra-strong light-matter interactions.

Commentaires: 17 pages, 4 figures

Ref HAL: hal-01767245_v1
DOI: 10.1038/s41598-018-22459-7
WoS: WOS:000426643600011
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4 Citations
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White lesions represent an early phase of caries formation. 20 human sound premolars were subjected to pH cycling procedure to induce subsurface lesions (SLs) in vitro. In addition, 2 teeth with naturally developed white spot lesions (WSLs) were used as references. All specimens characterized by confocal Raman microscopy being used for the first time in examining white & subsurface lesions and providing a high resolution chemical and morphological map based on phosphate peak intensity alterations at 960 cm−1. Nanoindentation technique was used to measure Hardness (H) and Young’s modulus (E) of enamel. Phosphate map of examined samples exhibited presence of intact surface layer (ISL) followed by severe depletion in (PO43−) peak in the area corresponding to the body of the lesion. In all examined groups, the mechanical properties of enamel were decreased in lesion area and found to be inversely related to penetration depth of indenter owing to enamel hierarchical structure. By combining the above two techniques, we linked mechanical properties of enamel to its chemical composition and ensured that the two methods are highly sensitive to detect small changes in enamel composition. Further work is required to bring these two excellent tools to clinical application to perceive carious lesions at an early stage of development.

Ref HAL: hal-01762923_v1
DOI: 10.5194/jsss-7-227-2018
WoS: 000429154300002
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3 Citations
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Nuclear magnetic resonance (NMR) experiments are usually carried out at very high and homogenous magnetic fields. However, portable systems have emerged in recent years at low fields, offering many advantages like easier maintenance and open design that permit us to imagine a variety of coil configurations in order to maximize the homogeneity of the static field B-0 and the radio-frequency field B-1. The wire winding distribution based on space harmonics suppression (SHS) of the cosine current density distribution offers new possibilities of producing homogenous static fields B-0. In this work, an extension of the SHS method, allowing homogeneity enhancement and flexibility of the coil design, is presented. The efficiency of the method is demonstrated through the construction of homogenous magnetic field coils dedicated to a very low magnetic field (8 mT) H-1 NMR spectrometer. The performance of these coils in terms of magnetic field intensity and homogeneity as well as self-heating behavior is compliant with in planta experiments for agronomic studies.

Ref HAL: hal-01762920_v1
PMID 29596455
DOI: 10.1371/journal.pone.0194845
WoS: 000428630000045
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3 Citations
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The climate warming implies an increase of stress of plants (drought and torrential rainfall). The understanding of plant behavior, in this context, takes a major importance and sap flow measurement in plants remains a key issue for plant understanding. Magnetic Resonance Imaging (MRI) which is well known to be a powerful tool to access water quantity can be used to measure moving water. We describe a novel flow-MRI method which takes advantage of inflow slice sensitivity. The method involves the slice selectivity in the context of multi slice spin echo sequence. Two sequences such as a given slice is consecutively inflow and outflow sensitive are performed, offering the possiblility to perform slow flow sensitive imaging in a quite straigthforward way. The method potential is demonstrated by imaging both a slow flow measurement on a test bench (as low as 10 μm.s−1) and the Poiseuille’s profile of xylemian sap flow velocity in the xylematic tissues of a tomato plant stem.

Ref HAL: hal-01740166_v1
Ref Arxiv: 1710.06666
DOI: 10.1103/PhysRevLett.120.086401
WoS: WOS:000425738500007
Ref. & Cit.: NASA ADS
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29 Citations
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We report a direct observation of temperature-induced topological phase transition between trivial and topological insulator in HgTe quantum well. By using a gated Hall bar device, we measure and represent Landau levels in fan charts at different temperatures and we follow the temperature evolution of a peculiar pair of "zero-mode" Landau levels, which split from the edge of electron-like and hole-like subbands. Their crossing at critical magnetic field Bc is a characteristic of inverted band structure in the quantum well. By measuring the temperature dependence of Bc, we directly extract the critical temperature Tc at which the bulk band-gap vanishes and the topological phase transition occurs. Above this critical temperature, the opening of a trivial gap is clearly observed.

Commentaires: 5 pages + Supplemental Materials