Deposition of La2Zr2O7 Film by Chemical Solution Deposition

La2Zr2O7 (LZO) formation of bulk powders and of films by Chemical Solution Deposition (CSD) process have been studied using propionates. The treatment involved a one step cycle in the reducing forming gas (Ar-5%H2) to be compatible with Ni-5at%W RABITS. Large amount of residual carbon was found in LZO powders formed in these conditions (10 wt %). The volume fraction of the cube texture in LZO films on Ni-5at%w RABITS was found to be a function of the speed of the gas flown above sample. This phenomenon is discussed in considering the C deposited from the carbon-containing gases emitted during the pyrolysis of the precursor. Using proper conditions (950 ^\circC and the speed of gas of 6.8\times10^{-2} m/s), LZO films with good surface crystallinity could be obtained on Ni-5at%W RABITS as demonstrated by X-ray diffraction, electron backscattered diffraction and RHEED. The existence of residual carbon in oxide films is a common question to films deposited by CSD processes under reducing condition

Nonlinear magnetic induction by helical motion in a liquid sodium turbulent flow

We report an experimental study of the magnetic field ~BB induced by a turbulent swirling flow of liquid sodium submitted to a transverse magnetic field ~BB0. We show that the induced field can behave nonlinearly as a function of the magnetic Reynolds number, Rm. At low Rm, the induced mean field along the axis of the flow, hBxi, and the one parallel to ~BB0, hByi, first behave like R2 m, whereas the third component, hBzi, is linear in Rm. The sign of hBxi is determined by the flow helicity. At higher Rm, ~BB strongly depends on the local geometry of the mean flow: hBxi decreases to zero in the core of the swirling flow but remains finite outside. We compare the experimental results with the computed magnetic induction due to the mean flow alone

Open questions about homogeneous fluid dynamos: the VKS experiments

We consider several problems that arise in the context of homogeneous fluid dynamos such as the e ect of turbulence on the dynamo threshold, the saturation level of the generated magnetic eld above the threshold and its dynamics. We compare some of our predictions with the recent experimental results of the Karlsruhe and Riga experiments. Finally, we present the VKS experiment that we have designed to answer some of the remaining open questions. We study, in particular, the response of a turbulent flow to an external magnetic eld

Risk factors, aetiology and outcome of ischaemic stroke in young adults: the Swiss Young Stroke Study (SYSS).

Ischaemic stroke (IS) in young adults has been increasingly recognized as a serious health condition. Stroke aetiology is different in young adults than in the older population. This study aimed to investigate aetiology and risk factors, and to search for predictors of outcome and recurrence in young IS patients. We conducted a prospective multicentre study of consecutive IS patients aged 16-55 years. Baseline demographic data, risk factors, stroke aetiology including systematic genetic screening for Fabry disease and severity were assessed and related to functional neurological outcome (modified Rankin Scale, mRS), case fatality, employment status, place of residence, and recurrent cerebrovascular events at 3 months. In 624 IS patients (60 % men), median age was 46 (IQR 39-51) years and median NIHSS on admission 3 (IQR 1-8). Modifiable vascular risk factors were found in 73 %. Stroke aetiology was mostly cardioembolism (32 %) and of other defined origin (24 %), including cervicocerebral artery dissection (17 %). Fabry disease was diagnosed in 2 patients (0.3 %). Aetiology remained unknown in 20 %. Outcome at 3 months was favourable (mRS 0-1) in 61 % and fatal in 2.9 %. Stroke severity (p < 0.001) and diabetes mellitus (p = 0.023) predicted unfavourable outcome. Stroke recurrence rate at 3 months was 2.7 %. Previous stroke or TIA predicted recurrent cerebrovascular events (p = 0.012). In conclusion, most young adults with IS had modifiable vascular risk factors, emphasizing the importance of prevention strategies. Outcome was unfavourable in more than a third of patients and was associated with initial stroke severity and diabetes mellitus. Previous cerebrovascular events predicted recurrent ones

MHD measurements in the von Kármán sodium experiment

We study the magnetic induction in a confined swirling flow of liquid sodium, at integral magnetic Reynolds numbers up to 50. More precisely, we measure in situ the magnetic field induced by the flow motion in the presence of a weak external field. Because of the very small value of the magnetic Prandtl number of all liquid metals, flows with even modest Rm are strongly turbulent. Large mean induction effects are observed over a fluctuating background. As expected from the von Kármán flow geometry, the induction is strongly anisotropic. The main contributions are the generation of an azimuthal induced field when the applied field is in the axial direction ~an V effect! and the generation of axial induced field when the applied field is the transverse direction ~as in a large scale a effect!. Strong fluctuations of the induced field, due to the flow nonstationarity, occur over time scales slower than the flow forcing frequency. In the spectral domain, they display a f21 spectral slope. At smaller scales ~and larger frequencies! the turbulent fluctuations are in agreement with a Kolmogorov modeling of passive vector dynamics

Magneto-optical investigations for LZO buffer layer thickness optimization in the simple low cost architecture NiW/LZO/YBCO

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Perturbed Angular Correlations Studies in the HgBa2CaCu2O6+delta high-T-C Superconductor

International audienceThe electric field gradients at Hg-199m nuclei have been measured via the perturbed angular correlation (PAC) technique, allowing a full characterization of the Hg neighborhood charge distribution at high oxygen doping on the Hg planes. The PAC technique has been applied to investigate the effect of high oxygen pressure during the measurement. Polycrystalline HgBa2CaCu2O6+delta (Hg-1212) samples have been annealed at 152 bar pressurized oxygen. The influence of oxygen pressure during the experiment was then investigated by measuring the samples at atmospheric pressure and under 152 bar oxygen pressure. The present set of PAC experiments shows that at high oxygen concentrations there is a non-uniform oxygen distribution. Moreover, the Hg environment is not free from oxygen and the results hint to a new type of ordering

Epitaxial Growth of Sputtered Ultra-thin NbN Layers and Junctions on Sapphire

International audienceHigh crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher Tc, higher Jc and lower resistivity have been obtained by growing untwined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 cm at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of untwined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600 to 300 during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb2Ny05-x ( 2.2 nm) at the unprotected NbN surface and of interfacial NbO ( 0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices

Oxygen ordering in the high-T-c superconductor HgBa2CaCu2O6+delta as revealed by perturbed angular correlation

International audienceLattice sites and collective ordering of oxygen atoms in HgBa2CaCu2O6+delta were studied using the perturbed angular correlation (PAC) technique at ISOLDE/CERN. The electric field gradients (EFG) at Hg-199m nuclei have been measured as functions of oxygen doping on the Hg planes, above and below T-c. In comparison with the results obtained for oxygen and fluorine doping in Hg-1201, the analysis shows a different oxygen ordering exhibited by Hg-1212. Moreover, for all studied cases, the experimental results show that at a local scale there is non uniform oxygen distribution. A series of ab initio EFG calculations allowed to infer that at low concentrations, regions without oxygen coexist with regions where O-2 delta dumbbell molecules are located at the center of the Hg mesh. On the other side, at high concentrations, O-2 delta dumbbell molecules coexist with single O-delta atoms occupying the center of the Hg mesh. The present results suggest that oxygen sits on the Hg planes in the form of a molecule and not as a single atom

Nonlinear magnetic induction by helical motion in a liquid sodium turbulent flow

We report an experimental study of the magnetic field ~BB induced by a turbulent swirling flow of liquid sodium submitted to a transverse magnetic field ~BB0. We show that the induced field can behave nonlinearly as a function of the magnetic Reynolds number, Rm. At low Rm, the induced mean field along the axis of the flow, hBxi, and the one parallel to ~BB0, hByi, first behave like R2 m, whereas the third component, hBzi, is linear in Rm. The sign of hBxi is determined by the flow helicity. At higher Rm, ~BB strongly depends on the local geometry of the mean flow: hBxi decreases to zero in the core of the swirling flow but remains finite outside. We compare the experimental results with the computed magnetic induction due to the mean flow alone