Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements

Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in perfect agreement with previous reports. The pinning force has been independently estimated from local depinning of individual vortices by lateral forces exerted by the MFM tip and from transport measurements. A good quantitative agreement between the two techniques shows that for low fields, B << Hc2, MFM is a powerful and reliable technique to probe the local variations of the pinning landscape. We also demonstrate that the monopole model can be successfully applied even for thin films with a thickness comparable to the penetration depth.Comment: 6 pages, 6 figures, 2 table

Choice of studies at university, personality and socio-economic status. Comparison between French students in a public social sciences and humanities faculty and Brazilian law students in a private faculty

Several studies have examined the links between personality and choice of studies, particularly from a longitudinal point of view. This original research proposes to study these aspects a posteriori. The Rotter questionnaire (1966) was completed by two groups of students in psychology (N=65) and law (N= 91). The other particularity of the samples concerns the location and the differences in socio-economic backgrounds: privileged for law students in Brazil, modest for students in psychology in France. Most of the hypotheses put forward, particularly those related to the citizenship and political aspects, are verified on the basis of the binomial test