1,315 research outputs found
Direct observation of superconducting vortex clusters pinned by a periodic array of magnetic dots in ferromagnetic/superconducting hybrid structures
Strong pinning of superconducting flux quanta by a square array of 1
m-sized ferromagnetic dots in a magnetic-vortex state was visualized by
low-temperature magnetic force microscopy (LT-MFM). A direct correlation of the
superconducting flux lines with the positions of the dots was derived. The
force that the MFM tip exerts on the individual vortex in the depinning process
was used to estimate the spatial modulation of the pinning potential. It was
found, that the superconducting vortices which are preferably located on top of
the Py dots experience about 15 times stronger pinning forces as compared to
the pinning force in the pure Nb film. The strong pinning exceeds the repulsive
interaction between the superconducting vortices and allows the vortex clusters
to be located at each dot. Our microscopic studies are consistent with global
magnetoresistace measurements on these hybrid structures.Comment: 4 pages, 4 figure
Refining Finite-Time Lyapunov Exponent Ridges and the Challenges of Classifying Them
While more rigorous and sophisticated methods for identifying Lagrangian based coherent structures exist, the finite-time Lyapunov exponent (FTLE) field remains a straightforward and popular method for gaining some insight into transport by complex, time-dependent two-dimensional flows. In light of its enduring appeal, and in support of good practice, we begin by investigating the effects of discretization and noise on two numerical approaches for calculating the FTLE field. A practical method to extract and refine FTLE ridges in two-dimensional flows, which builds on previous methods, is then presented. Seeking to better ascertain the role of a FTLE ridge in flow transport, we adapt an existing classification scheme and provide a thorough treatment of the challenges of classifying the types of deformation represented by a FTLE ridge. As a practical demonstration, the methods are applied to an ocean surface velocity field data set generated by a numerical model. (C) 2015 AIP Publishing LLC.ONR N000141210665Center for Nonlinear Dynamic
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
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
BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions
Josephson junctions with iron pnictides open the way for fundamental
experiments on superconductivity in these materials and their application in
superconducting devices. Here, we present hybrid Josephson junctions with a
BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter
electrode. The junctions show RSJ-like current-voltage characteristics up to
the critical temperature of the counter electrode of about 7.2K. The
temperature dependence of the critical current, IC, does not show an
Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at
microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200
{\mu}A at 4.2K we get an effective I_C R_N product of 6 {\mu}V.Comment: submitted to Appl. Phys. Let
Analytical and numerical analyses of the micromechanics of soft fibrous connective tissues
State of the art research and treatment of biological tissues require
accurate and efficient methods for describing their mechanical properties.
Indeed, micromechanics motivated approaches provide a systematic method for
elevating relevant data from the microscopic level to the macroscopic one. In
this work the mechanical responses of hyperelastic tissues with one and two
families of collagen fibers are analyzed by application of a new variational
estimate accounting for their histology and the behaviors of their
constituents. The resulting, close form expressions, are used to determine the
overall response of the wall of a healthy human coronary artery. To demonstrate
the accuracy of the proposed method these predictions are compared with
corresponding 3-D finite element simulations of a periodic unit cell of the
tissue with two families of fibers. Throughout, the analytical predictions for
the highly nonlinear and anisotropic tissue are in agreement with the numerical
simulations
Influence of the spreading resistance on the conductance spectrum of planar hybrid thin film SNS' junctions based on iron pnictides
To investigate the superconducting properties of iron pnictides we prepared
planar hybrid SNS' junctions in thin film technology with a pnictide base
electrode, a gold barrier layer and a lead counter electrode. Our design allows
characterization of the electrodes and the junction independently in a 4-probe
method. We show how both electrodes influence the measured spectra due to their
spreading resistance. While the Pb electrode has a constant resistance above
its , the contribution of the pnictide electrode is clearly
current-dependent and thus it needs a more advanced method to be corrected. We
present an empirical method, which is simple to apply and allows to deal with
the spreading resistance in our junctions to recalculate the actual conductance
and voltage of one junction at given temperature
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