FIG 2 - uploaded by Bernard Plaçais
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A sketch of the moving vortex lattice in an ideal slab perpendicular to the applied field. When driven by a low dc current, the configuration of the vortex and field lines, x (z) and B x (z), together with the supercurrent distribution J sy (z), are governed by the linearized set of equations 23II-IV, where J n Econst. Beyond a small healing length V defined in the text see Eq. 39 and Fig. 5 the vortex curvature and J s are uniform, and vortex lines coincide with field lines. The total current ii n i s is obtained on integrating J s and J n over the thickness d, and the experimental flux-flow conductivity, defined as the ratio i/(Ed) is given by Eq. 24.
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The dc and ac response of the ideal type-II superconductor in the mixed state is analyzed in the frame of a continuum electrodynamics, in which all fields are averaged on a scale exceeding the intervortex distance. The results of previous calculations are brought together and compared, while paying special attention to the role of the vortex line t...
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The aim of this work was to study the vortex dynamics in thin films at the vortex solid to vortex liquid transition. This region in the phase diagram has only been scarcely studied by sweep frequency measurements up to 20 GHz. We present a new cryogenic broadband calibration method. Its performance is shown by validating the method using metallic microstrips and by comparison with a usually used calibration technique. The interprétation of the results is performed by an ac mean field and a scaling analysis near Tg. Whereas the mean field theory is unable to account for the found results concerning all the dependences (frequency and magnetic field) of the impedance per unit length , a scaling of the complex ac conductivity up to our highest measurement frequencies is possible. This is in clear contradiction to the crossover from a scaling to a mean field behaviour found in the literature. As the critical exponents change when switching on the magnetic field, the universality class of the transition changes. Finally a new feature in the imaginary part of the ac conductivity o2 is shown. For temperatures over a certain value near Tc, o2 increases with increasing frequency. This behaviour is observed for all magnetic fields, only the crossover temperature changes according to the changes in Tc with magnetic field. This phenomenon does not have a theoretical explanation yet.
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The vortex contribution to the dc magnetic field (H) dependence of the
microwave surface impedance
Z~s(H)=Rs(H)+iXs(H) of
YBa2Cu3O7-δ thin films was
measured using suspended patterned resonators. Z~s(H) is
shown to be a direct measure of the flux density B(H) enabling a very
precise test of models of flux penetration. Three regimes of
field-dependent behavior were observed: (1) Initial flux penetration
occurs on very low-field scales Hi(4.2 K)~10 mT, (2) at
moderate fields the flux penetration into the virgin state is in
excellent agreement with calculations based upon the field-induced Bean
critical state for thin-film geometry, parametrized by a field scale
Hs(4.2 K)~Jcd~0.5 T, and (3) for very high fields
H>>Hs, the flux density is uniform and the measurements
enable direct determination of vortex parameters such as pinning force
constants αp and vortex viscosity η. However,
hysteresis loops are in disagreement with the thin-film Bean model, and
instead are governed by the low-field scale Hi, rather than
by Hs. Geometric barriers are insufficient to account for the
hysteresis loops.
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S
=15 nm, and a ferromagnetic PdNi layer of thicknesses d
F
=1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to H
c2 were applied. The microwave resistivity at fixed H/H
c2 increases with d
F
, eventually exceeding the Bardeen–Stephen flux-flow value. Vortex pinning is very weak at large d
F
.
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