236 research outputs found
Tunable Field Induced Superconductivity
We investigate the transport properties of a thin superconducting Al layer
covering a square array of magnetic dots with out-of-plane magnetization. A
thorough characterization of the magnetic properties of the dots allowed us to
fine-tune their magnetic state at will, hereby changing the influence of the
dots on the superconductor in a continuous way. We show that even though the
number of vortex-antivortex pairs discretely increases with increasing the
magnetization of the dots, no corresponding discontinuity is observed in the
resistance of the sample. The evolution of the superconducting phase boundary
as the magnetic state of the dots is swept permits one to devise a fully
controllable and erasable field induced superconductor
Model error estimation in ensemble data assimilation
A new methodology is proposed to estimate and account for systematic model error in linear filtering as well as in nonlinear ensemble based filtering. Our results extend the work of Dee and Todling (2000) on constant bias errors to time-varying model errors. In contrast to existing methodologies, the new filter can also deal with the case where no dynamical model for the systematic error is available. In the latter case, the applicability is limited by a matrix rank condition which has to be satisfied in order for the filter to exist. <br><br> The performance of the filter developed in this paper is limited by the availability and the accuracy of observations and by the variance of the stochastic model error component. The effect of these aspects on the estimation accuracy is investigated in several numerical experiments using the Lorenz (1996) model. Experimental results indicate that the availability of a dynamical model for the systematic error significantly reduces the variance of the model error estimates, but has only minor effect on the estimates of the system state. The filter is able to estimate additive model error of any type, provided that the rank condition is satisfied and that the stochastic errors and measurement errors are significantly smaller than the systematic errors. The results of this study are encouraging. However, it remains to be seen how the filter performs in more realistic applications
Tunable pinning in superconducting films with magnetic micro-loops
We study the flux pinning properties of superconductor/magnetic micro-rings
lattice hybrid structures. The used open triangular micromagnets represent an
eight-fold degree of freedom system, with six polarized and two flux-closure
possible states. By conveniently choosing the magnetic state of the underlying
rings it is possible to induce different pinning potentials. We show that the
magnetic vortex state with minimum stray field produces a weaker pinning in
comparison with the polarized states
On the origin of the reversed vortex ratchet motion
We experimentally demonstrate that the origin of multiply reversed rectified
vortex motion in an asymmetric pinning landscape is a consequence not only of
the vortex-vortex interactions but also essentially depends on the ratio
between the characteristic interaction distance and the period of the
asymmetric pinning potential. Our system consists of an Al film deposited on
top of a square array of size-graded magnetic dots with a constant lattice
period a=2\mu m. Four samples with different periods of the size gradient d
were investigated. For large d the dc voltage Vdc recorded under a sinusoidal
ac excitation indicates that the average vortex drift is from bigger to smaller
dots for all explored positive fields. As d is reduced a series of sign
reversals in the dc response are observed as a function of field. We show that
the number of sign reversals increases as d decreases. These findings are in
agreement with recent computer simulations and illustrate the relevance of the
different characteristic lengths for the vortex rectification effects.Comment: accepted in Phys. Rev. Let
Magnetic dipole induced guided vortex motion
We present evidence of magnetically controlled guided vortex motion in a
hybrid superconductor/ferromagnet nanosystem consisting of an Al film on top of
a square array of permalloy square rings. When the rings are magnetized with an
in-plane external field H, an array of point-like dipoles with moments
antiparallel to H, is formed. The resulting magnetic template generates a
strongly anisotropic pinning potential landscape for vortices in the
superconducting layer. Transport measurements show that this anisotropy is able
to confine the flux motion along the high symmetry axes of the square lattice
of dipoles. This guided vortex motion can be either re-routed by 90 degrees by
simply changing the dipole orientation or even strongly suppressed by inducing
a flux-closure magnetic state with very low stray fields in the rings.Comment: 5 pages, 3 figure
Guided nucleation of superconductivity on a graded magnetic substrate
We demonstrate the controlled spatial nucleation of superconductivity in a
thin film deposited on periodic arrays of ferromagnetic dots with gradually
increasing diameter. The perpendicular magnetization of the dots induces
vortex-antivortex molecules in the sample, with the number of (anti)vortices
increasing with magnet size. The resulting gradient of antivortex density
between the dots predetermines local nucleation of superconductivity in the
sample as a function of the applied external field and temperature. In
addition, the compensation between the applied magnetic field and the
antivortices results in an unprecedented enhancement of the critical
temperature
Enhanced pinning and proliferation of matching effects in a superconducting film with a Penrose array of magnetic dots
The vortex dynamics in superconducting films deposited on top of a five-fold
Penrose array of magnetic dots is studied by means of transport measurements.
We show that in the low pinning regime (demagnetized dots) a few periodic and
aperiodic matching features coexist. In the strong pinning regime (magnetized
dots) a richer structure of unforeseen periodic and aperiodic vortex patterns
appear giving rise to a clear enhancement of the critical current in a broader
field range. Possible stable vortex configurations are determined by molecular
dynamics simulations
Magnetic confinement of the superconducting condensate in superconductor/ferromagnet hybrid composites
The influence of an inhomogeneous magnetic field on the magnetoresistance of
thin Al films, used in different superconductor/ferromagnet hybrids, has been
investigated. Two contrasting magnetic textures with out-of-plane magnetization
are explored, namely (i) a plain film in a multidomain state and (ii) an array
of micro-sized dots. The stray fields of the ferromagnetic structures confine
the superconducting condensate and, accordingly, modify the condition for the
nucleation of superconductivity. By switching between different magnetic states
of the ferromagnet, this confinement can be tuned at will, hereby reversibly
changing the dependence of the critical temperature Tc on an external magnetic
field H. In particular, the continuous evolution from a conventional linear
Tc(H) dependence with a single maximum to a reentrant superconducting phase
boundary with multiple Tc peaks has been demonstrated
Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets
We investigate the transport properties of superconducting films with
periodic arrays of in-plane magnetized micromagnets. Two different magnetic
textures are studied: a square array of magnetic bars and a close-packed array
of triangular microrings. As confirmed by MFM imaging, the magnetic state of
both systems can be adjusted to produce arrays of almost point-like magnetic
dipoles. By carrying out transport measurements with ac drive, we observed
experimentally a recently predicted ratchet effect induced by the interaction
between superconducting vortices and the magnetic dipoles. Moreover, we find
that these magnetic textures produce vortex-antivortex patterns, which have a
crucial role on the transport properties of this hybrid system.Comment: 4 pages, 4 figure
Localization of superconductivity in superconductor-electromagnet hybrids
We investigate the nucleation of superconductivity in a superconducting Al
strip under the influence of the magnetic field generated by a current-carrying
Nb wire, perpendicularly oriented and located underneath the strip. The
inhomogeneous magnetic field, induced by the Nb wire, produces a spatial
modulation of the critical temperature T_c, leading to a controllable
localization of the superconducting order parameter (OP) wave function. We
demonstrate that close to the phase boundary T_c(B_ext) the localized OP
solution can be displaced reversibly by either applying an external
perpendicular magnetic field B_ext or by changing the amplitude of the
inhomogeneous field.Comment: 10 pages, 6 figure
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