437 research outputs found
Columnar defects acting as passive internal field detectors
We have studied the angular dependence of the irreversible magnetization of
several YBaCuO and 2H-NbSe single crystals with columnar
defects tilted off the c-axis. At high magnetic fields, the irreversible
magnetization exhibits a well known maximum when the applied
field is parallel to the tracks. As the field is decreased below , the peak shifts away from the tracks' direction toward either the
c-axis or the ab-planes. We demonstrate that this shift results from the
misalignment between the external and internal field directions due to the
competition between anisotropy and geometry effects.Comment: 5 figure
Angular dependent vortex dynamics in superconductors with columnar defects
We explore in detail the angular dependent vortex dynamics in type II
superconductors with aligned columnar defects introduced by irradiation with
very energetic heavy-ions. We use dc magnetization measurements deep in the
vortex solid phase, and ac susceptibility near the solid-liquid transition. We
show that aligned columnar defects are an excellent tool to test models for
vortex dynamics, particularly if they are tilted with respect to the
crystallographic axes, so their effects can be easily distinguished from those
arising from mass anisotropy, sample geometry, twin boundaries and intrinsic
pinning. This allows us, for instance, to use the uniaxial pinning of the
columnar defects as a probe to determine the orientation of the vortices inside
a bulk material, which in general is different from the orientation of the
applied fields. In some aspects we have found an excellent agreement with the
theoretical expectations of the Bose-glass model. The field dependence of the
lock-in angle follows remarkably well the 1/H prediction over the whole
temperature range of our measurements. In turn, the temperature dependence of
the lock-in angle gives strong support to the concept of an effective pinning
energy dominated by the entropic smearing effect. On the other hand, both our
ac and dc results show that columnar defects produce effective pinning over a
wide angular range, and that correlated pinning dominates the scenario for all
field orientations. One consequence of this is the existence of a rich variety
of vortex staircases.Comment: to be published in International Book series "Studies of High
Temperature Superconductors", edited by Anant Narlikar, Nova Science
Publishers, New York, Vol 49/50, (2003
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
An examination of certification and professional development of superintendents and principals across the fifty states
This study was designed to examine how superintendents and principals across the nation are certified, to determine how states are structured to handle certification and professional development of these administrators, to report the services and provisions utilized within the states for professional development of administrators and to determine the state funding levels for their professional development. The seven questions providing focus for this study addressed: (1) the requirement for certification, (2) levels of certification, temporary certification, renewal of certification and alternative certification, (3) reciprocity of certification, (4) state structures for handling certification and professional development, (5) services and provisions for professional development, (6) levels of funding for professional development and (7) factors that might influence practices across the fifty states related to certification and professional development of superintendents and principals;Information about administrator certification and professional development was obtained from two main sources: a survey instrument and documents obtained from the states. Responses were received from all fifty states, resulting in a 100% return rate. Descriptive statistical reports were generated from the data;The significant findings of the study were categorized in three areas: (1) certification requirements for superintendents, (2) certification requirements for principals and (3) factors associated with certification and professional development practices;(1) For superintendents: Forty-four states require certification for practice, twenty-five require levels of certification, fourteen provide temporary certification, thirty-eight require renewal of certification and twelve have written interstate certification agreements. (2) For principals: All states require certification for practice, twenty-seven require levels of certification, sixteen provide temporary certification, forty-one require renewal of certification and fourteen have written interstate certification agreements. (3) Factors found to be associated with certification and professional development practices were geographic region, state graduation rate and the number of state-level administrator improvement initiatives
Flux pinning properties of superconductors with an array of blind holes
We performed ac-susceptibility measurements to explore the vortex dynamics
and the flux pinning properties of superconducting Pb films with an array of
micro-holes (antidots) and non-fully perforated holes (blind holes). A lower
ac-shielding together with a smaller extension of the linear regime for the
lattice of blind holes indicates that these centers provide a weaker pinning
potential than antidots. Moreover, we found that the maximum number of flux
quanta trapped by a pinning site, i.e. the saturation number ns, is lower for
the blind hole array.Comment: 6 figures, 6 page
Dynamic Regimes in Films with a Periodic Array of Antidots
We have studied the dynamic response of Pb thin films with a square array of
antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac
drive amplitudes h, vortices moving inside the pinning potential give rise to a
frequency- and h-independent response together with a scarce dissipation. For
higher amplitudes, the average distance travelled by vortices surpasses the
pinning range and a critical state develops. We found that the boundary h*(H,T)
between these regimes smoothly decreases as T increases whereas a step-like
behavior is observed as a function of field. We demonstrate that these steps in
h*(H) arise from sharp changes in the pinning strength corresponding to
different vortex configurations. For a wide set of data at several fields and
temperatures in the critical state regime, we show that the scaling laws based
on the simple Bean model are satisfied.Comment: 7 pages, 5 figure
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
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