333 research outputs found
Single vortices observed as they enter NbSe
We observe single vortices as they penetrate the edge of a superconductor
using a high-sensitivity magneto-optical microscope. The vortices leap across a
gap near the edge, a distance that decreases with increasing applied field and
sample thickness. This behaviour can be explained by the combined effect of the
geometrical barrier and bulk pinning.Comment: 2 pages, 1 figure, M2S-Rio proceeding
Experimental study of the correlation length of critical-current fluctuations in the presence of surface disorder: Probing vortex long-range interactions
We report on critical currents and voltage noise measurements in Niobium
strips in the superconducting state, in the presence of a bulk vortex lattice
() and in the surface superconducting state ().
For homogeneous surfaces, the correlation length of the current fluctuations
can be associated with the electromagnetic skin depth of vortex superficial
instabilities. The modification of the surface state by means of low energy
irradiation induces a strong modification of the critical current and of the
noise. The appearance of a corner frequency in the spectral domain can be
linked with the low wave-vectors of the artificial corrugation. Since this
latter occurs only for , we propose that the long-range
interactions allow the correlation length to extend up to values imposed by the
surface topography.Comment: accepted for publication in PR
Edge and Bulk Transport in the Mixed State of a Type-II Superconductor
By comparing the voltage-current (V-I) curves obtained before and after
cutting a sample of 2H-NbSe2, we separate the bulk and edge contributions to
the transport current at various dissipation levels and derive their respective
V- I curves and critical currents. We find that the edge contribution is
thermally activated across a current dependent surface barrier. By contrast the
bulk V-I curves are linear, as expected from the free flux flow model. The
relative importance of bulk and edge contributions is found to depend on
dissipation level and sample dimensions. We further show that the peak effect
is a sharp bulk phenomenon and that it is broadened by the edge contribution
Vortex avalanches and self organized criticality in superconducting niobium
In 1993 Tang proposed [1] that vortex avalanches should produce a self
organized critical state in superconductors, but conclusive evidence for this
has heretofore been lacking. In the present paper, we report extensive
micro-Hall probe data from the vortex dynamics in superconducting niobium,
where a broad distribution of avalanche sizes scaling as a power-law for more
than two decades is found. The measurements are combined with magneto-optical
imaging, and show that over a widely varying magnetic landscape the scaling
behaviour does not change, hence establishing that the dynamics of
superconducting vortices is a SOC phenomenon.Comment: 3 pages + 4 figures, a reference added, citation typos fixe
Evolution of the fishtail-effect in pure and Ag-doped MG-YBCO
We report on magnetic measurements carried out in a textured
YBaCuO and YBa(CuAg)O (at
0.02) crystals. The so-called fishtail-effect (FE) or second
magnetization peak has been observed in a wide temperature range
0.4~~0.8 for . The origin of the FE arises for
the competition between surface barrier and bulk pinning. This is confirmed in
a non-monotonically behavior of the relaxation rate . The value
for Ag-doped crystals is larger than for the pure one due to the presence of
additional pinning centers, above all on silver atoms.Comment: 6 pages, 6 figure
Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet
We report Hall sensor measurements of the magnetic relaxation of Mn
acetate as a function of magnetic field applied along the easy axis of
magnetization. Data taken at a series of closely-spaced temperatures between
0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order''
transition between thermally-assisted relaxation and magnetic decay via quantum
tunneling.Comment: 4 pages, including 7 figure
Dynamic transition in driven vortices across the peak effect in superconductors
We study the zero-temperature dynamic transition from the disordered flow to
an ordered flow state in driven vortices in type-II superconductors. The
transition current is marked by a sharp kink in the
characteristic with a concomitant large increase in the defect concentration.
On increasing magnetic field , the follows the behaviour of the
critical current . Specifically, in the peak effect regime
increases rapidly along with . We also discuss the effect of varying
disorder strength on .Comment: 4 pages, 4 figure
Flow Induced Organization and Memory of a Vortex Lattice
We report on experiments probing the evolution of a vortex state in response
to a driving current in 2H-NbSe crystals. By following the vortex motion
with fast transport measurements we find that the current enables the system to
reorganize and access new configurations. During this process the system
exhibits a long-term memory: if the current is turned off the vortices freeze
in place remembering their prior motion. When the current is restored the
motion resumes where it stopped. The experiments provide evidence for a
dynamically driven structural change of the vortex lattice and a corresponding
dynamic phase diagram that contains a previously unknown regime where the
critical current can be either or by applying an
appropriate driving current.Comment: 5 pages, 4figure
Peak effect, vortex-lattice melting-line and order - disorder transition in conventional and high-T superconductors
We investigate the order - disorder transition line from a Bragg glass to an
amorphous vortex glass in the H-T phase diagram of three-dimensional type-II
superconductors with account of both pinning-caused and thermal fluctuations of
the vortex lattice. Our approach is based on the Lindemann criterion and on
results of the collective pinning theory and generalizes previous work of other
authors. It is shown that the shapes of the order - disorder transition line
and the vortex lattice melting curve are determined only by the Ginzburg
number, which characterizes thermal fluctuations, and by a parameter which
describes the strength of the quenched disorder in the flux-line lattice. In
the framework of this unified approach we obtain the H-T phase diagrams for
both conventional and high-Tc superconductors. Several well-known experimental
results concerning the fishtail effect and the phase diagram of high-Tc
superconductors are naturally explained by assuming that a peak effect in the
critical current density versus H signalizes the order - disorder transition
line in superconductors with point defects.Comment: 15 pages including 11 figure
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