975 research outputs found
Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T
We studied the tunneling density of states in YBCO films under strong
currents flowing along node directions. The currents were induced by fields of
up to 32.4T parallel to the film surface and perpendicular to the
planes. We observed a remarkable change in the tunneling conductance at high
fields where the gap-like feature shifts discontinuously from 15meV to a lower
bias of 11meV, becoming more pronounced as the field increases. The effect
takes place in increasing fields around 9T and the transition back to the
initial state occurs around 5T in decreasing fields. We argue that this
transition is driven by surface currents induced by the applied magnetic field.Comment: 4 pages, 7 figure
Metastability and uniqueness of vortex states at depinning
We present results from numerical simulations of transport of vortices in the
zero-field cooled (ZFC) and the field-cooled (FC) state of a type-II
superconductor. In the absence of an applied current , we find that the FC
state has a lower defect density than the ZFC state, and is stable against
thermal cycling. On the other hand, by cycling , surprisingly we find that
the ZFC state is the stable state. The FC state is metastable as manifested by
increasing to the depinning current , in which case the FC state
evolves into the ZFC state. We also find that all configurations acquire a
unique defect density at the depinning transition independent of the history of
the initial states.Comment: 4 pages, 4 figures. Problem of page size correcte
Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBaCuO Superconductor
We report planar tunneling measurements on thin films of
YBaCuO at various doping levels under magnetic fields. By
choosing a special setup configuration, we have probed a field induced energy
scale that dominates in the vicinity of a node of the d-wave superconducting
order parameter. We found a high doping sensitivity for this energy scale. At
Optimum doping this energy scale is in agreement with an induced
order parameter. We found that it can be followed down to low fields at optimum
doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Vortex trapping and expulsion in thin-film YBCO strips
A scanning SQUID microscope was used to image vortex trapping as a function
of the magnetic induction during cooling in thin-film YBCO strips for strip
widths W from 2 to 50 um. We found that vortices were excluded from the strips
when the induction Ba was below a critical induction Bc. We present a simple
model for the vortex exclusion process which takes into account the vortex -
antivortex pair production energy as well as the vortex Meissner and
self-energies. This model predicts that the real density n of trapped vortices
is given by n=(Ba-BK)/Phi0 with BK = 1.65Phi0/W^2 and Phi0 = h/2e the
superconducting flux quantum. This prediction is in good agreement with our
experiments on YBCO, as well as with previous experiments on thin-film strips
of niobium. We also report on the positions of the trapped vortices. We found
that at low densities the vortices were trapped in a single row near the
centers of the strips, with the relative intervortex spacing distribution width
decreasing as the vortex density increased, a sign of longitudinal ordering.
The critical induction for two rows forming in the 35 um wide strip was (2.89 +
1.91-0.93)Bc, consistent with a numerical prediction
Magnetic field of an in-plane vortex outside a layered superconductor
We present the solution to London's equations for the magnetic fields of a
vortex oriented parallel to the plane, and normal to a crystal face, of a
layered superconductor. These expressions account for flux spreading at the
superconducting surface, which can change the apparent size of the vortex along
the planes by as much as 30%. We compare these expressions with experimental
results.Comment: 13 pages, 5 figure
Vortices in magnetically coupled superconducting layered systems
Pancake vortices in stacks of thin superconducting films or layers are
considered. It is stressed that in the absence of Josephson coupling
topological restrictions upon possible configurations of vortices are removed
and various examples of structures forbidden in bulk superconductors are given.
In particular, it is shown that vortices may skip surface layers in samples of
less than a certain size R_c which might be macroscopic. The Josephson coupling
suppresses R_c estimates
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
Josephson junctions in thin and narrow rectangular superconducting strips
I consider a Josephson junction crossing the middle of a thin rectangular
superconducting strip of length L and width W subjected to a perpendicular
magnetic induction B. I calculate the spatial dependence of the gauge-invariant
phase difference across the junction and the resulting B dependence of the
critical current Ic(B).Comment: 4 pages, 6 figures, revised following referee's comment
Predicted field-dependent increase of critical currents in asymmetric superconducting nanocircuits
The critical current of a thin superconducting strip of width much larger
than the Ginzburg-Landau coherence length but much smaller than the Pearl
length is maximized when the strip is straight with
defect-free edges. When a perpendicular magnetic field is applied to a long
straight strip, the critical current initially decreases linearly with but
then decreases more slowly with when vortices or antivortices are forced
into the strip. However, in a superconducting strip containing sharp 90-degree
or 180-degree turns, the zero-field critical current at H=0 is reduced because
vortices or antivortices are preferentially nucleated at the inner corners of
the turns, where current crowding occurs. Using both analytic London-model
calculations and time-dependent Ginzburg-Landau simulations, we predict that in
such asymmetric strips the resulting critical current can be {\it increased} by
applying a perpendicular magnetic field that induces a current-density
contribution opposing the applied current density at the inner corners. This
effect should apply to all turns that bend in the same direction.Comment: Introduction rewritten to include additional references, 17 pages, 14
figure
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