Vortex dynamics in superconductors in the presence of anisotropic pinning
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Abstract
Vortex
dynamics in two different classes of superconductors with anisotropic
unidirected pinning sites was experimentally investigated by magnetoresistivity
measurements: YBCO−films with unidirected twins and Nb-films deposited on faceted $\mathrm Al_2O_3$ substrate surfaces. For the interpretation
of the experimental results a theoretical model based on the Fokker-Planck
equation was used.
It was proved by X-ray measurements that YBCO films prepared
on (001) $\mathrm NdGaO_3$ substrates exhibit only one twin orientation in contrast
to YBCO films grown on (100) $\mathrm SrTiO_$3 substrates. The magnetoresistivity
measurements of the YBCO films with unidirected
twin boundaries revealed the existence of two new magnetoresistivity components,
which is a characteristic feature of a guided vortex motion: an odd
longitudinal component with respect to the magnetic field sign reversal and
an even transversal component. However, due to the small coherence length
in YBCO and the
higher density of point-like defects comparing to
high-quality YBCO single crystals, the strength of the isotropic point
pinning was comparable with the strength of the pinning produced by twins.
This smeared out all effects caused by the pinning anisotropy. The behaviour
of the odd longitudinal component was found to be independent of the
transport current direction with respect to the twin planes.
The magnetoresistivity measurements of faceted Nb films demonstrated
the appearance of an odd longitudinal and even transversal component of
the magnetoresistivity. The temperature and magnetic field dependences of
all relevant magnetoresistivity components were measured. The angles between
the average vortex velocity vector and the transport current direction
calculated from the experimental data for the different transport current orientations
with respect to the facet ridges showed that the vortices moved
indeed along the facet ridges. An anomalous Hall effect, i.e. a
sign change of
the odd transversal magnetoresistivity, has been found in the temperature
and magnetic field dependences of the Hall resisitivity of the samples.
The theory developed by V.~A.~Shklovskij was used for the explanation
of the experimental data. It shows very good agreement with the experiment.
The temperature dependence of the even longitudinal magnetoresistivity
component of the samples could be very well fitted within the theoretical
approach, using for the isotropic and anisotropic pinning potential
simple potential with a symmetric triangular potential wells whose depths
were estimated from the experimental data.