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Slow & Fast Vortices in Iron Pnictide Superconductors
Using the attocube ANR31 rotator, a precise nano-rotator setup was designed to fit on a small( 25 mm diameter) standard sample carrier. We have investigated the vortex matter of the iron-pnictide high temperature superconductors [ 1 ]. We studied the mobility of magnetic vortices in the layered superconductor SmFeAs( O, F) and could show an enormous enhancement of vortex mobility associated with a transition of the vortex nature itself, changing from Abrikosov to Josephson-type. A perfectly in-plane Josephson vortex, centered in a“ non-superconducting” Sm( O, F) layer, can only be weakly pinned and thus experiences the mentioned enhancement in mobility.
This feature, however, is immediately lost if the field is tilted out of the FeAs planes and even the smallest misalignment(< 0.1 °) completely destroys the effect as the misaligned vortex is not parallel to the crystallographic layers anymore. As mobile vortices cause dissipation, their mobility is observed as a very sharp spike in voltage as shown in Fig. 1( see also [ 1 ]). Therefore angular precision and stability is the key to observing this effect. The discovered Abrikosov to Josephson transition was unexpected, as the materials’ electronic anisotropy is low. Moreover, Josephson vortices are believed to be a feature of highly anisotropic superconductors. This finding challenges our“ global” understanding of superconducting anisotropies and their relevance for the microscopic, intra-unit cell modulation of the order parameter.
[ 1 ] P. J. W. Moll, L. Balicas, V. Geshkenbein, G. Blatter, J. Karpinski, N. D. Zhigadlo, and B. Batlogg, Nature Materials 12, 134( 2013)( Data and Images courtesy of Philip Moll, et al. Laboratory of Solid State Physics, ETH Zurich, Switzerland)