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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 )