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Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors


Iron pnictides are layered high Tc superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosov-like to Josephson-like in the pnictide superconductor SmFeAs(O,F) with Tc~48–50 K on cooling below a temperature T*~41–42 K, despite its moderate electronic anisotropy γ~4–6. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the flux-flow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T* coincides well with the temperature where the coherence length ξc perpendicular to the layers matches half of the FeAs-layer spacing. For fields slightly out-of-plane (> 0.1°– 0.15°) the vortices are completely immobilized as well-pinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from well-pinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.

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Figure 1: Vortex dynamics in microcut SmFeAs(O,F) crystals probed by c-axis currents.
Figure 2: Transition from slow Abrikosov to fast Josephson vortices.
Figure 3: T* transition in LaFeAs(O,F) at a smaller fraction of Tc~20 K.
Figure 4: Non-monotonic field- and temperature dependence of the critical current.


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L.B. is supported by DOE-BES through award DE-SC0002613. The NHMFL is supported by NSF through NSF-DMR-0084173 and the State of Florida. This work was supported by the Swiss National Science Foundation and the National Center of Competence in Research MaNEP (Materials with Novel Electronic Properties). We thank the Electron Microscopy Center ETH Zurich (EMEZ), in particular P. Gasser, for support of the FIB work, and K. A. Moler and A. Koshelev for discussions.

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B.B. and P.J.W.M. designed the experiment and wrote the paper with G.B. and V.G. The FIB work and the low-field experiments were performed by P.J.W.M. L.B. and P.J.W.M. performed the experiments in high magnetic fields in Tallahassee. V.G. and G.B. contributed to the theoretical discussion, and added Supplementary Information SC. The crystals were grown by N.D.Z. and J.K.

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Correspondence to Philip J. W. Moll.

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Moll, P., Balicas, L., Geshkenbein, V. et al. Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors. Nature Mater 12, 134–138 (2013).

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