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Evidence for an Fulde-Ferrell-Larkin-Ovchinnikov State with Segmented Vortices in the BCS-BEC-Crossover Superconductor FeSe

We present resistivity and thermal-conductivity measurements of superconducting FeSe in intense magnetic fields up to 35 T applied parallel to the ab plane. At low temperatures, the upper critical field μ0Habc2 shows an anomalous upturn, while thermal conductivity exhibits a discontinuous jump at μ0H ≈ 24 T well below μ0Habc2, indicating a first-order phase transition in the superconducting state. This demonstrates the emergence of a distinct field- induced superconducting phase. Moreover, the broad resistive transition at high temperatures abruptly becomes sharp upon entering the high- field phase, indicating a dramatic change of the magnetic-flux properties. We attribute the high- field phase to the Fulde-Ferrel-Larkin- Ovchinnikov (FFLO) state, where the formation of planar nodes gives rise to a segmentation of the flux-line lattice. We point out that strongly orbital-dependent pairing as well as spin-orbit interactions, the multiband nature, and the extremely small Fermi energy are important for the formation of the FFLO state in FeSe.

resistivity ; thermal conductrivity ; FeSe ; high magnetic fields ; superconductivity ; FFLO state ; orbital-dependent pairing

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