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 μ0Hc2ab shows an anomalous upturn, while thermal conductivity exhibits a discontinuous jump at μ0H∗≈24 T well below μ0Hc2ab, 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.
@article{arxiv.1911.08237,
title = {Evidence for an FFLO state with segmented vortices in the BCS-BEC-crossover superconductor FeSe},
author = {S. Kasahara and Y. Sato and S. Licciardello and M. Čulo and S. Arsenijević and T. Ottenbros and T. Tominaga and J. Böker and I. Eremin and T. Shibauchi and J. Wosnitza and N. E. Hussey and Y. Matsuda},
journal= {arXiv preprint arXiv:1911.08237},
year = {2020}
}