Evidence for an FFLO state with segmented vortices in the BCS-BEC-crossover superconductor FeSe

TL;DR

This study provides evidence for an FFLO state in FeSe, characterized by a distinct high-field phase with segmented vortices, revealed through resistivity and thermal conductivity measurements under high magnetic fields.

Contribution

It demonstrates the emergence of an FFLO state with segmented vortices in FeSe, highlighting the roles of orbital-dependent pairing, spin-orbit interactions, and multiband effects.

Findings

Discontinuous jump in thermal conductivity at 24 T indicating a phase transition.

Anomalous upturn of the upper critical field at low temperatures.

Evidence of a field-induced phase with segmented flux-line lattice.

Abstract

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 $\mu_0 H_{c2}^{ab}$ shows an anomalous upturn, while thermal conductivity exhibits a discontinuous jump at $\mu_0 H^{\ast}\approx 24$ T well below $\mu_0 H_{c2}^{ab}$, 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.