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

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 µ0H ab c2 shows an anomalous upturn, while thermal conductivity exhibits a discontinuous jump at µ0H * ≈ 24 T well below µ0H ab c2 , 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 highfield 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.Exotic superconductivity with a nontrivial Cooperpairing state has been a longstanding issue of interest in condensed-matter physics. Among possible exotic states, a spatially nonuniform superconducting state in the presence of strong magnetic fields caused by the paramagnetism of conduction electrons has been the subject of great interest after the pioneering work by Fulde and Ferrell as well as Larkin and Ovchinnikov (FFLO) [1,2]. In the FFLO state, pair breaking due to the Pauli paramagnetic effect is reduced by forming a new pairing state (k↑, −k + q↓) with |q| ∼ gµ B H/ υ F (υ F is the Fermi velocity, g the g-factor, and µ B the Bohr magneton) between Zeeman split parts of the Fermi surface, instead of (k↑, −k↓) pairing in BCS superconductors [Figs. 1(a) and 1(b)]. The fascinating aspect of the FFLO state is that the superconducting order parameter, in its simplest form, is modulated as ∆ ∝ sin q · r, and periodic planar nodes appear perpendicular to the magnetic field near the upper critical field H c2 , leading to a segmentation of the vortices into pieces of length Λ = π/|q| [ Fig. 1(c)].Despite tremendous efforts in the search for the FFLO states in the past half century, indications of its experimental realization have been reported in only a few candidate materials, including quasi-two-dimensional (2D) organic superconductors and the heavy-fermion superconductor CeCoIn 5 [3-5]. In both systems, a thermodynamic phase transition occurs below H c2 and a high-field superconducting phase emerges at low temperatures [6][7][8][9]. In the former, each superconducting layer is very weakly coupled via the Josephson effect. The FFLO state is observed in a magnetic field H applied parallel to the layers, where the magnetic flux is concentrated in the regions between the layers forming coreless Josephson vortices. Therefore, the segmentation of the vortices by FFLO nodes, which is one of the most fascinating properties of the FFLO state, is not expected. The presence of the FFLO...

Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe

“…Considering the largest value of the superconducting gap of bulk FeSe of Δ~2 meV 44 , the ratio 2Δ/(k B T c )~5.3 is specific to the strong coupling limit. As a result, the Pauli limiting field can reach a value determined by~Δ/ ffiffiffi g p μ B of 24.4 T for g = 2 42 , which is close to the in-plane upper critical field of bulk FeSe 47 . As the inplane critical field of FeSe thin flakes is reduced to 20 T or below one can envisage that the dominant gap Δ is reduced as well, as observed for the thicker flakes (see Fig.…”

Control of Macromolecule Chains Structure in a Nanofiber

“…The compressed air was pumped into the tube via the two holes. After the air flow was pumped into the tube, it constantly collides with the tube wall in the form of turbulence, and finally formed an air vortex in the tube, like the vortex spinning [ 18 , 19 , 20 , 21 ]. Afterwards, the air vortex continues to flow downwards, and had an effect on the jet in the electrospinning process and caused the macromolecular chains in the jet to be entangled.…”

Simulating Artificial 1D Physics with Ultra‐Cold Fermionic Atoms: Three Exemplary Themes

“…[ 30,36 ] However, in spite of its significance, universally accepted experimental evidence for the FFLO pairing has still not been obtained, although a number of experiments conducted in solid‐state systems have shown results highly suggestive of the FFLO state. [ 32,34,37–43 ] Quasi‐1D ultra‐cold fermionic systems with attractive interactions offer another promising route to its experimental demonstration.…”