Vortex structure in FeTeSe single crystals

Vortex structure in FeTe0.66Se0.44 and FeTe0.6Se0.4 single crystals with Tc ∼ 11.7 and 14.5 K, respectively, has been studied using the decoration technique. It has been found that in single crystals with the simplest crystalline structure of 11-family iron-containing superconductors (without interlayers), no regular vortex lattice is observed, similar to the case of the previously studied 122 and 1111 families. Using transmission electron microscopy, the dislocation structure with a density of ∼109 cm−2 has been observed. The problem of pinning in iron-containing superconductor single crystals is discussed.