Vortex structure in FeTeSe single crystals

Vortex structure in FeTe$_{0.66}$Se$_{0.44}$ and FeTe$_{0.6}$Se$_{0.4}$ single crystals with $T_c \sim 11.7$ К 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 $\sim 10^9$ cm$^{-2}$ has been observed. The problem of pinning in iron-containing superconductor single crystals is discussed.