Highly effective and isotropic pinning in epitaxial Fe(Se,Te) thin films grown on CaF₂ substrates

We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF₂(001) substrate. High critical current density values - larger than 1 MA/cm² in self field and liquid helium - are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through transmission electron microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what was observed on SrTiO₃, where defects parallel to the c-axis enhance pinning in that direction.