Effects of high-energy Pb-ion irradiation on critical current and flux pinning in Fe(Se,Te) thin films

Irradiating a superconductor with ions is a powerful tool to create a controlled distribution of defects in it, with a morphology depending on the energy and the mass of the chosen particle. In this study, high-energy Pb-ion irradiation (1.15 GeV) was performed on Fe(Se,Te) thin films grown on CaF2 substrates, to introduce columnar defects and to enhance flux pinning capability and critical current density (Jc). The employed fluence was 2.9 × 1011 cm−2, corresponding to a dose equivalent field of 6 T. X-ray diffraction and scanning transmission electron microscopy analyses confirmed an increase of the defect density after irradiation, and the formation of columnar tracks. Despite a slight reduction in the critical temperature (around 1 K), the irradiated samples showed an increase of Jc up to 40%, in magnetic fields close to the dose equivalent field. Irradiation also induces a kink in the irreversibility line, which is consistent with a transition from a low-field single-vortex pinning regime provided by irradiation columnar defects to a high-field collective pinning regime. Accordingly, the analysis of the pinning force evidenced a shift in the peak position after irradiation, which can be associated to the active role of the irradiation tracks. These results demonstrate the effectiveness of irradiation in optimizing the performance of iron-based superconducting films.