Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)

Maximizing the sustainable supercurrent density, Jc, is crucial to high current applications of superconductivity and, to achieve this, preventing dissipative motion of quantized vortices is key. Irradiation of superconductors with high-energy heavy ions can be used to create nanoscale defects that act as deep pinning potentials for vortices. This approach holds unique promise for high current applications of iron-based superconductors because Jc amplification persists to much higher radiation doses than in cuprate superconductors without significantly altering the superconducting critical temperature. However, for these compounds virtually nothing is known about the atomic scale interplay of the crystal damage from the high-energy ions, the superconducting order parameter, and the vortex pinning processes. Here, we visualize the atomic-scale effects of irradiating FeSexTe1-x with 249 MeV Au ions and find two distinct effects: compact nanometer-sized regions of crystal disruption or 'columnar defects', plus a higher density of single atomic-site 'point' defects probably from secondary scattering. We show directly that the superconducting order is virtually annihilated within the former while suppressed by the latter. Simultaneous atomically-resolved images of the columnar crystal defects, the superconductivity, and the vortex configurations, then reveal how a mixed pinning landscape is created, with the strongest pinning occurring at metallic-core columnar defects and secondary pinning at clusters of pointlike defects, followed by collective pinning at higher fields.