Superconducting weak links at substrate steps in bilayer nickelate films

Ambient-pressure superconductivity above 40 K has been realized in bilayer nickelate thin films on SrLaAlO 4 (001), yet resistive measurements often exhibit incomplete zero-resistance states and/or two-step superconducting transitions of unresolved origin. We show that these features arise from superconducting weak links at substrate step-terrace structures. A direct correlation between the azimuthal-angle-dependent two-step transition and step-terrace orientation establishes this connection, further supported by atomic-scale visualization of structurally disrupted regions at step terraces. Modeling the films as pristine (weak-link) regions with higher (lower) transition temperature reproduces essential features of the two-step transition using both a simple model and finite-element simulations. Transport measurements further reveal a strong dependence of transition characteristics - and even the occurrence of superconductivity itself - on average terrace width. These results identify step-induced weak links as the microscopic origin of two-step transitions in bilayer nickelate films, providing a framework for understanding anisotropic and percolative transport, and a route to superconducting devices.