Enhancement of the lower critical field in FeSe-coated Nb structures for superconducting radio-frequency applications

Bulk Nb superconducting radio-frequency (SRF) cavities are widely used in accelerators, and their accelerating gradient and general performance are limited by the superheating field (B-sh). To push the theoretical limit of the B-sh, new multilayer structures are required. We fabricated FeSe-coated Nb films using pulsed laser deposition, performed structural characterizations, and measured the transport and magnetic properties for this superconductor-superconductor bilayer structure with smooth surface. Additionally, the measured B-c1 of FeSe-coated Nb film is greatly enhanced, while the B-sh of the FeSe layer is expected to be higher than that of bulk Nb, yet the superconducting transition temperature (T-c) is less than 5 K. This work presents the first fabrication of a new coating layer: FeSe deposited on Nb, showing the possibility of using iron-based materials for multilayer structures in SRF cavities.