In-plane Critical Field far beyond the Paramagnetic Limit for Thin Al Films on Topological Insulator Surface*

Heterostructures of an s-wave superconductor and a strong topological insulator are regarded as a viable platform for the creation of Majorana zero mode (MZM). However, the microscopic understanding of the proximity effect between the superconductor and topological surface state has yet to be fully elucidated. Here we report measurements of the in-plane critical field (H_C//) of thin Al, a superconductor with minimal spin-orbit interaction (SOI), on the surface of (Bi_0.5Sb_0.5)₂TeSe₂ (BSTS). H_C// of the Al films are directly compared with those of simultaneously grown control samples of Al films on SiO₂. Giant enhancement of H_C// beyond the paramagnetic limit was observed for the Al on BSTS. The enhancement diminishes with increasing Al thickness. The H_C//(T) of thin Al on SiO₂ is consistent with the Chanderasekha-Clogston theory, while the H_C//(T) of thin Al on BSTS can be well-described by a theory based the Maki equation invoking strong induced SOI in Al. The results provide direct evidence for an electrically transparent interface and strong proximity effect between the Al and BSTS. The Al/BSTS heterostructure may be a promising system for inducing topological superconductivity and controlled generation of MZMs.