Interface-Enhanced Superconductivity in Ultrathin TiN Proximitized by Topological Insulators

A high-quality topological insulator-superconductor (TI-SC) heterostructure with an atomically sharp and well-controlled interface is crucial for realizing topological superconductivity and a topological quantum qubit. In particular, many studies of TI-SC heterostructures have focused on inducing a superconducting gap in the TI layer via proximity effect, while the active manipulation of superconductivity in the SC layer remains largely unexplored. In this work, we fabricated TI/TiN heterostructures using highly air-stable, ultrathin TiN films as the SC layer and observed an interface-enhanced superconductivity that contrasts with the conventional proximity effect in the superconductor-normal metal interface. Band structure measurements reveal a consistent shift of the Dirac point with T_c enhancement. Interfacial charge transfer provides a plausible explanation for this shift based on the systematic analysis and is therefore a likely contributor to the observed T_c enhancement. First-principles calculations elucidate the charge transfer pathways, highlighting the critical role of the interfacial BiTe (BiSe) bilayer. Our results not only provide a tunable TI-SC hybrid system with robust superconductivity at ultrathin thickness but also offer a potential route for manipulating superconductivity in TI-SC heterostructures via interface engineering.