Anomalous Superconducting Proximity Effect in Bi₂ Se₃ /FeSe_0.5 Te_0.5 Thin-Film Heterojunctions

The superconducting proximity effect (SPE) induces a superconductivity transition in otherwise non-superconducting thin films in proximity with a superconductor. The SPE usually occurs in real space and decays exponentially with film thickness. Herein, an abnormal SPE in a topological insulator (TI)/superconductor heterostructure is unveiled, which is attributed to the topologically protected surface state. Surprisingly, such abnormal SPE occurs in momentum space regardless of the TI film thickness, as long as the topological surface states are robust and form a continuous conduction loop. Combining transport measurements and scanning tunneling microscopy/spectroscopy techniques, the SPE in Bi₂ Se₃ /FeSe_0.5 Te_0.5 heterostructures is explored, where Bi₂ Se₃ is an ideal 3D topological insulator and FeSe_0.5 Te_0.5 a typical iron-based superconductor. As the thickness of the Bi₂ Se₃ thin film exceeds 400 nm, there still exists SPE-induced superconductivity on the surface of Bi₂ Se₃ thin film with a transition temperature T_c not less than 10 K. Such an extraordinary behavior is induced by the unique properties of topologically protected surface states of Bi₂ Se₃ . This research deepens the understanding of the important role of topologically protected surface states in the SPE.