Proximity-Induced Superconductivity in a Topological Insulator using an Fe-based Superconductor

Interest in the superconducting proximity effect (SPE) has recently been reignited by theoretical predictions that it could be used to achieve topological superconductivity. However, small proximity-induced gaps (Δ_ind) of ~1 meV predominantly obtained using select few low-T_c superconductors have hindered the fundamental understanding of what drives the SPE across complex interfaces. We use molecular beam epitaxy to grow a prototypical topological insulator Bi₂Te₃ on a multi-gap high-T_c superconductor Fe(Te,Se), to achieve a significantly larger induced superconducting gap (Δ_ind) and transition temperature (T_c) compared to studies. Interestingly, in contrast to previous hypothesis suggesting the importance of Fermi surface alignment in achieving superconducting proximity effect, we discover that Δ_ind in Bi₂Te₃ is dictated by the superconducting gap in the momentum-mismatched parent superconductor’s Fermi surfaces. Our work suggests a concrete pathway to guide the synthesis of new heterostructures with larger Δ_ind and higher T_c using Fe-based superconductors.