Nanoscale Synthesis and Characterization of Topological Materials

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 have predominantly been obtained so far using select few low-T_c superconductors. In this talk, I will discuss how we use a combination of molecular beam epitaxy and scanning tunneling microscopy/spectroscopy to study topological insulators grown on high-T_c superconductors Fe(Te,Se) and Bi₂Sr₂CaCu₂O_8+x. On the surface of Bi₂Te₃ grown on Fe(Te,Se), we detect Δ_ind as high as ~3.5 meV, which is the largest reported gap induced by proximity to an s-wave superconductor to-date. We find that Δ_ind exponentially decays with Bi₂Te₃ thickness, but remains finite even after the topological surface states had been formed. By imaging the scattering and interference of surface state electrons, we provide a microscopic visualization of the fully gapped Bi₂Te₃ surface state due to Cooper pairing. We contrast this observation with the lack of observed superconducting gap in Bi₂Te₃/Bi₂Sr₂CaCu₂O_8+x heterostructures. We conclude by discussing the roles of various parameters in driving the SPE effect across complex interfaces.