Theory of Giant In-Plane Critical Field Enhancement in Superconducting Films on a Topological Insulator Surface

Majorana zero modes (MZMs) are of interest due to potential applications in topological quantum computing. Topological insulators (TIs), with spin-momentum locked surface states, may enable emergence of MZMs when coupled to an s-wave superconductor (SC) via the proximity effect. We recently observed a giant enhancement of the in-plane H_c in thin Al, a superconductor with minimal intrinsic spin-orbit coupling (SOC), evaporated onto a 3-dimensional strong TI.¹ Here we present a theoretical analysis of this enhancement. Superconductivity at in-plane fields well in excess of the Chanderasekhar-Clogston (CC) limit is understood as a consequence of strong SOC induced in the thin Al films from the topological surface state through the inverse proximity effect. The CC model is combined with the Maki equation to calculate H_c at different temperatures. The calculated H_c(T) shows excellent quantitative agreement with the experimental result. A relatively large value of the induced spin-orbit scattering rate (in comparison to the superconducting energy gap) is derived, indicating a strong proximity coupling in this TI-SC heterostructure which may be a promising platform for observing MZMs.
¹Z. Lin et. al., unpublished.