Giant enhancement of the in-plane critical field for thin Al films via proximity coupling to a topological insulator

Abstract

A topological superconducting state can be induced in the surface state of a topological insulator (TI) by way of proximity coupling to a conventional $s$-wave superconductor ($s\ensuremath{-}\mathrm{SC}$). Planar $s\ensuremath{-}\mathrm{SC}/\mathrm{TI}$ junction structures were proposed as a scalable platform for controlled generation and manipulation of Majorana zero mode (MZM), which holds intriguing promise for fault-tolerant quantum computing. Despite intensive research efforts, the presence of MZM has not been definitively demonstrated in $s\ensuremath{-}\mathrm{SC}/\mathrm{TI}/s\ensuremath{-}\mathrm{SC}$ lateral junctions. A key factor is a lack of direct measurement and quantitative understanding of the proximity coupling between the $s\ensuremath{-}\mathrm{SC}$ and TI. Here we report evidence for strong superconducting proximity effect between a three-dimensional strong TI and Al, a conventional $s\ensuremath{-}\mathrm{SC}$ with minimal intrinsic spin-orbit coupling, in the form of pronounced enhancement of the in-plane critical field (${H}_{c||}$) of the thin Al. Specifically, the ${H}_{c||}$ of a 6-nm-thick Al film deposited on a TI is found to be 2.7 times its Pauli limit and about three times that of a simultaneously deposited reference film on $\mathrm{Si}/\mathrm{Si}{\mathrm{O}}_{2}$. The analysis of the ${H}_{c||}$ enhancement within the Maki theory indicates significant induced spin-orbit interaction in the Al due to electronic coupling to the TI. Our results revealed a pathway for producing SC/TI devices of high interfacial electrical transparency conducive for MZM generation and manipulation.