Evidence for a robust sign-changing s-wave order parameter in monolayer films of superconducting Fe (Se,Te)/Bi2Te3

TL;DR

This study reveals that monolayer Fe(Se,Te) films on Bi2Te3 exhibit a robust, sign-changing s-wave superconducting order parameter, similar to bulk but with unique features, highlighting their potential for topological superconductivity.

Contribution

First direct phase-sensitive evidence of a sign-changing s-wave order parameter in monolayer Fe(Se,Te) films, demonstrating their unconventional superconductivity and topological potential.

Findings

Monolayer Fe(Se,Te)/Bi2Te3 hosts multigap superconductivity similar to bulk.

BQPI reveals a sign-changing s-wave order parameter.

Unique sign change patterns in BQPI not seen in bulk.

Abstract

The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with unconventional superconductivity and may be an ideal platform to realize exotic states such as high-order topological corner modes and Majorana modes. Thin films of Fe (Se,Te) are important for device fabrication, phase sensitive transport measurements and for realizing proposals to engineer higher-order modes. However, while bulk Fe (Se,Te) has been extensively studied with a variety of techniques, the nature of the superconducting order parameter in the monolayer limit has not yet been explored. In this work, we study monolayer films of Fe (Se,Te) on Bi2Te3 with scanning tunneling spectroscopy and Bogoliubov quasiparticle interference (BQPI). We discover that the monolayer Fe (Se,Te)/Bi2Te3 heterostructures host a robust, multigap superconducting state that strongly resembles the bulk. BQPI maps at the gap energies show a strong spatial modulation, oriented 45 degrees to the Fe-Se bond direction. Analysis of the phase-referenced quasiparticle interference signal reveals a sign-changing s-wave order parameter similar to the bulk. Moreover, we observe a unique pattern of sign changes in the BQPI signal which have not been observed in the bulk. Our work establishes monolayer Fe (Se,Te)/Bi2Te3 as a robust multi-band unconventional superconductor and sets the stage for explorations of non-trivial topology in this highly-tunable system.