Sublattice Dichotomy in Monolayer FeSe Superconductor

Abstract

The pairing mechanism behind the monolayer FeSe is one essential question for iron-based superconductors. In this work, we show the sublattice degree of freedoms of monolayer FeSe plays a special role in its pairing properties, namely the sublattice dichotomy. The high-quality monolayer FeSe samples with atomic flat $1\times1$ topography on the SrTiO$_3$(001) substrates are grown by molecular beam epitaxy. By comparing the tunneling spectra at $α$ and $β$ Fe sublattices, we find the coherence peak of $α$-Fe at the inner gap $+V_i$ is higher than $β$-Fe while the coherence peak of $β$-Fe at $-V_i$ is higher than $α$-Fe with a similar amount. We also observed a reversed effect at the outer gap $\pm V_o$. We propose the $η$-pairing mechanism between $k$ and $-k+Q$ is the key mechanism for this unconventional sublattice dichotomy effect.