Sublattice Dichotomy in Monolayer FeSe Superconductor

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 1×1 topography on the SrTiO₃(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 +Vᵢ is higher than β-Fe while the coherence peak of β-Fe at -Vᵢ is higher than α-Fe with a similar amount. We also observed a reversed effect at the outer gap ± Vₒ. We propose the η-pairing mechanism between k and $-k+Q$ is the key mechanism for this unconventional sublattice dichotomy effect.