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 \(1\times1\) topography on the SrTiO\(_3\)(001) substrates are grown by molecular beam epitaxy. By comparing the tunneling spectra at \(\alpha\) and \(\beta\) Fe sublattices, we find the coherence peak of \(\alpha\)-Fe at the inner gap \(+V_i\) is higher than \(\beta\)-Fe while the coherence peak of \(\beta\)-Fe at \(-V_i\) is higher than \(\alpha\)-Fe with a similar amount. We also observed a reversed effect at the outer gap \(\pm V_o\). We propose the \(\eta\)-pairing mechanism between \(k\) and \(-k+Q\) is the key mechanism for this unconventional sublattice dichotomy effect.