  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.
