Sublattice Dichotomy in Monolayer FeSe Superconductor

TL;DR

This paper investigates the role of sublattice degrees of freedom in monolayer FeSe superconductors, revealing a unique sublattice dichotomy in pairing properties linked to an unconventional $ ext{eta}$-pairing mechanism.

Contribution

It uncovers the sublattice dichotomy in pairing spectra of monolayer FeSe and proposes the $ ext{eta}$-pairing mechanism as the underlying cause.

Findings

Sublattice-dependent coherence peaks observed in tunneling spectra.

Reversal of spectral effects between inner and outer gaps.

Evidence supporting $ ext{eta}$-pairing as the pairing mechanism.

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 $\alpha$ 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.