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 $α$ 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.

💡 Research Summary

The authors investigate the superconducting pairing mechanism in monolayer FeSe grown on SrTiO₃(001) by combining molecular‑beam epitaxy (MBE) growth of atomically flat 1 × 1 films with ultra‑low‑temperature scanning tunneling microscopy and spectroscopy (STM/STS). High‑resolution topography reveals two inequivalent Fe sublattices, denoted α‑Fe and β‑Fe, which are distinguished by their positions relative to the upper (Se⁺) and lower (Se⁻) Se layers. Fourier‑transform analysis confirms the presence of both the 2‑Fe Brillouin‑zone Bragg peaks and additional Qₓ, Qᵧ spots associated with the 1‑Fe sublattice, establishing that the surface is free of the commonly observed (2 × 1) reconstruction.

Large‑bias (±500 mV) spectra show the expected band‑structure features of electron pockets centered at the M points, consistent with ARPES reports that the Γ hole pocket is absent in the monolayer. More importantly, when the bias is confined to the superconducting gap region, the differential conductance spectra display a striking “sublattice dichotomy.” Each Fe site exhibits two sets of coherence peaks: an inner gap at ±Vᵢ≈±10 meV and an outer gap at ±Vₒ≈±15–17 meV. On α‑Fe the positive‑bias inner peak (+Vᵢ) is markedly higher than the negative‑bias counterpart (‑Vᵢ), whereas on β‑Fe the opposite holds: the negative‑bias inner peak is stronger. The outer‑gap peaks show the reverse pattern, i.e., α‑Fe has a larger negative‑bias outer peak while β‑Fe has a larger positive‑bias outer peak. This particle‑hole asymmetry is complementary between the two sublattices and is confined to the superconducting gap; it disappears in the normal‑state spectra.

To quantify the asymmetry the authors define Z(r,Vᵢ)=g(r,+Vᵢ)/g(r,‑Vᵢ). Histograms show Z≈1.1–1.5 on α‑Fe and Z≈0.6–0.9 on β‑Fe. Spatial maps of g(r,±Vᵢ) and g(r,±Vₒ) reveal a periodic modulation within the 2‑Fe unit cell that mirrors the sublattice contrast. The second derivative of the I‑V curves (‑d³I/dV³) also displays peaks that invert between the two sublattices, further confirming the effect.

The authors propose that this dichotomy arises from the coexistence of two pairing channels made possible by broken inversion symmetry at the FeSe/SrTiO₃ interface. The first channel is the conventional intraband (k,‑k) pairing, which is even under parity. The second is an interband “η‑pairing” between states (k,‑k+Q) with Q = (π,π), which is odd under parity and becomes allowed when the inversion center is removed. Because Q is a reciprocal lattice vector of the true 2‑Fe unit cell, the η‑pairing does not break translational symmetry but introduces a sign change between the two Fe sublattices. A k·p model incorporating both even‑parity normal pairing and odd‑parity η‑pairing reproduces the experimentally observed sublattice‑dependent density of states, including the reversal of particle‑hole asymmetry between the inner and outer gaps.

The robustness of the effect is demonstrated by varying temperature, bias set‑points, and normalization procedures; the dichotomy persists across all conditions and even survives in regions where a (2 × 1) electronic modulation is present, albeit with reduced contrast. This indicates that the phenomenon is intrinsic to the superconducting state rather than an artifact of surface reconstruction or impurity scattering.

In summary, the work uncovers a novel sublattice‑dependent superconducting signature in monolayer FeSe, directly linking the broken inversion symmetry at the interface to a mixed pairing state that combines conventional even‑parity pairing with odd‑parity η‑pairing. The findings highlight the importance of the two‑Fe unit‑cell description for iron‑based superconductors and suggest that sublattice degrees of freedom can host unconventional pairing channels, potentially offering new routes to enhance superconducting transition temperatures in heterostructures.