Pairing symmetry of monolayer FeSe on $\mathrm{SrTiO_3}$
In plain words
Monolayer FeSe lacks the hole pocket that standard theories of iron-based superconductors rely on, so it is unclear whether the superconducting gap changes sign between its electron pockets.
Precise statement
With only electron pockets at $M$, candidate states are plain s-wave (phonon-assisted), nodeless d-wave (sign change between the two electron pockets), incipient $s\pm$ (pairing involving a hole band below $E_F$) and states with interband pairing. Determine the relative sign of the gap on the two electron pockets and whether it varies between the two Fe sublattices.
What would settle it
A phase-sensitive measurement (phase-referenced quasiparticle interference, impurity bound-state spectroscopy, or Josephson interferometry) giving the relative gap sign on the two electron pockets.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Scanning tunneling spectroscopy found coherence peaks that differ between the two Fe sublattices, interpreted as $\eta$-pairing between $k$ and $-k+Q$ (Ding et al., Physical Review Letters 136, 066502, 2026, arXiv 2406.15239).
Parent problems
- Special case of Pairing symmetry of heavily electron-doped FeSe without hole pockets · open · from revision 1 Monolayer FeSe/SrTiO3 is one of the hole-pocket-free FeSe systems covered The parent was revised since revision 1: it now states another question.
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