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Title: Pairing symmetry of monolayer FeSe on SrTiO3
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: contested
Status note: 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).
Posed since: (empty)