Pairing symmetry of monolayer FeSe on SrTiO3
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
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).
Related problems
- Special case of Pairing symmetry of heavily electron-doped FeSe without hole pockets · Monolayer FeSe/SrTiO3 is one of the hole-pocket-free FeSe systems covered