Theory ties Fe d_xy orbital in monolayer FeSe/SrTiO3 to nodeless d-wave superconductivity

A theoretical study of monolayer FeSe on SrTiO3 has been posted as an arXiv · preprint and has not yet been peer reviewed. The system shows high-temperature superconductivity with an electron-only Fermi surface. The authors use an experimentally constrained multiorbital theory to examine the role of the Fe d_xy orbital. They were motivated by recent Josephson scanning tunneling microscopy (JSTM) evidence for nodeless d-wave superconductivity in this material arxiv.orgarXiv · препринтarxiv.orgOpen source ↗.

Orbital structure and hopping

The authors constructed a tight-binding model that includes all five Fe orbitals in the physical two-Fe unit cell. It is fitted to quasiparticle interference (QPI) and angle-resolved photoemission spectroscopy (ARPES) data and reproduces the observed low-energy dispersions and electron-pocket structure arxiv.orgarXiv · препринтarxiv.orgOpen source ↗.

The model shows substantial d_xy character near the Fermi level. The extracted hopping parameters are dominated by nearest-neighbor hopping arxiv.orgarXiv · препринтarxiv.orgOpen source ↗. This differs from the conventional picture of iron-based superconductors, where kinetic frustration suppresses nearest-neighbor hopping of the d_xy orbital and next-nearest-neighbor hopping dominates. According to the authors, this frustration is substantially lifted in monolayer FeSe.

The band structure also differs from the bulk-like ordering. The orbital ordering of the doublets is reversed, and together with interorbital hybridization this reconstructs the low-energy bands. The resulting electron pockets carry substantial d_xy weight arxiv.orgarXiv · препринтarxiv.orgOpen source ↗.

Pairing and Josephson signatures

The authors compared pairing channels with two approaches: itinerant spin-fluctuation calculations and strong-coupling exchange approaches. Both favor a robust d-wave pairing state. Its interband pairing components allow a fully gapped quasiparticle spectrum arxiv.orgarXiv · препринтarxiv.orgOpen source ↗.

The study also derives a symmetry-based selection rule for spatial Josephson tunneling. It predicts bond-centered maxima and interference zeros enforced by mirror symmetry. These features distinguish d-wave pairing from competing s-wave states arxiv.orgarXiv · препринтarxiv.orgOpen source ↗.

The symmetry-enforced nodal lines survive near Fe-site impurities. Around vortices the mirror protection is lost, but a pronounced contrast between the d- and s-wave Josephson patterns persists, according to the authors arxiv.orgarXiv · препринтarxiv.orgOpen source ↗. They conclude that the d_xy electronic structure is connected to nodeless d-wave superconductivity and that spatial Josephson interference can resolve pairing symmetry in multiorbital superconductors.