Stripes, spin resonance, and nodeless d-wave pairing symmetry in Fe2Se2-based layered superconductors

We calculate RPA-BCS-based spin resonance spectra of the newly discovered iron-selenide superconductor by using the two-orbital tight-binding model in a 1-Fe-unit cell. The slightly squarish electron pocket Fermi surfaces at (pi,0)/(0,pi) momenta produce leading interpocket nesting instability at incommensurate vector q similar to (pi,0.5 pi) in the normal-state static susceptibility, pinning a strong stripe-like spin-density wave or antiferromagnetic order at some critical value of U. The same nesting also induces d(x2-y2) pairing in a 1-Fe-unit cell. The superconducting gap is nodeless and isotropic on the Fermi surfaces, as they lie concentric to the fourfold symmetric point of the d-wave gap maxima, in agreement with various experiments. This produces a slightly incommensurate spin resonance with upward dispersion, in close agreement with neutron data on chalcogenides. Finally, we demonstrate the conversion procedure from a 1-Fe-unit cell to a 2-Fe-unit cell in which the gap symmetry transformed simultaneously into a d(xy) pairing and the resulting resonance spectrum moves from the q similar or equal to (pi,pi) to the q similar or equal to (2 pi,0)/(2 pi,0) region.