Nodeless superconducting gap in AxFe2Se2 (A=K,Cs) revealed by angle-resolved photoemission spectroscopy

Abstract

Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(±)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A=K,Cs) with a superconducting transition temperature (Tc) of about 30 K. We found A(x)Fe(2)Se(2) (A=K,Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of ~10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(±) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.

Fermi-surface topology and low-lying electronic structure of the iron-based superconductor Ca10(Pt3As8)(Fe

“…Not only the in-plane lattice parameter a 0 but also the shapes, sizes and Fermi velocities of the Γ and X 0 Fermi pockets show very little difference with those of the 122 parent compounds. [21][22][23][24][25][26][27][28][29] This indicates that a universal electronic structure capturing the underlying superconducting mechanism may exist for different sub-families of the Fe-based superconductors (except for the K α Fe 2−β Se 2 series, where electron pockets instead of hole pockets are observed around the Γ point 30,31 ).…”

Robustness ofs-Wave Pairing in Electron-OverdopedA1−yFe2−xSe2Fang1, Wu2, Thomale3 et al. 2011Phys. Rev. X9751141View full textAdd to dashboardCite

“…(Some signature of the possible density of states at the À point is still under debate; in any case, this pocket, if present, is assumed to be very flat and shallow.) ARPES experiments have also reported large isotropic superconducting gaps at these pockets [5][6][7]. The absence of hole pockets around the À point of the BZ provides a new arena of Fermi-surface topology in which to investigate the pairing symmetries and mechanisms of superconductivity proposed for iron-based superconductors from a variety of approaches .…”

“…(''122 iron pnictides'' are those with the generic stoichiometric formula AFe 2 As 2 .) Importantly, both angle-resolved photoemission spectroscopy (ARPES) [5][6][7] and local-density approximation calculations [8][9][10] show the presence of only electron Fermi pockets located at the M point of the folded Brillouin zone (BZ). (Some signature of the possible density of states at the À point is still under debate; in any case, this pocket, if present, is assumed to be very flat and shallow.)…”

Magnetic phase diagram of an extendedJ1−J2model on a modulated square lattice and its implications for the antiferromagnetic phase of KyFeYu1, Goswami2, Si3 2011Phys. Rev. B311190View full textAdd to dashboardCite

“…In these 122 ironchalcogenides, there are parent compounds which are antiferromagnetically ordered and insulating 18,21 . Furthermore, both the angle resolved photoemission experiments [22][23][24] and band structure calculations [25][26][27][28] reveal unique fermiology. Unlike the pnictides, there are no hole Fermi pockets; the Fermi surface is entirely made up of electron Fermi pockets, with the dominant pieces located near the M points of the extended Brillouin zone.…”