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.

The superconducting gap is a pivotal character for a superconductor. While the cuprates and conventional phonon-mediated superconductors are characterized by distinct d-wave and s-wave pairing symmetry with nodal and nodeless gap distribution respectively, the superconducting gap distributions in iron-based superconductors are rather diversified. While nodeless gap distributions have been directly observed in Ba 1−x K x Fe 2 As 2 , BaFe 2−x Co x As 2 , K x Fe 2−y Se 2 , and FeTe 1−x Se x [1-4], the signatures of nodal superconducting gap have been reported in LaOFeP, LiFeP, KFe 2 As 2 , BaFe 2 (As 1−x P x ) 2 , BaFe 2−x Ru x As 2 and FeSe [5-12]. We here report the angle resolved photoemission spectroscopy (ARPES) measurements on the superconducting gap structure of BaFe 2 (As 1−x P x ) 2 in the momentum space, and particularly, the first direct observation of a circular line node on the largest hole Fermi surface around the Z point at the Brillouin zone boundary. Our data rules out the d-wave pairing origin of the nodal gap, and unify both the nodal and nodeless gaps in iron pnictides under the s ± pairing symmetry.The pairing symmetry of the Cooper pair in a superconductor is manifested in its gap structure. Particularly, nodes or nodal lines of the superconducting gap often imply unconventional (e.g. non-s-wave) pairing symmetries. For most ironbased superconductors, there are electron Fermi surfaces at the Brillouin zone corner and hole Fermi surfaces at the center. It has been proposed that the pairing interactions between the electron and hole Fermi surfaces will induce nodeless s-wave order parameter with opposite signs on them [13][14][15]. While this nodeless s ± -wave pairing symmetry has gained increasing experimental support [16][17][18], nodal gap has been reported in LaOFeP, LiFeP, KFe 2 As 2 , BaFe 2 (As 1−x P x ) 2 , BaFe 2−x Ru x As 2 , and FeSe by thermal conductivity, penetration depth, nuclear magnetic resonance, and scanning tunneling spectroscopy studies [5][6][7][8][9][10][11][12]. However, no direct measurement on any of these compounds has been reported regarding the gap structure so far, and especially the location of the nodes remains unknown. Since BaFe 2 (As 1−x P x ) 2 has relatively high superconducting transition temperature T c , it provides an opportunity for direct access of the detailed gap structure in the momentum space by angle resolved photoemission spectroscopy (ARPES).We have conducted ARPES measurements on BaFe 2 (As 0.7 P 0.3 ) 2 with a T c of 30 K (see Method section for details). As previous detailed polarization dependent studies have shown [19] and replicated here in Fig. 1a, there * Electronic address: dlfeng@fudan.edu.cn

The superconductivity discovered in iron-pnictides is intimately related to a nematic ground state, where the C4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with the polarization dependent angle-resolved photoemission spectroscopy. A uniaxial strain was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state, and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy-and dyz-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the dxz-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the development of orbitaldependent spin polarization is likely the dominant force to drive the nematicity, while the ferroorbital ordering between dxz and dyz orbitals can only play a minor role here.

The nature of the parent compound of a high-temperature superconductor (HTS) often plays a pivotal role in determining its superconductivity. The parent compounds of the cuprate HTSs are antiferromagnetically ordered Mott insulators, while those of the iron-pnictide HTSs are metals with spin-density-wave order. Here we report the electronic identification of two insulating parental phases and one semiconducting parental phase of the newly discovered family of K x Fe 2Ày Se 2 superconductors. The two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is even present in the superconducting and semiconducting K x Fe 2Ày Se 2 compounds. However, it is mesoscopically phaseseparated from the superconducting or semiconducting phase. Moreover, we find that both the superconducting and semiconducting phases are free of the magnetic and vacancy orders present in the insulating phases, and that the electronic structure of the superconducting phase could be developed by doping the semiconducting phase with electrons. The rich electronic properties discovered in these parental phases of the K x Fe 2Ày Se 2 superconductors provide the foundation for studying the anomalous behavior in this new class of iron-based superconductors.

The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multi-band/multi-orbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe1.85Co0.15As2 by studying the polarization and photon energy dependence of angle-resolved photoemission data. While the distributions of the dxz, dyz, and d 3z 2 −r 2 orbitals agree with the prediction of density functional theory, those of the dxy and d x 2 −y 2 orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations, and more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.