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The origin of high-temperature superconductivity in iron-based superconductors is still not understood; determination of the pairing symmetry is essential for understanding the superconductivity mechanism. In the iron-based superconductors that have hole pockets around the Brillouin zone centre and electron pockets around the zone corners, the pairing symmetry is generally considered to be s±, which indicates a sign change in the superconducting gap between the hole and electron pockets. For the iron-based superconductors with only hole pockets, however, a couple of pairing scenarios have been proposed, but the exact symmetry is still controversial. Here we determine that the pairing symmetry in KFe2As2—which is a prototypical iron-based superconductor with hole pockets both around the zone centre and around the zone corners—is also of the s±type. Our laser-based angle-resolved photoemission measurements have determined the superconducting gap distribution and identified the locations of the gap nodes on all the Fermi surfaces around the zone centres and the zone corners. These results unify the pairing symmetry in hole-doped iron-based superconductors and point to spin fluctuation as the pairing glue in generating superconductivity.

Recent studies have shown that layered compound EuCd2As2could exhibit diverse topological states depending on the different magnetic structures, such as Weyl semimetal, Dirac semimetal and topological insulator. In order to further study the interplay between magnetism and topology of EuCd2As2, it is necessary to figure out its magnetic structure. Here, by magnetization (M) measurements and negative magnetostriction (λ) along the [001] direction measured by scanning tunneling microscopy on EuCd2As2single crystals, we observe field-induced metamagnetic phase transition from A-type antiferromagnetic (AFM) ground state to field-polarized state, with canted AFM (CAFM) state in between. Magnetization and magnetostriction are more sensitive to the in-plane field than the out-of-plane field, indicating the magnetic moments lying in the abplane. The absence of abrupt jump on M-Hand λ-Hcurves demonstrates that the phase transition is a second-order type. In CAFM state, Mincreases linearly with the field and λis proportional to M2. Tunneling conductance spectra show the field-induced evolution of the electronic density of states. Our results provide experimental evidence for understanding the magnetic structure of EuCd2As2.