We have studied the electronic structure of the nonmagnetic LiFeAs (Tc$\sim${}18\,\,K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of 3, high density of states at the Fermi level caused by a van Hove singularity, and no evidence for either a static or a fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides.

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter [1–6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7–9] and spin fluctuations are not necessary [7,10], contrary to the models [1–6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity.

We report superconducting (SC) properties of stoichiometric LiFeAs (Tc=17\,\,K) studied by small-angle neutron scattering (SANS) and angle-resolved photoemission (ARPES). Although the vortex lattice exhibits no long-range order, well-defined SANS rocking curves indicate better ordering than in chemically doped 122 compounds. The London penetration depth $\lambda${}ab(0)=210\ifmmode\pm\else\textpm\fi{}20\,\,nm, determined from the magnetic field dependence of the form factor, is compared to that calculated from the ARPES band structure with no adjustable parameters. The temperature dependence of $\lambda${}ab is best described by a single isotropic SC gap $\Delta${}0=3.0\ifmmode\pm\else\textpm\fi{}0.2\,\,meV, which agrees with the ARPES value of $\Delta${}0ARPES=3.1\ifmmode\pm\else\textpm\fi{}0.3\,\,meV and corresponds to the ratio 2$\Delta${}/kBTc=4.1\ifmmode\pm\else\textpm\fi{}0.3, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor.

We present a systematic angle-resolved photoemission spectroscopy study of the superconducting gap in FeSe. The gap function is determined in a full Brillouin zone including all Fermi surfaces and $k_z$ dependence. We find significant anisotropy of the superconducting gap in all momentum directions. While the in-plane anisotropy can be explained by both nematicity-induced pairing anisotropy and orbital-selective pairing, the $k_z$ anisotropy requires an additional refinement of the theoretical approaches.