We have studied the electronic structure of the nonmagnetic LiFeAs (T(c)∼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

Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin-or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the lowenergy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOCinduced Fermi surfaces.In the presence of spin-orbit coupling, the electron's spin quantized along any fixed axis is no longer a good quantum number, but its total angular momentum is. This basic fact alone or in combination with a particular symmetry breaking may lead to a splitting of otherwise degenerate energy bands and is the origin of fascinating phenomena such as spin Hall effects A special role has been played by SOC in the field of superconductors. In low-dimensional or noncentrosymmetric systems it can promote and stabilize superconductivity [6], allow ferromagnetism to coexist with superconductivity [7] or even rise T c [8]. If SOC is large, some superconductors can host an elusive Fulde-Ferrell-Larkin-Ovchinnikov state [9] or topological superconductivity [4]. It is anticipated that SOC could be a very important ingredient in describing the superconducting state in Sr 2 RuO 4 [10]. Since k-dependent spin-orbit splitting is larger than the superconducting gap in this material, the SOC-induced spin anisotropy together with the orbital mixing should directly influence the orbital and spin angular momentum of the Cooper pairs. Singlet and triplet states could be strongly mixed, blurring the distinction between spin-singlet and spintriplet pairing [11].In multiband iron-based superconductors, where the low energy electronic structure is composed of different orbitals, the situation is even more complicated because of the presence of the sizeable Hund's coupling. When the electronic structure near the Fermi energy is composed of different orbitals and spins mixed via spin-orbit coupling, determination of the pairing symmetry becomes non-trivial. However, up to now SOC in iron pnictides and chalcogenides was considered weak.We start with the example of LiFeAs, which is a special representative of iron-based family of superconductors [12]. This material is one of the most studied due to its stoichiometry and non-polar surfaces. Its electronic structure is believed to be well understood from numerous angle-resolved photoemission experiments (ARPES) and the parameterization of its electronic dispersions has been used to test the most developed theoretical approaches [13][14][15]. To detect spin-orbit coupling in LiFeAs experimentally we first ne...

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