Abstract
In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors.
Highlights
Level both at the center and in the corner of the Brillouin zone and connected by the (π, π) nesting vector
It has been predicted that the presence of such singularities may lead to a significant enhancement of superconductivity[7]. Observation of this singular Fermi surface topology was made possible by the combination of high-quality SmFe0.92Co0.08AsO single crystals, which can only be grown by the laborious high-pressure technique with linear dimensions on the order of 300 μm (Ref. 8) and a synchrotron-based angle-resolved photoemission spectroscope with a smaller beam spot, operating below 900 mK with < 4 meV total energy resolution[9,10]
The former set of features does look similar to the Fermi surface predicted by our ab initio calculations of the electronic structure shown in Fig. 1d and reported previously[11]
Summary
Angle-resolved photoemission measurements were performed using synchrotron radiation (“13-ARPES” end-station at BESSY) within the range of photon energies 20–90 eV and various polarizations on cleaved surfaces of high-quality single crystals. High-quality single crystals of superconducting SmFe0.92Co0.08AsO with masses of a few micrograms were synthesized by the high-pressure high-temperature cubic anvil technique and were characterized by x-ray diffraction, transport and magnetization measurements[8]. The latter revealed a superconducting transition temperature of about 16 K. Calculations based on the local density approximation (LDA) tend to put partially filled 4f states of lanthanides at the Fermi level. In order to avoid this in the present calculations a Sm ion was replaced by La. We have verified that the band dispersions in the vicinity of the Fermi level calculated for LaFeAsO are very close to dispersions obtained for SmFeAsO with the same atomic positions provided that Sm 4f5 electrons are treated as localized quasi-core states and their exchange splitting is neglected. The effect of Co doping was simulated by using the virtual crystal approximation
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