  Electronically driven nematic order is often considered as an essential
ingredient of high-temperature superconductivity. Its elusive nature in
iron-based supercon- ductors resulted in a controversy not only as regards its
origin but also as to the degree of its influence on the electronic structure
even in the simplest representative material FeSe. Here we utilized
angle-resolved photoemission spectroscopy and density functional theory
calculations to study the influence of the nematic order on the electronic
structure of FeSe and determine its exact energy and momentum scales. Our
results strongly suggest that the nematicity in FeSe is electronically driven,
we resolve the recent controversy and provide the necessary quantitative
experimental basis for a successful theory of superconductivity in iron-based
materials which takes into account both, spin-orbit interaction and electronic
nematicity.
