  We present ARPES data taken from the structurally simplest representative of
iron-based superconductors, FeSe, in a wide temperature range. Apart from the
variations related to the nematic transition, we detect very pronounced shifts
of the dispersions on the scale of hundreds of kelvins. Remarkably, upon
warming the sample up, the band structure has a tendency to relax to the one
predicted by conventional band structure calculations, right opposite to what
is intuitively expected. Our findings shed light on the origin of the dominant
interaction shaping the electronic states responsible for high-temperature
superconductivity in iron-based materials.
