Anomalous temperature evolution of the electronic structure of FeSe

We present angle-resolved photoemission spectroscopy 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 degrees Kelvin. Remarkably, upon warming up the sample, the band structure has a tendency to relax to the one predicted by conventional band structure calculations, directly 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