  High-resolution time- and angle-resolved photoemission measurements were made
on FeSe superconductors. With ultrafast photoexcitation, two critical
excitation fluences that correspond to two ultrafast electronic phase
transitions were found only in the $d_{yz}$-orbit-derived band near the
Brillouin-zone center within our time and energy resolution. Upon comparison to
the detailed temperature dependent measurements, we conclude that there are two
equilibrium electronic phase transitions (at approximately 90 and 120 K) above
the superconducting transition temperature, and an anomalous contribution on
the scale of 10 meV to the nematic states from the structural transition is
experimentally determined. Our observations strongly suggest that the
electronic phase transition at 120 K must be taken into account in the energy
band development of FeSe, and, furthermore, the contribution of the structural
transition plays an important role in the nematic phase of iron-based
high-temperature superconductors.
