Experimental evidence for a metastable state in FeTe1−Se following coherent-phonon excitation

Changes of the electronic structure of FeTe1−xSex (x = 0, 0.3) upon ultrafast excitation with near-infrared laser pulses are investigated using time- and angle-resolved extreme ultraviolet photoemission spectroscopy. At different temperatures and for different doping levels we observe a global oscillation of energy bands near the Fermi energy driven by the coherent excitation of the A1g(Te) phonon mode in FeTe1−xSex. Prominently, photo-induced band shifts and a spectral weight reduction persist for at least 300 ps, long after the decay of the photo-excited carriers and the damping of the coherent phonons. We argue that the system escapes from the original equilibrium state and is transiently trapped in a possibly metastable state envisioning a purely optical means of manipulating the electronic structure in an iron-based superconductor.