Laser-switchable flat bands in the van der Waals ferromagnet Fe₅GeTe₂

The two-dimensional van der Waals ferromagnet Fe₅GeTe₂ can be found either in the Fe(1) site-ordered phase or Fe(1) site-disordered phase. By performing laser-based micrometer-scale angle-resolved photoemission spectroscopy (micro-ARPES) on the site-disordered phase, we reveal the emergence of sharp flat bands near the Fermi level around the Brillouin zone center, while the site-ordered phase shows only strongly-dispersive bands near zero momentum. The flat bands manifest a coherence-incoherence crossover around 100 K.

Local laser heating is used to investigate the temperature evolution of the two phases from 8 K to 730 K, as well as their temporal evolution under laser irradiation. By tuning the laser heating and exposure time, we controllably switch small areas of the sample (≈10 µm) from the site-disordered to the site-ordered phase (writing procedure), which could be of interest for reconfigurable spintronics applications. The reverse process is attained by thermally cycling the whole sample (erasing procedure). The kinetics of the writing phase transition highlights the role of diffusion in the domain nucleation and growth, setting a limit on the speed of the phase switching. This work establishes Fe₅GeTe₂ as the first laser-switchable flat-band system.