Ultrafast filling of an electronic pseudogap in an incommensurate crystal

We investigate the quasiperiodic crystal (LaS)_1.196VS ₂ by angle and time resolved photoemission spectroscopy. The dispersion of electronic states is in qualitative agreement with the band structure calculated for the VS₂ slab without incommensurate distortion. Nonetheless, the spectra display a temperature dependent pseudogap instead of quasiparticle crossing. The sudden photoexcitation at 50 K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable time scale. We attribute this surprisingly short time scale to a very strong electron-phonon coupling to the incommensurate distortion. This result sheds light on the electronic localization arising in aperiodic structures and quasicrystals.