Femtosecond dynamics of electronic states in the Mott insulator 1T-TaS2 by time resolved photoelectron spectroscopy

Photoexcitation of the Mott insulator 1T-TaS2 by an intense laser pulse leads to an ultrafast transition toward a gapless phase. Besides the collapse of the electronic gap, the sudden excitation of the charge density wave (CDW) mode results in periodic oscillations of the electronic states. We employ time resolved photoelectron spectroscopy to monitor the rich dynamics of electrons and phonons during the relaxation toward equilibrium. The qualitative difference between the oscillatory dynamics of the CDW and the monotonic recovery of the electronic gap proves that 1T-TaS2 is indeed a Mott insulator. Moreover the quasi-instantaneous build-up of mid gap states is in contrast with the retarded response expected from a Peierls insulating phase. Interestingly, the photoinduced electronic states in the midgap spectral region display a weak resonance that is reminiscent of a quasiparticle peak.