RESUMO
Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics of the charge-density wave in the Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1 ps, the system is driven across a phase transition from a long-range charge ordered state to a quasiequilibrium state with domainlike short-range charge and lattice order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic-site selectivity.
RESUMO
Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to directly determine the momentum-dependent electronic structure dynamics in the layered Peierls-Mott insulators 1T-TaS(2) and 1T-TaSe(2) on the sub-300 fs time scale. Extracted spectroscopic order parameters display a global two-time-scale dynamics indicating a quasi-instantaneous loss of the electronic orders and a subsequent coherent suppression of the lattice distortion on a time scale related to the frequency of the charge-density-wave amplitude mode. After one half-cycle of coherent amplitude-mode vibration, a crossover state between insulator and metal with partially filled-in and partially closed Mott and Peierls gaps is reached. The results are discussed within the wider context of electronic order quenching in complex materials.
RESUMO
Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the Mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time- and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. Specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-TaS(2) is a Peierls insulator and that the ultrafast response of 1T-TiSe(2) is highly suggestive of an excitonic insulator.