title: Time-domain classification of charge-density-wave insulators.
abstract: 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.
authors.affiliation: Institute of Experimental and Applied Physics, University of Kiel, D-24098 Kiel, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Hellmann
authors.forename: T
authors.initials: T
authors.lastname: Rohwer
authors.forename: M
authors.initials: M
authors.lastname: Kalläne
authors.forename: K
authors.initials: K
authors.lastname: Hanff
authors.forename: C
authors.initials: C
authors.lastname: Sohrt
authors.forename: A
authors.initials: A
authors.lastname: Stange
authors.forename: A
authors.initials: A
authors.lastname: Carr
authors.forename: M M
authors.initials: MM
authors.lastname: Murnane
authors.forename: H C
authors.initials: HC
authors.lastname: Kapteyn
authors.forename: L
authors.initials: L
authors.lastname: Kipp
authors.forename: M
authors.initials: M
authors.lastname: Bauer
authors.forename: K
authors.initials: K
authors.lastname: Rossnagel