Time-domain classification of charge-density-wave insulators

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-TaS2 is a Peierls insulator and that the ultrafast response of 1T-TiSe2 is highly suggestive of an excitonic insulator.