Distinguishing insulators by the dominant type of interaction is a central problem in condensedmatter physics. Basic models include the Bloch-Wilson and the Peierls insulator due toelectron–lattice interactions, the mott and the excitonic insulator caused by electron–electroninteractions, and the Anderson insulator arising from electron–impurity interactions. In realmaterials, however, all the interactions are simultaneously present so that classification is oftennot straightforward. Here, we show that time- and angle-resolved photoemission spectroscopycan directly measure the melting times of electronic order parameters and thus identify—viasystematic temporal discrimination of elementary electronic and structural processes—thedominant interaction. specifically, we resolve the debates about the nature of two peculiarcharge-density-wave states in the family of transition-metal dichalcogenides, and show that Rbintercalated 1T-Tas_2 is a Peierls insulator and that the ultrafast response of 1T-Tise_2 is highlysuggestive of an excitonic insulator.
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