Observation of the Charge Density Wave Excitonic Order Parameter in Topological Insulator Monolayer WTe₂

Strong electron-hole interactions in a semimetal or narrow-gap semiconductor may drive a ground state of condensed excitons. Monolayer WTe₂ has been proposed as a host material for such an exciton condensate, but the order parameter─the key signature of a macroscopic quantum-coherent condensate─has not been observed. Here, we use Fourier-transform scanning tunneling spectroscopy (FT-STS) to study quasiparticle interference (QPI) and periodic modulations of the local density of states (LDOS) in monolayer WTe₂. In WTe₂ on graphene, in which the carrier density can be varied via back-gating, FT-STS shows QPI features in the two-dimensional (2D) bulk bands, confirming the interacting nature of the bandgap in neutral WTe₂ and the semimetallic nature of highly n- and p-doped WTe₂. We observe additional nondispersive spatial modulations in the LDOS imprinted on the topological edge mode of neutral WTe₂ on metallic substrates (graphene and graphite), which we interpret as the interaction of the topological edge mode with the expected charge density wave order parameter of the excitonic condensate in WTe₂ at low interaction strength due to screening by the metallic substrates.