Observation of charge density wave excitonic order parameter in topological insulator monolayer WTe2

TL;DR

This study provides experimental evidence of charge density wave excitonic order in monolayer WTe2, revealing the interaction of topological edge modes with excitonic condensate signatures using advanced spectroscopic techniques.

Contribution

First observation of the excitonic order parameter in monolayer WTe2 through Fourier-transform scanning tunnelling spectroscopy.

Findings

QPI features confirm the interacting nature of the bandgap in WTe2.

Non-dispersive LDOS modulations indicate charge density wave order.

Interaction of topological edge modes with excitonic condensate signatures observed.

Abstract

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 tunnelling spectroscopy (FT-STS) to study quasi-particle 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 2D bulk bands, confirming the interacting nature of the bandgap in neutral WTe2 and the semi-metallic nature of highly n- and p-doped WTe2. We observe additional non-dispersive 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.