Local spectroscopy of a gate-tunable energy gap in monolayer 1T’-WTe₂

The interplay between strong correlation and topology leads to new quantum phases. In monolayer 1T’-WTe₂ non-trivial topology gives rise to a quantum spin Hall insulator (QSHI) phase, characterized by helical 1D edge-states surrounding an insulating 2D bulk. Experimental evidence supports quantized conductance through such edge-states, but the nature of the insulating bulk and whether it is dominated by spin-orbit coupling (SOC) or strong correlation remains under debate. Here we have employed scanning tunneling microscopy and spectroscopy (STM/S) on gate-tunable 1T’-WTe₂ devices to explore this issue. Our samples were fabricated using a combination of molecular beam epitaxy (MBE) and van der Waals (vdW) stacking, which allows us to synthesize high-quality monolayer 1T’-WTe₂ films on a gate-tunable graphene field-effect transistor supported by hBN. Gate-dependent STS reveals a substantial energy gap in 1T’-WTe₂ at charge neutrality, but the gap diminishes when the Fermi level is tuned into either the conduction or valence band. STS across the sample edges shows that edge-states persist at all gate voltages, and Fourier-Transform-STM measurement of the bulk states shows an evolution of the bulk band structure with carrier density. We will discuss the comparison of our data with existing theoretical models such as SOC-induced gapping and a proposed excitonic insulator phase.