Resolving a THz-induced Phase Transition in WTe₂ on the Sub-Atomic Scale with THz-STM

The discovery of topologically protected states has had a lasting impact on condensed matter research, leading to countless theoretical and experimental discoveries of new topological phases in materials. Amongst these, the transition metal dichalcogenide WTe₂ has been proposed as a candidate type-II Weyl semimetal that hosts Weyl points at the contact points of electron and hole pockets. More recently, an experimental THz pump / ultrafast electron diffraction probe study of WTe₂ has indicated that strong THz fields can drive a structural phase transition from the Weyl semimetal ground state into a trivial semimetal phase through an interlayer shear motion that restores lattice inversion symmetry. However, this study primarily relied on measurements of the lattice symmetry to deduce the topological transition and did not have access to the electronic properties of either phase.<br/>Here, we show that terahertz scanning tunneling microscopy (THz-STM) can both drive the phase transition of WTe₂ via the enhanced THz fields at the STM tip apex and distinguish the electronic phases. We find evidence for the phase transition through THz-induced changes to the local density of states and real-space imaging, both supported by DFT calculations. The spatial contrast between the phases enables us to perform THz-STM imaging with unprecedented spatial resolution – down to the 10 picometer scale – revealing subtle differences in the surface electronic wavefunctions as the atomic positions distort and the lattice planes shift across the transition.<br/>The possibility of finely adjusting the density of states of a material with an ultrafast light field and simultaneously resolving the spatial dependence of the transition on the sub-atomic scale presents a novel way of studying topological phase transitions. Overall, our finding that THz-STM is extremely sensitive to differences between electronic phases is an exciting prospect for further studies of topological materials with THz-driven transitions.