Terahertz field control of surface topology probed with subatomic resolution

Abstract

Light-induced phase transitions offer a method to dynamically modulate topological states in bulk complex materials. Yet, next-generation devices demand nanoscale architectures with contact resistances near the quantum limit and precise control over local electronic properties. The layered material WTe2 has gained attention as a probable Weyl semimetal, with topologically protected linear electronic band crossings hosting massless chiral fermions. Here we demonstrate a local phase transition facilitated by the light-induced shear motion of a single atomic layer at the surface of bulk WTe2, thereby opening the door to nanoscale device concepts. Ultrafast terahertz fields enhanced at the apex of an atomically sharp tip couple to the key interlayer shear mode of WTe2 via a ferroelectric dipole at the interface, inducing a structural phase transition at the surface to a metastable state. Subatomically resolved differential imaging, combined with hybrid-level density functional theory, reveals a shift of 7 ± 3 pm in the top atomic plane. Tunnelling spectroscopy links electronic changes across the phase transition with the electron and hole pockets in the band structure, suggesting a reversible, light-induced annihilation of the topologically protected Fermi arc surface states in the top atomic layer. A terahertz field exceeding 1 V nm−1 induced a structural phase transition in the top atomic layer of a bulk WTe2 crystal. Differential imaging revealed a surface shift of 7 ± 3 pm and an electronic signature consistent with a topological phase transition.