Terahertz control of surface topology probed with subatomic resolution

TL;DR

This study demonstrates a light-induced topological phase transition in WTe₂ via ultrafast terahertz fields, enabling nanoscale control of surface electronic states with potential for advanced device applications.

Contribution

It reveals a novel method to induce and observe topological phase transitions at the atomic scale using terahertz light and surface shear motion in WTe₂.

Findings

Achieved a reversible topological phase transition at the surface of WTe₂.

Observed a 7 pm shift in the top atomic layer during the transition.

Linked electronic structure changes to the annihilation of Fermi arc surface states.

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 WTe$_2$ has gained attention as a likely Weyl semimetal, with topologically protected linear electronic band crossings hosting massless chiral fermions. Here, we demonstrate a topological phase transition facilitated by light-induced shear motion of a single atomic layer at the surface of bulk WTe$_2$, thereby opening the door to nanoscale device concepts. Ultrafast terahertz fields enhanced at the apex of an atomically sharp tip resonantly couple to the key interlayer shear mode of WTe$_2$ 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 $\pm$ 3 picometres 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.