Mode-resolved reciprocal space mapping of electron-phonon interaction in the Weyl semimetal candidate Td-WTe$_2$

TL;DR

This study demonstrates how selective excitation of a shear phonon mode in Td-WTe$_2$ can modulate its electronic structure and potentially induce a topological phase transition, providing real-time experimental insights into electron-phonon interactions.

Contribution

First direct experimental observation of mode-resolved effects of coherent phonons on the electronic structure in Td-WTe$_2$, highlighting the shear mode's role in topological phase control.

Findings

Coherent shear phonon excitation modulates electronic states near Weyl points.

Periodic reduction in spin splitting indicates non-centrosymmetric ground state.

Shear mode-selectivity confirmed by comparison with other phonon modes.

Abstract

The selective excitation of coherent phonons provides unique capabilities to control fundamental properties of quantum materials on ultrafast time scales. For instance, in the presence of strong electron-phonon coupling, the electronic band structure can become substantially modulated. Recently, it was predicted that by this means even topologically protected states of matter can be manipulated and, ultimately, be destroyed: For the layered transition metal dichalcogenide Td-WTe$_2$, pairs of Weyl points are expected to annihilate as an interlayer shear mode drives the crystalline structure towards a centrosymmetric phase. By monitoring the changes in the electronic structure of Td-WTe$_2$ with femtosecond resolution, we provide here direct experimental evidence that the coherent excitation of the shear mode acts on the electronic states near the Weyl points. Band structure data in comparison with our results imply, furthermore, the periodic reduction in the spin splitting of bands near the Fermi energy, a distinct electronic signature of the non-centrosymmetric Td ground state of WTe$_2$. The comparison with higher-frequency coherent phonon modes finally proves the shear mode-selectivity of the observed changes in the electronic structure. Our real-time observations reveal direct experimental insights into electronic processes that are of vital importance for a coherent phonon-induced topological phase transition in Td-WTe$_2$.