High-pressure phases of van der Waals Weyl semimetal transition metal ditellurides

Previous high-pressure studies about WTe 2 have reported divergent critical pressures for structural and electronic phase transitions, obscuring a comprehensive understanding about the complex quantum phases. In this work, we precisely assign the structural phase evolutions and relevant electronic changes of type-II Weyl semimetals, WTe 2 and MoTe 2 , using various optical methods. We confirm that the T d to 1T′ structural phase transition occurs at about 2.5 GPa concomitantly with the previously reported decrease of magnetoresistance and emergence of superconductivity. Notably, electron-phonon coupling remains intact despite pressure-dependent variations in structural symmetry and atomic bond strengths, providing crucial insights into the origin of superconductivity in transition metal ditellurides. We also demonstrate an additional structural transition at about 10 GPa, possibly to a triclinic 1T″ structure, which has a significant influence on the electronic structure due to intra-layer distortion of atomic positions. Our findings on distinct evolutions of inter-layer and intra-layer structural parameters offer a generic understanding of the relationship between anisotropic bond strength in the van der Waals materials and pressure-dependent structural changes.