Experimental observation of phonon- and band-specific electron-phonon coupling in a topological semi-metal

Light excitation of quantum materials can drive phase transitions such as insulator-to-metal transitions or the melting of magnetic orders. It can also lead to ultrafast topological phase transitions, as it was demonstrated recently in the topological semi-metal WTe₂ using ultrafast electron diffraction [1]. The photoinduced transition in this system occurs when a shear phonon mode is driven sufficiently strongly to recover the inversion symmetry of the lattice. In this context we studied the electron dynamics of WTe₂ using time- and angle-resolved photoemission spectroscopy. We report the observation of band oscillations at four different frequencies that are due to A_1g coherent phonon modes and include the shear mode causing the photoinduced topological transition. By isolating the effect of each phonon on the different electronic bands, we reveal the complexity of electron-phonon coupling in the electronic band structure.

[1] E. J. Sie, et al. Nature 565, 61 (2019).