Experimental realization of a transient highly-metallic inverted-charge-density-wave state in 1<i>T</i>-TaSe₂

Charge density wave (CDW) materials are characterized by a periodic lattice distortion and an associated charge density modulation. A femtosecond laser pulse could coherently excite the amplitude mode, to steer the material into thermally inaccessible states and alter the electron-phonon interactions. Combining time- and angle-resolved photoemission spectroscopy and time-dependent density functional theory simulations, we uncover a new transient inverted CDW state in photoexcited 1T-TaSe₂. This novel CDW state is accessed by over-driving the periodic star-of-David lattice distortion during the amplitude mode oscillation. The dynamic band structure indicates that the Ta 5d band width is maximum for the normal state, while it is reduced for both the usual CDW state and the inverted CDW state, which is associated with a dramatic change in the momentum-dependent electron-phonon coupling. Another important observation is that the inverted CDW state has more density of state at the Fermi level than the normal state. Our results demonstrate how ultrafast lasers can be used to generate unique lattice distortions and manipulate band- and mode-projected electron-phonon couplings.