Altering the electron-phonon coupling by strong photoexcitation in Ta₂NiSe₅

In condensed matter physics, the assumption that electrons and phonons live on distinct energy and time scales fails in the limit where the electron-phonon coupling strength g is much larger than the phonon frequency ω.

We report here a direct signature of deep-strong electron-phonon coupling in Ta₂NiSe₅ using time- and angle-resolved photoemission spectroscopy. At low photoexcitation, we observe the generation of 5 different coherent phonons and the resulting band oscillations are well reproduced by frozen phonon calculations: we are in the perturbative regime. At high photoexcitation, the B_2g phonon mode at 2 THz undergoes a transient renormalization, with an observed softening of more than 20% of its frequency. This partial softening lasts only for a few coherent wiggles, <1.5 ps.

The very large electronic excitation following the light pulse and an extraordinarily high g/ω ratio for that B_2g phonon mode are considered responsible for our observations, which can be modeled by non-perturbative calculations.

We’ll discuss the implications of our findings to the interpretation of the insulating transition of Ta₂NiSe₅, whose origin is still debated between primarily structural or excitonic.