The spontaneous condensation of excitons in the excitonic insulating phase has been reported in Ta2NiSe5 below 325 K. In this context, we present the temperature-dependent optical pump optical probe spectroscopy of Ta2NiSe5, with a focus on coherent phonon dynamics. In addition to the fast relaxation process involving excitonic recombination, we observe a systematic behavior for the slow relaxation process associated with the relaxation of hot phonons. The asymmetry parameter and cubic anharmonicity of the 3 THz mode demonstrate the structural transition across TC=325 K, whereas the order parameter nature and asymmetry of 2 THz modes reveal its coupling with the excitonic phase of Ta2NiSe5. Coherent phonon modes display less anharmonicity compared to the corresponding Raman modes. Continuous Wavelet Transform (CWT) reveals that the peak time tpeak of phonons is similar for all modes except the 3 THz mode. The temperature dependence of tpeak for the M3 mode exhibits a possible role of excitonic condensate below Tc in the formation of quasiparticle (phonon). CWT analysis supports the time-dependent asymmetry of the M3 mode caused by photoexcited carriers. This study illustrates the role of photoexcited carriers in depicting a structural transition and dressing of coherent phonons and, hence, demonstrating many-body effects.
@article{arxiv.2411.18839,
title = {Ultrafast Dynamics of Coherent Phonon Modes in Excitonic Insulator Ta$_2$NiSe$_5$},
author = {Vikas Arora and Sukanya Pal and Luminita Harnagea and D. V. S. Muthu and A K Sood},
journal= {arXiv preprint arXiv:2411.18839},
year = {2024}
}