Ultrafast Dynamics of Coherent Phonon Modes in Excitonic Insulator Ta$_2$NiSe$_5$

Abstract: The spontaneous condensation of excitons in the excitonic insulating phase has been reported in Ta$2$NiSe$_5$ below 325 K. In this context, we present the temperature-dependent optical pump optical probe spectroscopy of Ta$_2$NiSe$_5$, 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 T$_C$=325 K, whereas the order parameter nature and asymmetry of 2 THz modes reveal its coupling with the excitonic phase of Ta$_2$NiSe$_5$. Coherent phonon modes display less anharmonicity compared to the corresponding Raman modes. Continuous Wavelet Transform (CWT) reveals that the peak time t${peak}$ of phonons is similar for all modes except the 3 THz mode. The temperature dependence of t$_{peak}$ for the M3 mode exhibits a possible role of excitonic condensate below T$_c$ 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.