Ultrafast dynamics of coherent phonon modes in the excitonic insulator Ta2NiSe5

Abstract

The spontaneous condensation of excitons in the excitonic insulating phase has been reported in Ta2NiSe5 (TNSe) below 325 K. In this context, we present the temperature-dependent optical pump–optical probe spectroscopy of Ta2NiSe5, with the focus on coherent phonon (CP) dynamics. In addition to the fast relaxation process involving excitonic recombination, we observe a systematic behaviour 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 nature of the order parameter and asymmetry of 2 THz modes reveal its coupling with the excitonic phase of TNSe. Coherent phonon modes display less anharmonicity than the corresponding Raman modes. Continuous wavelet transform (CWT) reveals that the peak time tpeak of the 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, hence, demonstrating many-body effects.