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

TL;DR

This study investigates the ultrafast coherent phonon dynamics in Ta$_2$NiSe$_5$, revealing how excitonic condensation influences phonon behavior and structural transitions through temperature-dependent optical spectroscopy.

Contribution

It provides new insights into the coupling between excitonic phases and phonon modes, highlighting the role of photoexcited carriers in structural and many-body effects in Ta$_2$NiSe$_5$.

Findings

Coherent phonon modes show less anharmonicity than Raman modes.

Temperature-dependent peak times suggest excitonic influence on phonon formation.

Structural transition is evidenced by asymmetry parameters and anharmonicity changes.

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.