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Pressure-Induced Exciton States Mediate Coherent Phonon Dynamics in Ta2NiSe5
Xiaowei Wang1,2, Guangming Niu1, Xin Liu1,3
1State Key Laboratory of Chemical Reaction Dynamics and Dalian Coherent Light Source, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
Abstract:
Ta2NiSe5 is a room-temperature excitonic insulator (EI) with strong electron-phonon coupling. However, the evolution of this coupling across the phase transition remains elusive. Here we investigated the nonequilibrium dynamics of Ta2NiSe5 under pressure using time-resolved reflectance spectroscopy (TRS). Below 1 GPa, the 2 THz order-parameter-coupled coherent phonon (OPCCP) exhibits an extended coherence lifetime, which indicates the protection of the exciton condensate and is consistent with the rapid recovery of the EI phase. In contrast, the onset of the OPCCP is delayed at higher pressures due to inefficient energy transfer mediated by preformed excitons. Furthermore, the presence of quasi-elastic scattering in Raman spectra and the enhancement of the 2 THz mode asymmetry factor 1/|q| suggest that the system enters a preformed exciton-dominated regime at higher pressures. This study reveals the regulatory effects of different exciton states on coherent phonons.
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