Probing Anisotropic Quasiparticle Dynamics and Topological Phase Transitions in Quasi-1D Topological Insulator ZrTe₅

The transition metal pentatelluride ZrTe₅ exhibits rich lattice-sensitive topological electronic states, and demonstrates great potential in photoelectric and thermoelectric devices. However, a comprehensive investigation of electron-phonon coupling and phonon scattering process remains limited, despite their importance for transport properties and device optimization. Here, the hot carrier dynamics and a 1.15 THz A_g mode coherent phonon in ZrTe₅ are investigated by femtosecond transient spectroscopy across 10-300 K. Notably, polarization-dependent measurements explicitly decouple a strong anisotropic transient response, which is attributed to the effects of excited-state electron relaxation and reflectivity modulation by displacive excited coherent phonons. The temperature dependence of electron relaxation time in ZrTe₅ shows an inflection point, first offering the ultrafast dynamical signature of a temperature-driven Lifshitz transition. At low temperatures, a long-lived electron relaxation component emerges in the transient response, providing possible evidence of topological surface states in ZrTe₅. In addition, the temperature-dependent coherent phonon is also analyzed, revealing that its scattering is dominated by three-phonon interactions and exhibits a relatively long lifetime compared to other modes. This work deepens the understanding of ultrafast processes in ZrTe₅, resolves longstanding questions, paves the way for studying electronic phase transitions, and advances ZrTe₅'s application in optoelectronic and quantum devices.