Extremely low-energy collective modes in a quasi-one-dimensional system

We have investigated the quasiparticle dynamics and collective excitations in the quasi-one-dimensional material ZrTe$_5$ using ultrafast optical pump-probe spectroscopy. Our time-domain results reveal two coherent oscillations having extremely low energies of $\hbar\omega_1\sim$0.33 meV (0.08 THz) and $\hbar\omega_2\sim$1.9 meV (0.45 THz), which are softened as the temperature approaches two different critical temperatures ($\sim$54 K and $\sim$135 K). We attribute these two collective excitations to the amplitude mode of charge density wave instabilities in ZrTe$_5$ with tremendously small nesting wave vectors. Furthermore, scattering with the $\hbar\omega_2$ mode may result in a peculiar quasiparticle decay process with a timescale of $\sim$1-2 ps below the transition temperature $T^*$ ($\sim$135 K). Our findings provide pivotal information for studying the fluctuating order parameters and their associated quasiparticle dynamics in various low-dimensional topological systems and other materials.