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

Abstract

We have investigated the quasiparticle dynamics and collective excitations in the quasi-one-dimensional material ZrTe5 using ultrafast optical pump-probe spectroscopy. Our time-domain results reveal two coherent oscillations having extremely low energies of ħω1 ∼0.33 meV (0.08 THz) and ħω2 ∼1.9 meV (0.45 THz), which are softened as the temperature approaches two different critical temperatures (∼54 K and ∼135 K). We attribute these two collective excitations to the amplitude mode of photoinduced dynamic charge density waves in ZrTe5 with tremendously small nesting wave vectors. Furthermore, a peculiar quasiparticle decay process associated with the ħω2 mode with a timescale of ∼1–2 ps is found below the transition temperature T* (∼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.