Raman Spectroscopic and Dynamic Electrical Investigation of Multi-State Charge-Wave-Density Phase Transitions in 1 T-TaS2.

Two-dimensional layered 1 T-TaS2 exhibits rich charge-density-wave (CDW) states with distinct electronic structures and physical properties, leading to broad potential applications, such as phase-transition memories, electrical oscillators and photodetectors. Besides the various CDW ground states at different temperatures, multiple intermediate phases in 1 T-TaS2 have been observed by applying optical and electrical stimulations. Here, we investigated the electric-field-driven multistate CDW phase transition by Raman spectroscopy and voltage oscillations in 1 T-TaS2. Strong correlation was observed between electrical conductivity and intensity of fold-back acoustic and optical phonon modes in 1 T-TaS2. This indicates that the multistate transitions arise from serial transitions, from the nearly commensurate (NC) CDW phase to out-of-equilibrium intermediate states, and finally to the incommensurate (IC) CDW phase. The dynamics of phase transition under an electric field was investigated. As the electrical field increased, the dwell time of different CDW states changed. At lower temperatures, the multistate oscillations disappeared because of higher-energy barriers between the intermediate phases and/or lower thermal excitation energies at lower temperatures.