Anomalous Amplitude Mode Dynamics Below the Expected Charge-Density-Wave Transition in 1T-VSe2

A charge-density-wave (CDW) is characterized by a dynamical order parameter consisting of a time-dependent amplitude and phase, which manifest as optically-active collective modes of the CDW phase. Studying the behavior of such collective modes in the time-domain, and their coupling with electronic and lattice order, provides important insight into the underlying mechanisms behind CDW formation. This work reports on femtosecond broadband transient reflectivity experiments of bulk 1T-VSe2 using near-infrared excitation. At low temperature, coherent oscillations associated with the CDW amplitude mode and phonons of the distorted lattice are observed. Across the expected transition temperature at 110 K, signatures of a rearrangement of the electronic structure are evident in the quasiparticle dynamics. However, the amplitude mode instead softens to zero frequency at 80 K, possibly indicating an additional phase transition at this temperature. In addition, photoinduced CDW melting, associated with a collapse of the electronic and lattice order, is found to occur at moderate excitation densities, consistent with a dominant electron-phonon CDW mechanism.