Massive-to-massless parametric amplification of phasons in an unconventional charge density wave insulator (TaSe₄)₂I

In a mechanism analogous to the Higgs, the collective phase oscillations (phasons) of an incommensurate charge density wave (CDW) may acquire mass through coulomb interactions. While the existence of massive phasons was theorized decades ago, it is only recently that THz emissions from an unconventional CDW insulator, (TaSe₄)₂I, have been detected and linked to this mode. To confirm their presence directly, local charge oscillations need to be measured with femtosecond temporal resolution, which has remained challenging. In this work we use a recently developed multimodal ultrafast scanning tunneling microscope to observe local charge dynamics in (TaSe₄)₂I. We detect coherent current oscillations in the time domain, exhibiting a characteristic temperature dependence that provides direct evidence for a massive longitudinal phason at 0.22 THz. Intriguingly, we also observe an equally intense mode at 0.11THz. Comparisons with complementary optical pump-probe reflectance (OPPR) data from the same sample suggest that the 0.11 THz mode is a parametrically amplified massless transverse phason which competes with and suppresses the amplitudon seen in OPPR at this frequency. This work not only confirms the existence of massive phasons but also uncovers their complex interactions with other modes, setting the stage for future research into dynamic phenomena in quantum materials, including light-induced superconductivity.