Observation of the massive Lee-Fukuyama phason in a charge density wave insulator (TaSe₄)₂I

The lowest-lying fundamental excitation of an incommensurate charge density wave (CDW) material is widely believed to be a massless phason, i.e. a collective modulation of the phase of the CDW order parameter. However, as first pointed out by Lee and Fukuyama in 1978, long-range Coulomb interactions should push the phason energy up to the plasma energy of the CDW condensate, making the phason massive and a fully gapped spectrum. Whether such behavior occurs in a CDW system has been unresolved for more than four decades. Using time-domain THz emission spectroscopy, we investigate this issue in the material (TaSe₄)₂I, a classical example of a quasi-one-dimensional CDW insulator. Upon transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrow-band THz radiation. The frequency, polarization and temperature-dependence of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interaction (U). Our observations constitute the first direct evidence of the massive "Lee-Fukuyama" phason and highlight the potential applicability of fundamental collective modes of correlated materials as compact and robust sources of THz radiation.