Phys.org  March 9, 2023 The lowest-lying fundamental excitation of an incommensurate charge-density-wave material is believed to be a massless phason—a collective modulation of the phase of the charge-density-wave order parameter. However, long-range Coulomb interactions should push the phason energy up to the plasma energy of the charge-density-wave condensate, resulting in a massive phason and fully gapped spectrum. A team of researchers in the US (University of Illinois, SLAC National Accelerator Laboratory) used time-domain terahertz emission spectroscopy, to investigate this issue in (TaSe4)2I, a quasi-one-dimensional charge-density-wave insulator. On transient photoexcitation at low temperatures, they found the material strikingly emits coherent, narrowband […]