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 terahertz radiation. The frequency, polarization and temperature dependences of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interactions. According to the researchers their observations will have a profound impact on the field of strongly correlated materials, and in the understanding of the interplay between interactions, density-wave ordering and superconductivity in materials… read more. TECHNICAL ARTICLE
Collective modes of an incommensurate CDW phase. Credit: Nature Materials (2023)Â