High-temperature charge density wave in rare-earth tetratelluride

Charge density waves (CDWs) are broken-symmetry ground states that are driven by the electron-phonon interaction in quasi one-dimensional (1D) or two-dimensional (2D) metals. The subject has generated considerable interest as it offers important insights into electron-phonon physics, its competition or coexistence with other ground states, as well as its potential role in the phase diagram of high-Tc cuprates. Recently, a CDW state was found in a newly synthesized quasi-2D compound, rare-earth tetratelluride. Here, by high-resolution angle-resolved photoemission spectroscopy, we demonstrate that in the CDW state, the Fermi surface is fully gapped but the gap size is momentum-dependent. Surprisingly, the CDW persists to above 400 K, providing a remarkable system for the room-temperature study of 2D semiconductors with collective electronic states.