Observation of strong phonon-phonon coupling in one-dimensional van der Waals crystals

The phenomena of pronounced electron-electron and electron-phonon interactions in one-dimensional (1D) systems are ubiquitous, which are well described by frameworks of Luttinger liquid, Peierls instability and concomitant charge density wave. However, the experimental observation of strong phonon-phonon coupling in 1D was not demonstrated. Herein we report the first observation of strong phonon-phonon coupling in 1D condensed matters by using double-walled carbon nanotubes (DWNTs), representative 1D van der Waals crystals, with combining the spectroscopic and microscopic tools as well as the ab initio density functional theory (DFT) calculations. We observe uncharted phonon modes in one commensurate and three incommensurate DWNT crystals, three of which concurrently exhibit strongly-reconstructed electronic band structures. Our DFT calculations for the experimentally observed commensurate DWNT (7,7) @ (12,12) reveal that this new phonon mode originates from a (nearly) degenerate coupling between two transverse acoustic eigenmodes (ZA modes) of constituent inner and outer nanotubes having trigonal and pentagonal rotational symmetry along the nanotube circumferences. Such coupling strongly hybridizes the two phonon modes in different shells and leads to the formation of a unique lattice motion featuring evenly distributed vibrational amplitudes over inner and outer nanotubes, distinct from any known phonon modes in 1D systems. All four DWNTs that exhibit the pronounced new phonon modes show small chiral angle twists, closely matched diameter ratios of 3/5 and decreased frequencies of new phonon modes with increasing diameters, all supporting the uncovered coupling mechanism. Our discovery of strong phonon-phonon coupling in DWNTs open new opportunities for engineering phonons and exploring novel phonon-related phenomena in 1D condensed matters.