Observation of superdiffusive phonon transport in aligned atomic chains

Fascinating phenomena can occur as charge and/or energy carriers are confined in one dimension(1-4). One such example is the divergent thermal conductivity (kappa) of one-dimensional lattices, even in the presence of anharmonic interatomic interactions-a direct consequence of the Fermi-Pasta-Ulam-Tsingou paradox proposed in 1955(5). This length dependence of kappa, also known as superdiffusive phonon transport, presents a classical anomaly of continued interest(6-9). So far the concept has remained purely theoretical, because isolated single atomic chains of sufficient length have been experimentally unattainable. Here we report on the observation of a length-dependent kappa extending over 42.5 mu m at room temperature for ultrathin van der Waals crystal NbSe3 nanowires. We found that kappa follows a 1/3 power law with wire length, which provides experimental evidence pointing towards superdiffusive phonon transport. Contrary to the classical size effect due to phonon-boundary scattering, the observed kappa shows a 25-fold enhancement as the characteristic size of the nanowires decreases from 26 to 6.8 nm while displaying a normal-superdiffusive transition. Our analysis indicates that these intriguing observations stem from the transport of one-dimensional phonons excited as a result of elastic stiffening with a fivefold enhancement of Young's modulus. The persistent divergent trend of the observed thermal conductivity with sample length reveals a real possibility of creating novel van der Waals crystal-based thermal superconductors with kappa values higher than those of any known materials.

Automated summary

This study observed length-dependent thermal conductivity in ultrathin van der Waals crystal NbSe3 nanowires, providing experimental evidence for superdiffusive phonon transport. Contrary to the classical size effect due to phonon-boundary scattering, the observed thermal conductivity showed an enhancement as the characteristic size of the nanowires decreased.