Origin of superdiffusive thermal transport in one-dimensional van der Waals atomic chains

As a unique transport regime at the nanoscale, superdiffusive thermal transport has demonstrated its significance in nanophysics and potential applications in nanoscale thermal management. However, notable controversies between theoretical and experimental works persist regarding its origin in one-dimensional (1D) systems. Here we investigate the superdiffusive thermal transport behavior in 1D van der Waals $\mathrm{NbS}{\mathrm{e}}_{3}$ nanowires using molecular dynamics and lattice dynamics simulations combined with a machine learning potential. As a signature of superdiffusive thermal transport, a power-law length-dependent thermal conductivity $\ensuremath{\kappa}\ensuremath{\propto}{L}^{\ensuremath{\beta}}$ is uncovered at small diameters. The effects of phonon confinement and elastic stiffening on superdiffusive thermal transport are elucidated. We demonstrate that the stiffening of the elastic constant only appears along the nanowires, affecting a subset of phonon modes with minimal impact on thermal transport. The low-frequency phonons, which significantly contribute to thermal transport, are predominantly affected by phonon confinement. Our theoretical analysis reveals that phonon confinement enhances the momentum-conserving scattering events, inducing phonon hydrodynamics and enabling the observed superdiffusive thermal transport. Our results provide valuable insights into nanoscale thermal transport, which are crucial for phonon transport and thermal management at the nanoscale.