Observation of superdiffusive phonon transport in aligned atomic chains.

Fascinating phenomena can occur as charge and/or energy carriers are confined in one dimension1-4. One such example is the divergent thermal conductivity (κ) of one-dimensional lattices, even in the presence of anharmonic interatomic interactions-a direct consequence of the Fermi-Pasta-Ulam-Tsingou paradox proposed in 19555. This length dependence of κ, also known as superdiffusive phonon transport, presents a classical anomaly of continued interest6-9. 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 κ extending over 42.5 µm at room temperature for ultrathin van der Waals crystal NbSe3 nanowires. We found that κ 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 κ 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 κ values higher than those of any known materials.

Distinct Signatures of Electron-Phonon Coupling Observed in the Lattice Thermal Conductivity of NbSe3 Nanowires.

The last two decades have seen tremendous progress in quantitative understanding of several major phonon scattering mechanisms (phonon-phonon, phonon-boundary, phonon-defects), as they are the determinant factors in lattice thermal transport, which is critical for the proper functioning of various electronic and energy conversion devices. However, the roles of another major scattering mechanism, electron-phonon (e-ph) interactions, remain elusive. This is largely due to the lack of solid experimental evidence for the effects of e-ph scattering in the lattice thermal conductivity for the material systems studied thus far. Here we show distinct signatures in the lattice thermal conductivity observed below the charge density wave transition temperatures in NbSe3 nanowires, which cannot be recaptured without considering e-ph scattering. Our findings can serve as the cornerstone for quantitative understanding of the e-ph scattering effects on lattice thermal transport in many technologically important materials.