Dimensionality transition and divergent thermal conductivity observed in thin NbSe₃ nanowires

Fundamental understanding of thermal transport remains an intriguing research topic and a classical anomaly is the divergent thermal conductivity of one-dimensional (1D) lattices, a direct consequence of the classical Fermi-Pasta-Ulam-Tsingou (FPUT) paradox. Extensive theoretical and numerical studies have been carried out, and results indicate superdiffusive phonon transport with the thermal conductivity increasing with the chain length. However, solid experimental evidence has been scarce because of the challenge of measuring thermal transport through single atomic chains of sufficient length. Quasi-1D van der Waals (vdW) crystals with covalently bonded atomic chains assembled together via weak vdW interactions provide unique opportunities to probe the divergent thermal conductivity of 1D phonons. Here we report on systematic experimental studies of thermal transport through quasi-1D NbSe₃ nanowires. Results show distinct signatures of electron-phonon scattering in the lattice thermal conductivity of NbSe₃ nanowires as charge density wave phase transition occurs. Interestingly, contrary to the classical size effect, the observed thermal conductivity demonstrates a 25-fold enhancement as the nanowire diameter reduces from 26 to 6.8 nm. Examination of the temperature dependence indicates that 1D phonons dominate the thermal transport at elevated temperatures for thin NbSe₃ nanowires. At room temperature, the thermal conductivity displays a length dependence extending over 42.5 µm and follows a 1/3 power law with the wire length, which provides experimental evidence for superdiffusive transport. Atomic force microscopy characterization discloses a drastic elastic stiffening effect with a five-fold enhancement of the Young’s modulus for ultr-thin nanowires, which triggers the transition to 1D phonon transport.