Super diffusive length dependent thermal conductivity in one-dimensional materials with structural defects: longitudinal to transverse phonon scattering leads to $\kappa \propto L^{1/3}$ law

Abstract

Structural defects in one-dimensional heat conductors couple longitudinal (stretching) and transverse (bending) vibrations. This coupling results in the scattering of longitudinal phonons to transverse phonons and backwards. We show that the decay rate of longitudinal phonons due to this scattering scales with their frequencies as $\omega^{3/2}$ within the long wavelength limit ($\omega \rightarrow 0$), which is more efficient scattering compared to the traditionally considered Rayleigh scattering within the longitudinal band ($\omega^2$). This scattering results in temperature independent thermal conductivity depending on the size as $\kappa \propto L^{1/3}$ for sufficiently long materials. This predicted length dependence is observed in nanowires, though the temperature dependence is seen there possibly because of deviations from pure one-dimensional behavior. The significant effect of interaction of longitudinal phonons with transverse phonons is consistent with the earlier observations of a substantial suppression of thermal energy transport by kinks, obviously leading to such interaction, though anharmonic interaction can also be significant.