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Anomalous transport and phonon renormalization in a chain with transverse and longitudinal vibrations

We study thermal transport in a chain of coupled atoms, which can vibrate in longitudinal as well as transverse directions. The particles interact through anharmonic potentials upto cubic order. The problem is treated quantum mechanically. We first calculate the phonon frequencies self-consistently taking into account the anharmonic interactions. We show that for all the modes, frequencies must have linear dispersion with wave-vector $q$ for small $q$ irrespective of their bare dispersions. We then calculate the phonon relaxation rates $Γ_i(q)$, where $i$ is the polarization index of the mode, in a self-consistent approximation based on second order perturbation diagrams. We find that the relaxation rate for the longitudinal phonon, $Γ_x(q) \propto q^{3/2}$, while that for the transverse phonon $Γ_y(q) \propto q^2$. The consequence of these results on the thermal conductivity $κ(N)$ of a chain of $N$ particles is that $κ(N) \propto N^{1/2}$.