Extreme in-plane phonon blocking in simple chainlike crystals

Taking chainlike $\mathrm{Tl}X{\mathrm{Se}}₂$ $(X=\mathrm{Tl}, \mathrm{In}, \mathrm{Al})$ as examples, we find that weak in-plane atomic interactions in these crystals intrinsically lead to extreme in-plane phonon blocking and heat flux confinement. We explicitly evaluate the propagative and diffusive lattice thermal transports based on the unified theory on top of the fourth-order lattice anharmonicity, and the results manifest that high-frequency phonons make negligible contributions to two channel heat transfer along the in-plane directions. This phenomenon provides a feasible approach for an effective suppression of the overall lattice thermal conductivity $({\ensuremath{\kappa}}_{\mathrm{L}})$. The findings are also applicable to a series of quasi-one-dimensional solids with different crystal structures. This work provides fundamental insight into the understanding and minimization of low ${\ensuremath{\kappa}}_{\mathrm{L}}$ of quasi-one-dimensional crystals by focusing on their harmonic lattice dynamics.