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Ultralow Lattice Thermal Conductivity of the Random Multilayer Structure with Lattice Imperfections
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Randomizing the layer thickness of superlattices (SL) can lead to localization of coherent phonons and thereby reduces the lattice thermal conductivity kappa l. In this work, we propose strategies that can suppress incoherent phonon transport in the above random multilayer (RML) structure to further reduce kappa l. Molecular dynamics simulations are conducted to investigate phonon heat conduction in SLs and RMLs with lattice imperfections. We found that interfacial species mixing enhances thermal transport across single interfaces and few-period SLs through the phonon "bridge" mechanism, while it substantially reduces the kappa l of many-period SLs by breaking the phonon coherence. This is a clear manifestation of the transition from incoherent-phonon-dominated to coherent-phonon-dominated heat conduction in SLs when the number of interface increases. In contrast, interfacial species mixing always increases the kappa l of RMLs owing to the dominance of incoherent phonons. Moreover, we found that doping a binary RML with impurities can reduce kappa l significantly, especially when the impurity atom has an atomic mass lower or higher than both of the two base elements. This work reveals the critical effect of lattice imperfections on thermal transport in SLs and RMLs, and provides a unique strategy to hierachically suppress coherent and incoherent phonon transport concurrently.