Engineering the Phonon Transport through Structural Phase Transitions in Oxide Superlattices

A superlattice (SL) composed of different materials can exhibit multi-electronic and phononic properties, inherited from each layer within the SL. In particular, the change of interface number within SLs can facilitates an achievement of ultra-low thermal conductivity κ along the cross-plane, which can lead to a large thermoelectric figure of merit. However, the wave interference effect induced by a coherent phonon transport along the artificial periodicity limits the lowest value of κ. In this study, we demonstrate a method to reduce the coherent nature of phonon transport in SLs by adjusting the phonon mean free path λ through structural phase transitions. In SLs composed of metallic SrRuO₃ (SRO) and insulating SrTiO₃ (STO), we clearly observed the coherent-incoherent crossover of the phonon transport at room temperature, being correlated with the structural symmetry change within the SL. With decreasing temperature, we identified complex structural phase transitions in the STO layer accompanying the reduction of λ, leading to the weakening of phonon coherence and a further decrease of the κ in SLs compared to the bulk limit. Our findings can be an important guide to potentially overcome limitations in achieving ultra-low κ, thereby contributing to the development of highly efficient thermoelectric devices.