Anisotropic heat conduction of coherently transported phonons in InGaO3(ZnO)m single crystal films with superlattice structures

Superlattices provide a great platform for studying coherent transportation of low-frequency phonons, which are the main issues in mastering the manipulation of heat conduction. Studies have shown that the dominating characteristics in the thermal conductivity of superlattice can be adjusted between wave-like and particle-like phonon properties depending on the superlattice period. However, the phonon coherence length and the phonon mean free path from Umklapp processes have not been defined in one superlattice system, and the transition from wave-like and particle-like behavior is not clear to date despite the extensive research efforts. In this study, we use InGaO3(ZnO)m (m = integer) single crystal films with superlattice structure to experimentally characterize the phonon coherence length as well as the Umklapp mean free path in one system. According to the results, the nature of heat conduction in superlattice can change in three different ways depending on the ratio between the phonon coherence length and the superlattice period. We also discuss the role of the phonon characteristic lengths in the heat conduction of superlattices and its anisotropy.