Tuning coherent-phonon heat transport in oxide superlattices.

Accessing the regime of coherent phonon propagation in nanostructures opens new possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. Here we discuss the contribution of coherent phonons to the thermal conductivity of LaCoO₃/SrTiO₃ and PbTiO₃/SrTiO₃ superlattices, from 25K to 300 K, and over a wide range of periods and thicknesses. We demonstrate that the contribution of coherent phonons to these superlattices is relevant in the whole period length and can be substantially reduced by small variations of the periodicity. Actually, by using slightly aperiodic strucures, we have being able to reduce the thermal conductivity of the superlattices by 20%, at room temperature. This may have an interesting application in the development of low thermal conductivity devices, in which maintaining a relatively large thickness and clean interfaces is important for not deteriorating electrical transport, as in thermoelectrics. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices, as well as the effect of cooperative ferroelectric domains in PbTiO₃/SrTiO₃ superlattices.