Large phonon drag thermopower boosted by massive electrons and phonon leaking in LaAlO/LaNiO/LaAlO heterostructure.

An unusually large thermopower () enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO. The phonon-drag effect, which is not observed in bulk LaNiO, enhances for thin films compressively strained by LaAlO substrates. By a reduction in the layer thickness down to three unit cells and subsequent LaAlO surface termination, a 10 times enhancement over the bulk value is observed due to large phonon drag (), and the contribution to the total occurs over a much wider temperature range up to 220 K. The enhancement originates from the coupling of lattice vibration to the d electrons with large effective mass in the compressively strained ultrathin LaNiO, and the electron-phonon interaction is largely enhanced by the phonon leakage from the LaAlO substrate and the capping layer. The transition-metal oxide heterostructures emerge as a new playground to manipulate electronic and phononic properties in the quest for high-performance thermoelectrics.