Abstract
An unusually large thermopower (S) enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO3. The phonon-drag effect, which is not observed in bulk LaNiO3, enhances S for thin films compressively strained by LaAlO3 substrates. By a reduction in the layer thickness down to three unit cells and subsequent LaAlO3 surface termination, a 10 times S enhancement over the bulk value is observed due to large phonon drag S (Sg), and the Sg contribution to the total S occurs over a much wider temperature range up to 220 K. The Sg enhancement originates from the coupling of lattice vibration to the d electrons with large effective mass in the compressively strained ultrathin LaNiO3, and the electron–phonon interaction is largely enhanced by the phonon leakage from the LaAlO3 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.
Highlights
An unusually large thermopower (S) enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO3
The atomic-scale control of thin-film heterostructures based on transition-metal oxides (TMOs) is a fruitful way to elicit exotic electronic properties that are not accessible in bulk materials.[1−3] By control of the layer thickness and chemical composition, band structures and electronic correlations can be largely tailored, which allows a systematic tuning of the electronic properties such as the metal−insulator transitions, magnetic order, and superconductivity
We report the discovery of an unusually large thermopower enhancement by a phonon drag effect in ultrathin compressively strained LaNiO3 (LNO) films, which is further enhanced by phonon leaking with large penetration depth due to a heterostructure composed of similar perovskitetype structure oxides: LaAlO3 (LAO) capping layer/LNO ultrathin film/LAO substrate
Summary
M.K., X.H., Ta.K., T.T., J.M.T., M.M., R.A., H.Y., K.I., S.U., H.K., and To.K. contributed to the film growth, characterization, and DFT calculations of LaNiO3 films and heterostructures. All authors discussed the results and commented on the study. Ta.K. and To.K. proposed the idea and supervised the entire project
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