Unraveling Structure–Strain–Defect Relationships in Thermopower Modulation of Epitaxial Double Perovskite Oxide

Epitaxial strain engineering in oxide thin films offers a powerful strategy for tuning the electronic and thermoelectric properties by modulating the defect density, electronic band structure, and phonon scattering. In this study, we show that the gradual relaxation of in-plane epitaxial strain in layered double perovskite GdBaCo₂O_5.5+δ thin films grown on SrTiO₃ (001) substrates leads to a pronounced enhancement in thermoelectric power near the critical film thickness, where the epitaxial strain begins to relax, and a structural phase transition begins to occur. Our analysis suggests that this enhancement arises from an increase in carrier effective mass, likely caused by the reconstruction of electronic bands.