Improving 2D-ness to enhance thermopower in oxide superlattices

The thermoelectric performance of a material is determined by the fundamental transport dynamics of itinerant charge carriers and their interactions with the environment. For two-dimensional oxide thermoelectrics, predominantly represented by doped SrTiO₃-based superlattices (SLs), reduced spatial dimensions and increased effective mass are known to enhance thermopower (S). However, because of their large effective Bohr radius resulting from the high dielectric constant, SrTiO₃-based systems have limitations in exhibiting the 2D characteristic. Here, we focus on EuTiO₃as an alternative perovskite platform in which fractional La_xEu_1-xTiO₃/EuTiO₃artificial SLs demonstrate the improvement in 2D nature for the dimensionality-induced enhancement ofS. We observed a quasi-2D thermopowerS_2Dof -950μV K^-1andS_2D/S_3Dof ∼20 resulting from the improved 2D confinement. Thermopower measurements, combined with hybrid density functional theory calculations, show the enhancedSoriginates from the confinement of Ti 3d_xy-orbitals within the La_xEu_1-xTiO₃layers and the associated increase in the 2D density of states. A smaller effective Bohr radius and modified electronic band structures, in conjunction with the presence of the Eu 4f-states in EuTiO₃modified the local electronic potential and strengthened the spatial confinement of Ti 3d-states. This approach to improving the dimensional confinement establishes a small effective Bohr radius and 4f-state assisted 2D confinement provides valuable insights into the design of high-performance applications in artificial oxide SLs.