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 (<i>S</i>). 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<i>_x</i>Eu_1-<i>x</i>TiO₃/EuTiO₃artificial SLs demonstrate the improvement in 2D nature for the dimensionality-induced enhancement of<i>S</i>. We observed a quasi-2D thermopower<i>S</i>_2Dof -950<i>μ</i>V K^-1and<i>S</i>_2D/<i>S</i>_3Dof ∼20 resulting from the improved 2D confinement. Thermopower measurements, combined with hybrid density functional theory calculations, show the enhanced<i>S</i>originates from the confinement of Ti 3<i>d_xy</i>-orbitals within the La<i>_x</i>Eu_1-<i>x</i>TiO₃layers 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 4<i>f</i>-states in EuTiO₃modified the local electronic potential and strengthened the spatial confinement of Ti 3<i>d</i>-states. This approach to improving the dimensional confinement establishes a small effective Bohr radius and 4<i>f</i>-state assisted 2D confinement provides valuable insights into the design of high-performance applications in artificial oxide SLs.