Improving 2D-ness to enhance thermopower in oxide superlattices

This is the authors' abstract. We don't add key points for this paper.

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 SrTiO 3 -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, SrTiO 3 -based systems have limitations in exhibiting the 2D characteristic. Here, we focus on EuTiO 3 as an alternative perovskite platform in which fractional La x Eu 1− x TiO 3 /EuTiO 3 artificial SLs demonstrate the improvement in 2D nature for the dimensionality-induced enhancement of S . We observed a quasi-2D thermopower S 2D of −950 μ V K −1 and S 2D / S 3D of ∼20 resulting from the improved 2D confinement. Thermopower measurements, combined with hybrid density functional theory calculations, show the enhanced S originates from the confinement of Ti 3 d xy -orbitals within the La x Eu 1− x TiO 3 layers 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 f -states in EuTiO 3 modified the local electronic potential and strengthened the spatial confinement of Ti 3 d -states. This approach to improving the dimensional confinement establishes a small effective Bohr radius and 4 f -state assisted 2D confinement provides valuable insights into the design of high-performance applications in artificial oxide SLs.