Enhanced Seebeck coefficient of quantum-confined electrons in SrTiO ₃/SrTi_0.8Nb_0.2O₃ superlattices

We report two-dimensional Seebeck coefficients (∫S∫2D) of [(SrTi O3) x (Sr Ti0.8 Nb0.2 O3) y] 20 (x=1-60, y=1-20) superlattices, which were grown on the (100) face of insulating LaAl O3 substrates to clarify the origin of the giant ∫S∫2D values of the SrTi O3 superlattices [H. Ohta, Nat. Mater. 6, 129 (2007)]. The ∫S∫2D values of the [(SrTi O3) 17 (Sr Ti0.8 Nb0.2 O3) y] 20 superlattices increased proportionally to y-0.5 and reached 320 μV K-1 (y=1), which is approximately five times larger than that of the Sr Ti0.8 Nb0.2 O3 bulk (∫S∫3D =61 μV K-1). The slope of the log ∫S∫2D -log y plots was -0.5, proving that the density of states in the ground state for SrTi O3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25 nm (16 unit cells of SrTi O3).