Fig. 3. Atomic origin and structure of APBs in BM-SCO.

(A) HAADF-STEM image displaying a typical APB where CoO₆ and CoO₄ sublayers are shifted by one half-cell. (B) Atomic-resolution EELS maps verifying the step terrace structure on the Nb-STO surface and the resulting APB. (C) HAADF-STEM image corresponding to the yellow dashed box area in (A) and out-of-plane Sr-Sr atomic distance variations across the APB, highlighting local lattice distortions. (D) Schematic structure model of the APB interface, showing the octahedral-to-tetrahedral sublayer transition. (E and F) Density of states (DOS) calculations comparing the APB region to a perfect crystal, revealing a bandgap reduction (from ~1 to ~0.3 eV) that could enable two-dimensional conduction pathways.