Thin Films by Atomic-Scale Interface Engineering*

Ruddlesden-Popper (RP) oxides (A_n+1B_nO_3n+1, n = 1, 2, ...) have been widely studied with tunable properties such as high-T_c superconductivity and colossal magnetoresistance.^[1,2] However, high-quality RP film growth has been disturbed by extended defects, so-called out-of-phase boundaries (OPBs).^[3] Since OPB formation hampers the functionalities of films, OPB suppression is highly required to carry out high-performance RP-based functional devices. ^[4]

Here, we suppressed OPBs in RP thin films using single-terminated LaSrAlO₄ (LSAO) substrates. We developed a surface treatment recipe of LSAO substrates, which exhibited the uniform surface reconstruction having LaO-terminated surfaces. As a model system, the high-T_c cuprates La_1.85Sr_0.15CuO₄ (LSCO) films were employed. On mixed-terminated LSAO surfaces, LSCO films exhibited huge OPB formations. In contrast, on single-terminated LSAO surfaces, the OPBs were suppressed. These OPB-free LSCO films exhibited significantly enhanced superconductivity (T_c^zero ~ 30 K) than the film with huge OPBs (T_c^zero ~ 5 K), under given thickness (~ 6.5 nm). By reflection high-energy electron diffraction (RHEED) and density functional theory (DFT) calculation, we observed that the LSCO stacking sequence can be controlled by LSAO surface termination.