Atomic and electronic structures of superconducting BaFe2As2/SrTiO3 superlattices

The nanoscale structural, chemical, and electronic properties of artificial engineered superlattice thin films consisting of superconducting Co-doped BaFe2As2 (Ba-122) and nonsuperconducting SrTiO3 (STO) layers are determined by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy. The bonding of Ba-122/STO occurring between As (Ba) and SrO (TiO2) terminated layers has been identified. The thin STO (3 unit cell) insertion layers are a mixture of cations (Ba, Sr, Fe, and Ti) and rich in oxygen vacancies and the Ba-122 layers (10 unit cell) are free of vertical second phases. Our results explain why these superlattices show anisotropic transport response to an external magnetic field, i.e., strong ab-axis pinning (enhancing critical current density) and no c-axis pinning, which is opposite to single layer Ba-122 thin films. These findings reveal physical and chemical properties of superconducting/nonsuperconducting heterostructures and provide important insights into engineering of superconducting devices.