A first-principles study on the intrinsic asymmetric ferroelectricity of the SrTiO₃-BaTiO₃-CaTiO₃ tricolor superlattice at the nanoscale

We report a systematic theoretical study on the ferroelectric behavior of ultrathin three-component ferroelectric films, e.g., CaTiO₃- BaTiO₃-SrTiO₃, sandwiched between electrodes. Using first-principles calculations we demonstrate that such structures have intrinsic asymmetric ferroelectricity which is robust even at the nanoscale. In addition, there exists a certain relationship between the polarization directions and geometric stacking sequences of the superlattices. Specifically, the lowest energy states always have polarizations pointing from CaTiO₃ via BaTiO₃ to SrTiO₃, while the sequence in the metastable states is SrTiO₃-BaTiO₃-CaTiO₃. Therefore we were able to distinguish one ferroelectric state from its opposite state by means of their geometric stackings along the polarization directions. Besides this, band alignment analysis reveals that such structures are well behaved at the metal/ferroelectric interface, confirming the credibility and reliability of our first-principles calculation. Our finding may suggest a controllable and unambiguous way to build ferroelectric and multiferroic tunnel junctions.