The Quest for High(er) Tc in BaBiO3-Based Heterostructures

Almost all known superconductors with high Tc have layered structure and are quasi two-dimensional (2D) electronically. Among those that are not, hole-doped BaBiO3 has the highest Tc (and also the highest among all oxides other than cuprates). Coincidentally, Bi is a valence-skipping element and a natural candidate for a negative-U center, with the potential of enhancing superconductivity, as often pointed out by Ted Geballe. With this motivation, we have been trying to synthesize quasi-2D embodiments of BaBiO3. The technique we use is atomic-layer-by-layer molecular beam epitaxy (ALL-MBE), which allows for “digital” synthesis of atomically precise heterostructures and superlattices. We have succeeded in synthesizing some novel, artificial, metastable, layered compounds such as Ba3Bi2LaO9. So far, neither has shown superconductivity, but we will persist in trying.