First molecular-beam-epitaxy-grown superconducting BaPb_1-xBi_xO₃ (BPBO) thin films and discovery of a new growth mode: element-exchange epitaxy

BaPb_1-xBi_xO₃ (BPBO) is one of the very first tunable high temperature (T_c hovering over 10 K) oxide superconductors, which preluded the famous high T_c cuprate superconductor family by over a decade. This system has a unique standing with many of its own interesting properties bridging the big temperature gap between the two famous doped oxide superconductors, the cuprate superconductors with ambient T_c well over 100 K and the doped SrTiO₃ superconductor with T_c below ~0.5 K. In particular, BPBO has the strongest spin-orbit coupling strength among all known oxide superconductors, so it is one of the strong candidates for topological superconductor. Also, despite this system being known as a BCS superconductor, whether and how its strong spin-orbit coupling and the proximity to the charge-ordered BaBiO₃ system affects the superconductivity is little known. As such, despite the long history of the BPBO system and its potential importance, this system is under-investigated partly due to lack of quality thin film systems.

It is particularly notable that no literature on molecular-beam-epitaxy (MBE)-grown BPBO films exists. Here, we report the very first growth of superconducting BPBO films by MBE. Furthermore, we have discovered a new growth mode, dubbed element-exchange epitaxy, which allows formation of the desired BPBO phase by exchanging Pb in BaPbO₃ films with Bi. Although we achieved similar quality films using both the conventional co-deposition method and the new element-exchange epitaxy, this element-exchange epitaxy mode might be particularly useful when stabilizing otherwise metastable structures in other platforms. The availability of high-quality MBE films will spark renewed interest in this important family of strongly-spin-orbit-coupled tunable oxide superconductors and that the element-exchange epitaxy mode could be also utilized for other families of materials.