Unusual structural rearrangement and superconductivity in infinite layer cuprate superlattices

Abstract

Epitaxial stabilization of thermodynamically metastable phases and advances in atomic control of complex oxide thin-film growth can be used effectively for realizing novel phenomena and as an alternative for bulk synthesis under extreme thermodynamic conditions. Here, we investigate infinite layer (IL) based cuprate superlattices, where 7--8 unit cells of ${\mathrm{Sr}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Cu}{\mathrm{O}}_{2}$ (SCCO) are sandwiched between ultrathin spacer layers of $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO), $\mathrm{SrRu}{\mathrm{O}}_{3}$, or $\mathrm{BaCu}{\mathrm{O}}_{2}$ (BCO) and only observe superconductivity in the pure [SCCO/BCO] superlattice (SL) without spacer layers. Apparently, the insertion of an additional STO spacer layer in the latter SL prevents the occurrence of superconductivity. The observed superconductivity in [SCCO/BCO] SL is discussed in terms of a structural model involving the interplay between the $\mathrm{Cu}{\mathrm{O}}_{2}$ plane and the CuO chain similar to the bulk $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ superconductor. The structural origin was found by the identification of a metastable IL-$\mathrm{BaCu}{\mathrm{O}}_{2}$ variant, which deviates highly from its parent bulk crystal structure and exhibits a relatively larger out-of-plane lattice parameter (around $7\phantom{\rule{0.16em}{0ex}}\AA{}$) when sandwiched with SCCO in the form of [SCCO/BCO] SL. However, this variant is absent when STO spacer layers are introduced between SCCO and BCO layers. X-ray absorption spectra of the Cu $L$ edge for BCO exhibits a slightly higher energy satellite peak as compared to the $3{d}^{9}L$ Zhang-Rice character observed in SCCO. This result indicates the existence of contrasting plane and chain-type Cu-O blocks in SCCO and BCO, respectively, which is further corroborated using annular bright field scanning transmission electron microscopy imaging. This work unravels an unexpected structure of BCO which helps in realizing superconductivity in [SCCO/BCO] SL and provides a wider perspective in the growth and design of cuprate-based hybrid structures.