Superconducting Sr₂RuO₄ Thin Films without Out-of-Phase Boundaries by Higher-Order Ruddlesden-Popper Intergrowth.

Abstract

Ruddlesden-Popper (RP) phases (<i>A</i>_<i>n</i>+1<i>B</i>_<i>n</i>O_3<i>n</i>+1, <i>n</i> = 1, 2,···) have attracted intensive research with diverse functionalities for device applications. However, the realization of a high-quality RP-phase film is hindered by the formation of out-of-phase boundaries (OPBs) that occur at terrace edges, originating from lattice mismatch in the <i>c</i>-axis direction with the <i>A</i>'<i>B</i>'O₃ (<i>n</i> = ∞) substrate. Here, using strontium ruthenate RP-phase Sr₂RuO₄ (<i>n</i> = 1) as a model system, an experimental approach for suppressing OPBs was developed. By tuning the growth parameters, the Sr₃Ru₂O₇ (<i>n</i> = 2) phase was formed in a controlled manner near the film-substrate interface. This higher-order RP-phase then blocked the subsequent formation of OPBs, resulting in nearly defect-free Sr₂RuO₄ layer at the upper region of the film. Consequently, the Sr₂RuO₄ thin films exhibited superconductivity up to 1.15 K, which is the highest among Sr₂RuO₄ films grown by pulsed laser deposition. This work paves the way for synthesizing pristine RP-phase heterostructures and exploring their unique physical properties.