Epitaxial Growth and Strain-Dependent Phase Stability of Layered Carpy-Galy A₂B₂O₇ Films for Polar Metallicity

Layered perovskite-based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. While most renowned for the superconductivity observed in the Ruddlesden-Popper phases, many of these layered compounds are also ferroelectric and exhibit a sizeable in-plane polarization. Among these, the Carpy-Galy phases A_nB_nO_3n+2^1,2 characterized by 110-oriented perovskite planes interleaved with additional oxygen layers have been debated as platforms for hosting both metallicity and polar displacements³. However, the challenges associated with the synthesis of ultrathin Carpy-Galy films and understanding the impact of strain on their properties limit their integration into devices. Addressing this, our study focuses on La₂Ti₂O₇, an n=4 representative of the Carpy-Galy family, exploring its growth with pulsed laser deposition and concurrent phase stability under different epitaxial strain conditions. Our approach includes in-situ monitoring that enables us to delineate the coexistence of the targeted layered phase, the parent perovskite phase, and other kinetically favored growth polymorphs. Remarkably, we demonstrate that a 3% tensile strain from DyScO₃ (100) substrates promotes a controlled layer-by-layer growth mode, while SrTiO₃ (110) and LSAT (110) that exert negligible and compressive strains, respectively, require post-deposition annealing to achieve similar results. We show polarization enhanced by a factor of four compared to bulk crystals, map the polarization displacements using scanning transmission electron microscopy, and emphasize the significant role of epitaxial strain in achieving optimal properties. Lastly, we show that the metallicity in Carpy-Galy films can be induced in a controlled fashion through interfacial redox processes with a reactive metal, laying the foundation for exploring the Carpy-Galy phases as functional polar metals.

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2. Núñez Valdez, M. et al. Polyhedron 171, 181 (2019).
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