Polarization boost and ferroelectricity down to one unit cell in layered Carpy-Galy La${}_2$Ti${}_2$O${}_7$ thin films

Abstract

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${}_n$B${}_n$O${}_{3n+2}$), characterized by 110-oriented perovskite planes interleaved with additional oxygen layers, have been debated as platforms for hosting not only a robust polarization but also multiferroicity and polar metallicity. 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 issue, our study focuses on La${}_2$Ti${}_2$O${}_7$, an $n$=4 (A${}_2$B${}_2$O${}_7$) representative of the Carpy-Galy family, exploring its growth and concurrent phase stability on various substrates under different strain conditions. Remarkably, we demonstrate that a 3% tensile strain from DyScO${}_3$ (100) substrates promotes a controlled layer-by-layer growth mode, while SrTiO${}_3$ (110) and LaAlO${}_3$-Sr${}_2$TaAlO${}_6$ (110), that exert negligible and compressive strains respectively, require post-deposition annealing to achieve similar results. Using scanning probe microscopy, X-ray diffraction, scanning transmission electron microscopy, and polarization switching experiments, we confirm that these films possess exceptional ferroelectric properties, including a polarization of 18 $\mu$C/cm${}^2$ - more than three times higher than previously reported - as well as persistence of ferroelectricity down to a single-unit-cell thickness. This study not only advances our understanding of Carpy-Galy phases in thin films but also lays a foundation for their application in advanced ferroelectric device architectures.

19 pages, 5 main-text figures, 6 figures in Supporting Information

References in corpus (4)

• Restoring the density-gradient expansion for exchange in solids and surfaces
• Generalized gradient approximation for solids and their surfaces
• Atomically flat single terminated oxide substrate surfaces
• Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics

Cited by in corpus (1)

• Revival of Layered Ferroelectrics in Thin Films