Nanoscale and Mesoscale Curvature in Multidomain Ferroelectric Superlattices

Over the years, the combination of strain and electrostatic engineering has proven extremely effective at tuning the properties, inducing new functionality and creating exotic polarization patterns in epitaxial heterostructures of ferroelectric oxides. For example, in compressively strained superlattices composed of ferroelectric and non-ferroelectric oxides, the electrostatic boundary conditions lead to the formation of very dense, ordered nanoscale domains that give rise to highly enhanced and even (locally) negative susceptibilities [1]. Under tensile strain, on the other hand, the tendency to form ferroelastic domain structures while minimizing the depolarizing field can lead to complex, 3-dimensionally ordered ‘domain crystals’ or ‘supercrystals’ [2].<br/>Yet another useful control parameter is curvature, which inherently breaks inversion symmetry and can lead to a range of interesting flexoelectric effects. Unlike strain and electrostatics, however, large curvatures are more difficult to induce and manipulate. We have investigated both strained and free-standing multidomain ferroelectric superlattices with nanoscale and mesoscale curvatures respectively. We demonstrate how the polarization patterns in domain supercrystals lead to periodic modulations of the lattice, inducing large local curvatures with tunable periodicity in the non-ferroelectric spacer layers. We also discuss how the competition between strain and electrostatics leads to mesoscale curvature in free-standing ferroelectric superlattices, which, in turn, results in an usual asymmetry and depth evolution of the domain structure.<br/>[1] P. Zubko et al. Nature 534, 524 (2016).<br/>[2] M. Hadjimichael et al. Nature Materials 20, 495 (2021).