Gradient engineering in functional complex oxide heterostructures

Gradient engineering represents a revolutionary framework for the design of functional complex oxide heterostructures with properties beyond those achievable in homogeneous bulk materials. Emerging from research on strain gradients and flexoelectricity, this paradigm has been extended to include field and chemical gradient strategies. Pioneering research has demonstrated that flexoelectricity can induce ferroelectric-like behavior in centrosymmetric materials and impart various functionalities, including the flexo-photovoltaic effect and functional domain walls. This breakthrough has established strain gradients as powerful design parameters and has inspired the exploration of other gradient types in oxide systems. This article provides a comprehensive review of various types of gradient engineering, including strain, field, and chemical gradient engineering, focusing on their origin, methods of induction, and functionalities. As the field matures, gradient engineering has the potential to advance the development of oxide-based technologies for electronics, energy conversion, and information processing.