Frozen Epitaxial Strain in Complex Oxide Freestanding Membranes.

Transition metal complex oxides exhibit a diverse array of emergent physical properties engineered through epitaxial strain. However, the interpretation of strain-driven phenomena is often obscured by structural modifications─particularly under in-plane tension─which can trigger unexpected departures from bulk behavior. Here, we report the observation of "frozen epitaxial strain" in complex oxide heterostructures, where epitaxial tensile strain induces a unit-cell volume expansion that remains locked even after complete substrate release. Using synchrotron X-ray diffraction, we quantify this irreversible structural response, demonstrating that the lattice fails to recover its bulk-like dimensions in the freestanding state. X-ray photoelectron spectroscopy and first-principles calculations reveal that this expansion is stabilized by a high density of tensile strain-promoted oxygen vacancies. These findings establish a fundamental link between epitaxial tension, defect chemistry, and structural "memory," providing critical design rules for the controlled fabrication of functional freestanding membranes and strain-engineered oxide electronics.