The classical-to-quantum crossover in the strain-induced ferroelectric transition in SrTiO membranes.

Mechanical strain presents an effective control over symmetry-breaking phase transitions. In quantum paraelectric SrTiO, strain can induce ferroelectric order via modification of the local Ti potential energy landscape. However, brittle bulk materials can only withstand limited strain range (~0.1%). Taking advantage of nanoscopically-thin freestanding membranes, we demonstrate an in-situ strain-induced reversible ferroelectric transition in freestanding SrTiO membranes. We measure the ferroelectric order by detecting the local anisotropy of the Ti 3d orbital signature using X-ray linear dichroism at the Ti-K pre-edge, while the strain is determined by X-ray diffraction. With reduced thickness, the SrTiO membranes remain elastic with >1% tensile strain cycles. A robust displacive ferroelectricity appears beyond a temperature-dependent critical strain. Interestingly, we discover a crossover from a classical ferroelectric transition to a quantum regime at low temperatures, which enhances strain-induced ferroelectricity. Our results offer new opportunities to strain engineer functional properties in low dimensional quantum materials and provide new insights into the role of ferroelectric fluctuations in quantum paraelectric SrTiO.