Auxetic piezoelectric effect in heterostructures.

Inherent symmetry breaking at the interface has been fundamental to a myriad of physical effects and functionalities, such as efficient spin-charge interconversion, exotic magnetic structures and an emergent bulk photovoltaic effect. It has recently been demonstrated that interface asymmetry can induce sizable piezoelectric effects in heterostructures, even those consisting of centrosymmetric semiconductors, which provides flexibility to develop and optimize electromechanical coupling phenomena. Here, by targeted engineering of the interface symmetry, we achieve piezoelectric phenomena behaving as the electrical analogue of the negative Poisson's ratio. This effect, termed the auxetic piezoelectric effect, exhibits the same sign for the longitudinal (d 33 ) and transverse (d 31 , d 32 ) piezoelectric coefficients, enabling a simultaneous contraction or expansion in all directions under an external electrical stimulus. The signs of the transverse coefficients can be further tuned via in-plane symmetry anisotropy. The effects exist in a wide range of material systems and exhibit substantial coefficients, indicating potential implications for all-semiconductor actuator, sensor and filter applications.