Theoretical study on piezoresponse of ultrathin ferroelectric films

Piezoelectric effect is crucial in some nano devices, but it usually decreases with the reduction of film thickness. A comprehensive study of the nonlinear piezoresponse under an applied voltage has been carried out within the framework of the Landau phenomenological theory. For expitaxial heterostructures, polarization exists even below the critical thickness of a few atomic layers due to the large compressive strain provided by the substrate. Piezoelectric coefficient could be very large in the vicinity of the critical thickness due to the appearance of a dielectric susceptibility peak at the phase transition point. Under an applied voltage, the susceptibility peak is reduced and broadened, accompanying a nonlinear piezoresponse even below the critical thickness.