Tunable strain effect and ferroelectric field effect on the electronic transport properties of La _0.5Sr _0.5CoO ₃ thin films

Tensiled La _0.5Sr _0.5CoO ₃ (LSCO) thin films were epitaxially grown on piezoelectric 0.67Pb (Mg ₁₃Nb _2/3)O ₃-0.33PbTiO ₃ (PMN-PT) single-crystal substrates. Due to the epitaxial nature of the interface, the lattice strain induced by ferroelectric poling or the converse piezoelectric effect in the PMN-PT substrate is effectively transferred to the LSCO film and thus reduces the tensile strain of the film, giving rise to a decrease in the resistivity of the LSCO film. We discuss these strain effects within the framework of the spin state transition of Co ³⁺ ions and modification of the electronic bandwidth that is relevant to the induced strain. By simultaneously measuring the strain and the resistivity, quantitative relationship between the resistivity and the strain was established for the LSCO film. Both theoretical calculation and experimental results demonstrate that the ferroelectric field effect at room temperature in the LSCOPMN-PT field-effect transistor is minor and could be neglected. Nevertheless, with decreasing temperature, the ferroelectric field effect competes with the strain effect and plays a more and more important role in influencing the electronic transport properties of the LSCO film, which we interpreted as due to the localization of charge carriers at low temperature.