  The insulating state is one of the most basic electronic phases in condensed
matter. This state is characterised by an energy gap for electronic excitations
that makes an insulator electrically inert at low energy. However, for complex
oxides, the very concept of an insulator must be re-examined. Complex oxides
behave differently from conventional insulators such as SiO2, on which the
entire semiconductor industry is based, because of the presence of multiple
defect levels within their band gap. As the semiconductor industry is moving to
such oxides for high-dielectric (high-k) materials, we need to truly understand
the insulating properties of these oxides under various electric field
excitations. Here we report a new class of material called nonlinear insulators
that exhibits a reversible electric-field-induced metal-insulator transition.
We demonstrate this behaviour for an insulating LaAlO3 thin film in a
metal/LaAlO3/Nb-SrTiO3 heterostructure. Reproducible transitions were observed
between a low-resistance metallic state and a high-resistance non-metallic
state when applying suitable voltages. Our experimental results exclude the
possibility that diffusion of the metal electrodes or oxygen vacancies into the
LaAlO3 layer is occurring. Instead, the phenomenon is attributed to the
formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that
forms a continuum state with the conduction band of the Nb-SrTiO3. Once this
continuum (metallic) state is formed, the state remains stable even when the
voltage bias is turned off. An opposing voltage is required to deplete the
charges from the defect states. Our ability to manipulate and control these
defect states and, thus, the nonlinear insulating properties of complex oxides
will open up a new path to develop novel devices.
