Colloquia

Memristive devices are nonlinear dynamical systems that exhibit continuous, reversible and nonvolatile resistance changes that depend on the polarity, magnitude and duration of an applied electric field. The memristive properties of metal/metal oxide/metal (MOM) materials systems were discovered in the 1960s and studied without reaching a consensus on the physical mechanism, while the theoretical foundation of memristance was derived by Chua in 1971 without realizing there were physical examples of this circuit property. Recent studies on the mechanism revealed that memristive switching is caused by electric field-driven motion of charged dopants that define the interface position between conducting and semiconducting regions of the film. There have also been multiple reports of current-controlled negative differential resistance (CC-NDR) in electroformed MOM devices since the early 1960s (e.g. oxides of V, Nb, Ta, Ti and Fe), and there have been a variety of proposals for the physical mechanism. Current work presents persuasive evidence that CC-NDR in these materials is due to a Joule-heating induced metal-insulator transition (MIT). We have found that both memristance and CC-NDR coexist in many transition metal oxides, and the fact that both effects have been called "switching" has caused a great deal of confusion in the literature and prevented comprehensive understanding of these systems. I will explain the origin of both effects in titanium oxides and show some potential applications of combining the two effects in a single nanoscale device.