Subthreshold firing in Mott nanodevices.

Resistive switching, a phenomenon in which the resistance of a device can be modified by applying an electric field, is at the core of emerging technologies such as neuromorphic computing and resistive memories. Among the different types of resistive switching, threshold firing is one of the most promising, as it may enable the implementation of artificial spiking neurons. Threshold firing is observed in Mott insulators featuring an insulator-to-metal transition, which can be triggered by applying an external voltage: the material becomes conducting ('fires') if a threshold voltage is exceeded. The dynamics of this induced transition have been thoroughly studied, and its underlying mechanism and characteristic time are well documented. By contrast, there is little knowledge regarding the opposite transition: the process by which the system returns to the insulating state after the voltage is removed. Here we show that Mott nanodevices retain a memory of previous resistive switching events long after the insulating resistance has recovered. We demonstrate that, although the device returns to its insulating state within 50 to 150 nanoseconds, it is possible to re-trigger the insulator-to-metal transition by using subthreshold voltages for a much longer time (up to several milliseconds). We find that the intrinsic metastability of first-order phase transitions is the origin of this phenomenon, and so it is potentially present in all Mott systems. This effect constitutes a new type of volatile memory in Mott-based devices, with potential applications in resistive memories, solid-state frequency discriminators and neuromorphic circuits.