(Invited) Emulating Neurological and Psychological Functions with Solid State Ionic/Electronic Conductors

By proper doping, strontium titanate (STO) can be tuned to be an ionic conductor, an n-type or p-type semiconductor, or a mixed conductor. Memristive devices with a simple structure of Ni/Nb-STO/Ti are fabricated. Owing to the high work function of Ni, a Schottky barrier exists at the Ni/STO interface, while the Ti/STO contact is Ohmic. The resistive switching can be achieved by modulating the Schottky barrier height. Since the Schottky barrier height can be continuously varied by applying electric voltages, the electric conductance of the Ni/Nb-STO/Ti memristive device is continuously adjustable. With the Ni/Nb-STO/Ti memristive device, we achieved: (1) Emulation of the plasticity of synaptic connections between neurons. The synaptic plasticity, especially the spike-timing-dependent plasticity (STDP), is the neurological foundation for many advanced psychological functions. (2) Emulation of the learning/forgetting processes, and the resultant explicit/implicit memories. The Wickelgren’s power law, the best characterization of the forgetting process, is experimentally demonstrated for the forgetting process based on the loss of the explicit memory. In particular, the “time-saving” effect of the implicit memory in the relearning process is also demonstrated. (3) Emulation of the Pavlovian conditioning. The Pavlovian conditioning helps a biological body to prepare for an expected or likely event. The Pavlovian conditioning is systematically demonstrated using the memristive device in the framework of the asymmetric STDP. Three basic features of the Pavlovian conditioning, namely, acquisition, extinction and recovery, are fully implemented in detail. The effects of the temporal relation between conditioned and unconditioned stimuli as well as the time interval between individual training trials on the Pavlovian conditioning are investigated. The resulting change of the response strength, the number of training trials necessary for acquisition and the number of extinction trials are illustrated. Lithium lanthanum titanate (LLTO) is a mixed conductor of lithium ions and electrons; the ionic conduction occurs by the migration of lithium ions, and the electronic conduction occurs primarily via polaron hopping. Memristive devices based on LLTO are fabricated in the structure of Pt/LLTO/Pt. Behavioral plasticity builds up an association between information and an appropriate behavioral response, enabling a biological body to adjust its behavior in response to stimulation from external environments. Habituation is the elementary and ubiquitous form of the behavioral plasticity. Because the physical processes of ion migration and polaron hopping in LLTO are similar to the neuromorphic processes proposed in the dual-process theory of the habituation, the Pt/LLTO/Pt memristor enables a direct emulation of the habituation. More importantly, the nine parametric definitions in neuroscience for the habituation are fully realized. The emulation of various neurological and psychological functions in a single memristive device simplifies the construction of artificial neural networks. Moreover, our works establish a direct relation between solid state ionics, neurology and psychology.