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Perception, decisions, and sensations are all encoded into trains of action potentials in the brain. The relation between stimulus strength and all-or-nothing spiking of neurons is widely believed to be the basis of this coding. This initiated the development of spiking neuron models; one of today's most powerful conceptual tool for the analysis and emulation of neural dynamics. The success of electronic circuit models and their physical realization within silicon field-effect transistor circuits lead to elegant technical approaches. Recently, the spectrum of electronic devices for neural computing has been extended by memristive devices, mainly used to emulate static synaptic functionality. Their capabilities for emulations of neural activity were recently demonstrated using a memristive neuristor circuit, while a memristive neuron circuit has so far been elusive. Here, a spiking neuron model is experimentally realized in a compact circuit comprising memristive and memcapacitive devices based on the strongly correlated electron material vanadium dioxide (VO2) and on the chemical electromigration cell Ag/TiO2-x /Al. The circuit can emulate dynamical spiking patterns in response to an external stimulus including adaptation, which is at the heart of firing rate coding as first observed by E.D. Adrian in 1926.

In this work we present a phenomenological model for synaptic plasticity suitable to describe common plasticity measurements of memristive devices. We show evidence that the presented model is basically compatible with advanced biophysical plasticity models, which account for a large body of experimental data on spike-timing-depending plasticity (STDP) as an asymmetric form of Hebbian learning. The basic characteristics of our model are a saturation of the synaptic weight growth and a weight dependent learning rate. Moreover, it accounts for common resistive switching behaviors of memristive devices under voltage pulse application and allows to study essential requirements of individual memristive devices for the emulation of Hebbian plasticity in neuromorphic circuits. In this respect, memristive devices based on mixed ionic/electronic and one exclusively electronic mechanism are explored. The ionic/electronic devices consist of the layer sequence metal/isolator/metal and represent today's most popular devices. The electronic device is a MemFlash-cell which is based on a conventional floating gate transistor in a diode configuration wiring scheme exhibiting a memristive (pinched) I-V characteristic.

Among recently discovered ferroelectricity-related phenomena, the tunnelling electroresistance (TER) effect in ferroelectric tunnel junctions (FTJs) has been attracting rapidly increasing attention owing to the emerging possibilities of non-volatile memory, logic and neuromorphic computing applications of these quantum nanostructures. Despite recent advances in experimental and theoretical studies of FTJs, many questions concerning their electrical behaviour still remain open. In particular, the role of ferroelectric/electrode interfaces and the separation of the ferroelectric-driven TER effect from electrochemical ('redox'-based) resistance-switching effects have to be clarified. Here we report the results of a comprehensive study of epitaxial junctions comprising BaTiO(3) barrier, La(0.7)Sr(0.3)MnO(3) bottom electrode and Au or Cu top electrodes. Our results demonstrate a giant electrode effect on the TER of these asymmetric FTJs. The revealed phenomena are attributed to the microscopic interfacial effect of ferroelectric origin, which is supported by the observation of redox-based resistance switching at much higher voltages.

Bipolar switching behaviours of electrochemical metallization (ECM) cells with dual-layer solid electrolytes (SiOx-Ge0.3Se0.7) were analyzed. Type 1 ECM cell, Pt (bottom electrode)/SiOx/Ge0.3Se0.7/Cu (top electrode), exhibited typical eightwise current-voltage (I-V) hysteresis of ECM cells whereas Type 2 ECM cell, Pt (bottom electrode)/Ge0.3Se0.7/SiOx/Cu(top electrode), showed counter-eightwise hysteresis. In addition, absolute off-switching voltage in Type 2 cell is lower than that in Type 1 cell while on-switching voltage in both cells is almost the same. An attempt to understand this electrolyte-stack-sequence-depending switching polarity reversal was made in terms of the ECM cell potential change upon the electrolyte stack sequence and the consequent change in Cu filament growth direction. Relevant experimental evidence for the hypothesis was obtained regarding the switching behaviours. Furthermore, given the switching polarity reversal, feasibility of serial complementary resistive switches was also demonstrated.

Ferroelectric nanodomains were created in BaTiO(3) thin films by applying a voltage to a sharp conducting tip of a scanning force microscope (SFM). The films were epitaxially grown on SrRuO(3)-covered (001)-oriented SrTiO(3) substrates by a high-pressure sputtering. They appeared to be single-crystalline with the (001) crystallographic orientation relative to the substrate. Using the piezoresponse mode of the SFM to detect the out-of-plane film polarization, the domain sizes were measured as a function of the applied writing voltage and the pulse time. It was found that the time dependence of the domain diameter in a 60 nm thick BaTiO(3) film deviates significantly from the logarithmic law observed earlier in Pb(Zr(0.2)Ti(0.8))O(3) (PZT) films. At a given writing time, the domain size increases nonlinearly with increasing applied voltage, in contrast to the linear behavior reported earlier for PZT films and LiNbO(3) single crystals. The dynamics of domain growth is analyzed theoretically taking into account the strong inhomogeneity of the external electric field in the film and the influence of the bottom electrode. It is shown that the observed writing time and voltage dependences of the domain size can be explained by the domain-wall creep in the presence of random-bond disorder.

We fabricated high quality Nb/Al2O3/Ni(0.6)Cu(0.4)/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microm (1.3lambda(J)) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying approximately 6.7% of the magnetic flux quantum Phi(0). The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.

The operation of a novel, nonvolatile memory device based on a conductive ferroelectric/semiconductor thin film multilayer stack is simulated numerically. The simulation involves the self-consistent steady-state solution of the transport equation for electrons assuming a drift-diffusion transport mechanism and the Poisson equation. Special emphasis is put on the screening of the spontaneous polarization by conduction electrons as a function of the applied voltage. Depending on the orientation of the polarization in the ferroelectric layer, a high and a low resistive state are found, giving rise to a hysteretic I-V characteristic. The switching ratio, ranging from > 50% to several orders of magnitude, is calculated as a function of the dopant content. The suggested model provides one possible physical explanation of the I-V hysteresis observed for single-layer ferroelectric devices, if interfacial layers are taken into consideration. The approach will allow one to develop guidelines to improve the performance of these devices.