Complex dynamics of memristive circuits: Analytical results and universal slow relaxation.

Networks with memristive elements (resistors with memory) are being explored for a variety of applications ranging from unconventional computing to models of the brain. However, analytical results that highlight the role of the graph connectivity on the memory dynamics are still few, thus limiting our understanding of these important dynamical systems. In this paper, we derive an exact matrix equation of motion that takes into account all the network constraints of a purely memristive circuit, and we employ it to derive analytical results regarding its relaxation properties. We are able to describe the memory evolution in terms of orthogonal projection operators onto the subspace of fundamental loop space of the underlying circuit. This orthogonal projection explicitly reveals the coupling between the spatial and temporal sectors of the memristive circuits and compactly describes the circuit topology. For the case of disordered graphs, we are able to explain the emergence of a power-law relaxation as a superposition of exponential relaxation times with a broad range of scales using random matrices. This power law is also universal, namely independent of the topology of the underlying graph but dependent only on the density of loops. In the case of circuits subject to alternating voltage instead, we are able to obtain an approximate solution of the dynamics, which is tested against a specific network topology. These results suggest a much richer dynamics of memristive networks than previously considered.

Surface trap mediated electronic transport in biofunctionalized silicon nanowires.

Silicon nanowires (SiNWs), fabricated via a top-down approach and then functionalized with biological probes, are used for electrically-based sensing of breast tumor markers. The SiNWs, featuring memristive-like behavior in bare conditions, show, in the presence of biomarkers, modified hysteresis and, more importantly, a voltage memory component, namely a voltage gap. The voltage gap is demonstrated to be a novel and powerful parameter of detection thanks to its high-resolution dependence on charges in proximity of the wire. This unique approach of sensing has never been studied and adopted before. Here, we propose a physical model of the surface electronic transport in Schottky barrier SiNW biosensors, aiming at reproducing and understanding the voltage gap based behavior. The implemented model describes well the experimental I-V characteristics of the device. It also links the modification of the voltage gap to the changing concentration of antigens by showing the decrease of this parameter in response to increasing concentrations of the molecules that are detected with femtomolar resolution in real human samples. Both experiments and simulations highlight the predominant role of the dynamic recombination of the nanowire surface states, with the incoming external charges from bio-species, in the appearance and modification of the voltage gap. Finally, thanks to its compactness, and strict correlation with the physics of the nanodevice, this model can be used to describe and predict the I-V characteristics in other nanostructured devices, for different than antibody-based sensing as well as electronic applications.

Memcomputing with membrane memcapacitive systems.

We show theoretically that networks of membrane memcapacitive systems-capacitors with memory made out of membrane materials-can be used to perform a complete set of logic gates in a massively parallel way by simply changing the external input amplitudes, but not the topology of the network. This polymorphism is an important characteristic of memcomputing (computing with memories) that closely reproduces one of the main features of the brain. A practical realization of these membrane memcapacitive systems, using, e.g., graphene or other 2D materials, would be a step forward towards a solid-state realization of memcomputing with passive devices.

Absorption and injection models for open time-dependent quantum systems.

In the time-dependent simulation of pure states dealing with transport in open quantum systems, the initial state is located outside of the active region of interest. Using the superposition principle and the analytical knowledge of the free time evolution of such a state outside the active region, together with absorbing layers and remapping, a model for a very significant reduction of the computational burden associated with the numerical simulation of open time-dependent quantum systems is presented. The model is specially suited to study (many-particle and high-frequency effects) quantum transport, but it can also be applied to any other research field where the initial time-dependent pure state is located outside of the active region. From numerical simulations of open quantum systems described by the (effective mass) Schrödinger and (atomistic) tight-binding equations, a reduction of the computational burden of about two orders of magnitude for each spatial dimension of the domain with a negligible error is presented.

Dynamic computing random access memory.

The present von Neumann computing paradigm involves a significant amount of information transfer between a central processing unit and memory, with concomitant limitations in the actual execution speed. However, it has been recently argued that a different form of computation, dubbed memcomputing (Di Ventra and Pershin 2013 Nat. Phys. 9 200-2) and inspired by the operation of our brain, can resolve the intrinsic limitations of present day architectures by allowing for computing and storing of information on the same physical platform. Here we show a simple and practical realization of memcomputing that utilizes easy-to-build memcapacitive systems. We name this architecture dynamic computing random access memory (DCRAM). We show that DCRAM provides massively-parallel and polymorphic digital logic, namely it allows for different logic operations with the same architecture, by varying only the control signals. In addition, by taking into account realistic parameters, its energy expenditures can be as low as a few fJ per operation. DCRAM is fully compatible with CMOS technology, can be realized with current fabrication facilities, and therefore can really serve as an alternative to the present computing technology.

Distribution and biokinetic analysis of 210Pb and 210Po in poultry due to ingestion of dicalcium phosphate.

Dicalcium phosphate (DCP) is used as a calcium supplement for food producing animals (i.e., cattle, poultry and pig). When DCP is produced via wet acid digestion of the phosphate rock and depending on the acid used in the industrial process, the final product can result in enhanced (210)Pb and (210)Po specific activities (approximately 2000 Bq.kg(-1)). Both (210)Pb and (210)Po are of great interest because their contribution to the dose received by ingestion is potentially large. The aims of this work are to examine the accumulation of (210)Pb and (210)Po in chicken tissues during the first 42 days of life and to build a suitable single-compartment biokinetic model to understand the behavior of both radionuclides within the entire animal using the experimental results. Three commercial corn-soybean-based diets containing different amounts and sources of DCP were fed to broilers during a period of 42 days. The results show that diets containing enhanced concentrations of (210)Pb and (210)Po lead to larger specific accumulation in broiler tissues compared to the blank diet. Radionuclides do not accumulate homogeneously within the animal body: (210)Pb follows the calcium pathways to some extent and accumulates largely in bones, while (210)Po accumulates to a large extent in liver and kidneys. However, the total amount of radionuclide accumulation in tissues is small compared to the amounts excreted in feces. The single-compartment non-linear biokinetic model proposed here for (210)Pb and (210)Po in the whole animal takes into account the size evolution and is self-consistent in that no fitting parameterization of intake and excretions rates is required.