By now, there have been many demonstrations of resistive switching (“memristive”) devices based on organic, chalcogenide, transition metal-oxide and silicon materials and involving different physical switching mechanisms, such as phase-change transitions and modulation of the ionic concentration profiles by electrical and/or thermal forces. The conductance of some properly engineered memristors can be continuously tuned with relatively large electrical bias, and retained, effectively indefinitely, when small stress is applied. Such analog nonvolatile memory functionality combined with potentially very high device density, achieved by lateral scaling and/or monolithical vertical integration, enable many new exciting applications of memristors in neuromorphic and other types of computing. In my talk, I will review several such applications based on metal-oxide memristors which were a recent focus of my group. I will start with stateful material implication logic, which was originally suggested by HPL group. In our work, we showed that 3D version of this logic allows resolving Feynman grand challenge of implementing 8-bit adder in a volume smaller than 50-nm cube. I will then review our experimental work on memristor-based security primitives, in which we utilized device variations and their nonlinear I-Vs to demonstrate functionality and physical performance superior to those of conventional approaches. Finally, I will discuss Race Logic, a novel computing paradigm in which information is encoded in timing of the signals, so that a particular problem is mapped to a circuit with controllable memristor-based delay elements, and the solution is provided by measuring the time for the injected signals to propagate via circuit. Due to direct encoding of the computation to the underlying physics, Race Logic is extremely energy efficient for a variety of problems, such as bioinformatics and neurocomputing. I will conclude my talk with a summary on the device/material challenges, and potential future work, specific to the discussed applications.
6Optimized chalcogenide medium for inherently activated resistive switching device
Optimized chalcogenide medium for inherently activated resistive switching device
61Ionic structure in liquids confined by dielectric interfaces.
The behavior of ions in liquids confined between macromolecules determines the outcome of many nanoscale assembly processes in synthetic and biological materials such as colloidal dispersions, emulsions, hydrogels, DNA, cell membranes, and proteins. Theoretically, the macromolecule-liquid boundary is often modeled as a dielectric interface and an important quantity of interest is the ionic structure in a liquid confined between two such interfaces. The knowledge gleaned from the study of ionic structure in such models can be useful in several industrial applications, such as in the design of double-layer supercapacitors for energy storage and in the extraction of metal ions from wastewater. In this article, we compute the ionic structure in a model system of electrolyte confined by two planar dielectric interfaces using molecular dynamics simulations and liquid state theory. We explore the effects of high electrolyte concentrations, multivalent ions, dielectric contrasts, and external electric field on the ionic distributions. We observe the presence of non-monotonic ionic density profiles leading to a layered structure in the fluid which is attributed to the competition between electrostatic and steric (entropic) interactions. We find that thermal forces that arise from symmetry breaking at the interfaces can have a profound effect on the ionic structure and can oftentimes overwhelm the influence of the dielectric discontinuity. The combined effect of ionic correlations and inhomogeneous dielectric permittivity significantly changes the character of the effective interaction between the two interfaces.
Read more1Heating power lowering by downscaling the cell dimensions in nanoscale filamentary resistive switching devices
In this work, we theoretically investigate the size dependence of the heat process in thermochemical filamentary resistive switching memories of crossbar structure. The equivalent heat resistance of the system increases with the device dimensions scaled down because of the size-dependent electric and thermal conductivity and geometry configurations. The higher equivalent heat resistance by diminishing the cell sizes induces an enhanced self-heating effect of the filament. It promises lower operation voltage and heating power to trigger the thermally activated dissolution of the filament in RESET process. These results strengthen the advantage of filamentary memories in lateral and longitudinal scaling down technologies where less power consumption has long been urged. Our results also show the opposite dependence of the driven electric field on the linewidth and thickness of the device.
Read moreThermal Transport in Phase Change Memory Materials
Phase change memory uses brief pulses of electrical current to induce phase transitions in chalcogenide material regions with dimensions near or even far below 50 nm. The strongly differing electrical conductivities of the crystalline and amorphous phases allow data storage at densities in excess of terabits per square inch. Nanoscale conduction heat transfer governs the figures of merit in these devices, which include the energy and time required for switching, and has received much attention through both measurements and simulations over the last two decades. This chapter reviews the recent progress on thermal conduction phenomena relevant for phase change memory, including a summary of the physical mechanisms involved as well as the most useful simulation and measurement techniques. Experimental work has focused on separating the intrinsic and boundary resistances of thin film phase change materials, as well as the individual contributions of electrons and phonons to the effective conductivity of the hexagonal crystalline phase. Simulations have focused on improving device geometries and switching characteristics, and continue to need improvements in the areas of crystallization modeling and the impact of phase distribution on electrical and thermal transport. Future research requires a more detailed understanding of electron-phonon coupling and its impact on electrical and thermal conduction in the crystalline phase, as well as greater insight into thermoelectric transport and its impact on device behavior. This progress will be critical for the development of innovative memory strategies including multibit storage.
Read more624Stem-water Potential as a Sensitive Indicator of Water Stress in Prune Trees (Prunus domestica L. cv. French)
The relative sensitivity of plant- and soil-based measures of water availability were compared for prune trees subjected to a range of irrigation regimes under field conditions. Over the growing season, leaf- and stem-water potentials (ψ) measured at midday exhibited clear differences between frequently irrigated trees and unirrigated trees that were growing on stored soil moisture. Stem ψ was less variable than leaf ψ, and the daily variability in stem ψ was closely related to daily variability in evaporative demands, as measured by vapor pressure deficit (VPD). As a result of lower variability, stem ψ reflected the small stress effect of a moderate, 50% soil moisture depletion irrigation interval, whereas leaf ψ did not. The relation between soil water content and estimated orchard evapotranspiration (ET) was influenced by local differences in soil texture within the experimental plot. The relation between stem ψ and ET, however, was not influenced by soil texture and, in addition, was very similar to the relation between stem ψ and leaf stomatal conductance. Both relationships indicated that a 50% reduction in leaf and canopy level water loss characteristics was associated with relatively small reductions (0.5 to 0.6 MPa) in stem ψ. Stem ψ appears to be a sensitive and reliable plant-based measure of water stress in prune and maybe a useful tool for experimental work and irrigation scheduling.
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