Multilevel States Research Articles

Highly reliable multilevel resistive switching in a nanoparticulated In2O3 thin-film memristive device

The present report deals with the development of a cost-effective, solution-processable and nanoparticulated In2O3 thin-film memristive device for application in multilevel resistive random access memory (RRAM). The structural, morphological, elemental and electrical characterizations are carried out with the help of x-ray diffractometry, scanning electron microscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy and a memristor characterization system. The electrical characterizations reveal that the nanoparticulated In2O3 thin-film memristive device is free from the electroforming process and shows self-rectifying and self-compliance memory effects. Furthermore, we have demonstrated multilevel resistive switching with excellent memory properties of endurance (10 000 switching cycles) and retention (2 ks). Interestingly, the device shows a good memory window (10 for all cases), and multilevel resistance states are not degraded during endurance and retention memory tests. The uniformity and stability in multilevel resistive switching are further confirmed by statistical calculations. Furthermore, the nanoparticulated In2O3 thin-film memristive device mimics the property of paired-pulse facilitation of a biological synapse. A detailed analysis of electrical results suggests that the Schottky effect is responsible for the device conduction. In a nutshell, the nanoparticulated In2O3 memristive device is suitable for future multilevel RRAM application and is a promising candidate for development of an electronic synaptic device for application in neuromorphic computing.

Compliance-current-modulated resistive switching with multi-level resistance states in single-crystalline LiNbO3 thin film

Resistive switching behavior of a ca. 600 nm thick single-crystalline LiNbO3 (LNO) film has been investigated after vacuum-annealing. Oxygen vacancies (OVs) were generated in the LNO thin film during the annealing process. After electro-forming, filamentary resistive switching has been observed, and the performance of switching can be tuned by the compliance current level. Multi-level resistance states including four different low resistance states, were realized by setting different compliance currents, revealing that both concentration of OVs within the conductive filament and the geometry of the conductive filament influence the switching behavior. The conduction mechanisms of the charge transport during switching is discussed based on the current-voltage curves.

Spin-orbit-torque-driven multilevel switching in Ta/CoFeB/MgO structures without initialization

Spin-orbit torque (SOT) has been proposed as an alternative writing mechanism for the next-generation magnetic random access memory (MRAM), due to its energy efficiency and high endurance in perpendicular magnetic anisotropic materials. However, the three-terminal structure of SOT-MRAM increases the cell size and consequently limits the feasibility of implementing high density memory. Multilevel storage is a key factor in the competitiveness of SOT-MRAM technology in the nonvolatile memory market. This paper presents an experimental characterization of a multilevel SOT-MRAM cell based on a perpendicularly magnetized Ta/CoFeB/MgO heterostructure and addresses the initialization-free issue of multilevel storage schemes. Magneto-optical Kerr effect microscopy and micromagnetic simulation studies confirm that the multilevel magnetization states are created by changing a longitudinal domain wall pinning site in the magnet. The realization of robust intermediate switching levels in the commonly used perpendicularly magnetized Ta/CoFeB/MgO heterostructure provides an efficient way to switch magnets for low-power, high-endurance, and high-density memory applications.

Polymer Analog Memristive Synapse with Atomic-Scale Conductive Filament for Flexible Neuromorphic Computing System.

With the advent of artificial intelligence (AI), memristors have received significant interest as a synaptic building block for neuromorphic systems, where each synaptic memristor should operate in an analog fashion, exhibiting multilevel accessible conductance states. Here, we demonstrate that the transition of the operation mode in poly(1,3,5-trivinyl-1,3,5-trimethyl cyclotrisiloxane) (pV3D3)-based flexible memristor from conventional binary to synaptic analog switching can be achieved simply by reducing the size of the formed filament. With the quantized conductance states observed in the flexible pV3D3 memristor, analog potentiation and depression characteristics of the memristive synapse are obtained through the growth of atomically thin Cu filament and lateral dissolution of the filament via dominant electric field effect, respectively. The face classification capability of our memristor is evaluated via simulation using an artificial neural network consisting of pV3D3 memristor synapses. These results will encourage the development of soft neuromorphic intelligent systems.

Physical Unclonable Function: Multilevel States of Nano-Electromechanical Switch for a PUF-Based Security Device (Small 3/2019)

Physical Unclonable Function: Multilevel States of Nano-Electromechanical Switch for a PUF-Based Security Device (Small 3/2019)

Electric-field-controllable nonvolatile multilevel resistance switching of Bi0.93Sb0.07/PMN-0.29PT(111) heterostructures

Electric-field switchable multilevel nonvolatile resistance states are achieved at room temperature in Bi0.93Sb0.07/0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3(111) (PMN-0.29PT) heterostructures. During the initial poling of the PMN-0.29PT, the variation of the resistance of the Bi0.93Sb0.07 film with the electric field tracks the variation of the electric-field-induced in-plane strain of the PMN-0.29PT effectively, revealing that the resistance switching is dominated by the ferroelectric-domain-switching-induced lattice strain but not the domain-switching-induced polarization charges. A relative resistance change ΔR/R ∼ 7% at 300 K and up to ∼10% at 180 K were achieved near the coercive field EC of the PMN-0.29PT(111) substrate. At least five stable resistance states with good endurance properties could be obtained at room temperature by precisely controlling the electric-field pulse sequence as a result of the nonvolatile remnant strain transferring from the PMN-0.29PT to the film, providing a simple and energy efficient way to construct multistate resistive memory.

Multilevel States of Nano-Electromechanical Switch for a PUF-Based Security Device.

A nano-electromechanical (NEM) switch using multilevel states based on the high security physical unclonable function (PUF) is proposed and experimentally demonstrated. Using the asymmetric random stiction of a silicon nanowire (SiNW), the conventional binary state is simply expanded to a quaternary-state encryption key without increasing chip area. The multiple states are determined by the asymmetrically bent direction and stiction of the SiNW. The experimental results show that the fabricated NEM-PUF with multistates retains unique, random, and robust characteristics, while the key capacity is doubled, even with the same array size footprint.

Ultrafast Multilevel Switching in Au/YIG/n‐Si RRAM

AbstractResistive random access memory (RRAM) with ultrafast and multilevel switching is extremely promising for next‐generation nonvolatile memory. Here, ultrafast unipolar resistive switchings (≈540 ps) with high off/on resistance ratio (≈104) are obtained in yttrium iron garnet Y3Fe5O12 (YIG)‐based resistive memory on n‐Si substrate. The sub‐nanosecond operation is also successfully performed up to 85 °C with an off/on resistance ratio of ≈103. In addition, by using different compliance currents for the set process, five discrete resistance levels with ultrafast switchings among them are achieved and the multilevel states show reliable retention (>104 s). The large, stable, reproducible, and reliable switching behaviors of the Au/YIG/n‐Si RRAM cell shows its great potential for ultrafast multilevel memory applications.

Ferroelastic-strain-induced multiple nonvolatile resistance states in GeTe/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructures

We prepared 300-nm GeTe thin films on (111)-oriented and piezoelectrically active 0.71 Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29 PT) single-crystal substrates by the pulsed laser deposition and investigated the effects of in situ electric-field-controllable non-180° ferroelastic domain switching of the PMN-0.29 PT on the electronic properties of the GeTe films. The in-plane strain of the PMN-0.29 PT could be modulated continuously and reversibly by electric fields in a nonvolatile manner and could be effectively transferred to the GeTe films. Based on this, we realized reversible and nonvolatile resistance switching and obtained multilevel stable nonvolatile resistance states with good stability and endurance at T = 300 K by applying appropriate asymmetrical bipolar electric fields to the PMN-0.29 PT(111) substrates along the thickness direction. Such heterostructures may be used for multilevel data storage that allows each unit to store multiple bits of information and thus improve the memory density. Our investigation would be beneficial for the fabrication of nonvolatile memory devices using PMN-xPT-based heterostructures.

Intergovernmental relations on immigrant integration in multi-level states. A comparative assessment

ABSTRACTThe study of intergovernmental relations (IGR) is a classical research area in scholarship on federalism and territorial politics. However, it has largely ignored the relatively new, and recently decentralized area of immigrant integration. The aim of this Special Issue is twofold. First, it aims to analyse how governments in multi-level states coordinate on immigrant integration. Second, it wishes to explain the dynamics that shape the features of intergovernmental relations. In doing so, we focus on four multi-level states; two of which are federal (Belgium and Canada) and two that are decentralized (Italy and Spain). Whilst we engage with the established literature on intergovernmental relations to formulate hypotheses about the nature and dynamics of intergovernmental relations, we also formulate less explored hypotheses. Our overarching argument is that the scholarship on IGR benefits from in-depth comparative case studies comparing IGR not just across countries, but also across policy areas and over time.

Experimental measurement of a nonlinear entanglement witness by hyperentangling two-qubit states

We demonstrate that non-linear entanglement witnesses can be made particularly useful for entanglement detection in hyper-entangled or multilevel states. We test this idea experimentally on the platform of linear optics using a hyper-entangled state of two photons. Instead of several simultaneous copies of two-photon entangled states, one can directly measure the witness on single copy of a hyper-entangled state. Our results indicate that hyper-entanglement can be used for quick entanglement detection and it provides a practical testbed for experiments with non-linear entanglement witnesses.

Development of self-rectifying ZnO thin film resistive switching memory device using successive ionic layer adsorption and reaction method

In the present report, a simple and cost-effective successive ionic layer adsorption and reaction method is employed to develop self-rectifying ZnO thin film memory device. The nature of pinched hysteresis loop and frequency dependent I–V characteristics depicts that the developed device behaves like a memristive device. Moreover, significant pinched hysteresis loop at 1 MHz was observed which could be further exploited for the development of new class of high-frequency circuits by using ZnO memristive device. The observed analog memory with scan rate dependent synaptic weights behavior suggests that the ZnO memristive device is a potential candidate for the development of electronic synaptic devices for neuromorphic computing application. Furthermore, multilevel resistive switching with good memory window was obtained at 0.2 V read voltage. The developed device switched successfully in consecutive 10 k resistive switching cycles and can retain multilevel resistance states over 1000 s without any observable degradation in the resistance states. The insights drawn from electrical characterization indicates that the device charge and charge–magnetic flux relations depend upon the frequency of the applied signal. Furthermore, we have presented the criteria for differentiating the experimental device as a memristor or memristive device based on the nature of time domain charge and double valued charge–magnetic flux relation. The resistive switching effect of the present device is manifested due to the unified effect of the Ohmic and Schottky conduction mechanisms.

Flexible, multilevel, and low-operating-voltage resistive memory based on MoS2–rGO hybrid

With the growing demand for wearable electronic devices in recent years, flexible memory devices have attracted great interest among researchers. Here we report a flexible resistive memory device based on a hybrid of MoS2 and reduced graphene oxide (rGO). The device shows not only a low operating voltage (about 0.4 V), but also multilevel states by controlling the compliance current. In addition, it exhibits long retention time, high repeatability, and bending endurance. Further analysis indicates that the resistive-switching phenomenon can be attributed to the effect of electron trapping and de-trapping in the MoS2–rGO heterojunction. Our findings suggest that the MoS2–rGO hybrid has the potential to be applied in flexible, low-power, and high-density nonvolatile memory.

Integration of Oxide Semiconductor Thin Films with Relaxor-Based Ferroelectric Single Crystals with Large Reversible and Nonvolatile Modulation of Electronic Properties.

We report the fabrication of 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29PT)-based ferroelectric field effect transistors (FeFETs) by the epitaxial growth of cobalt-doped tin dioxide (SnO2) semiconductor thin films on PMN-0.29PT single crystals. Using such FeFETs we realized in situ, reversible, and nonvolatile manipulation of the electron carrier density and achieved a large nonvolatile modulation of the resistance (∼330%) of the SnO2:Co films through the polarization switching of PMN-0.29PT at 300 K. Particularly, combining the ferroelectric gating with piezoresponse force microscopy, X-ray diffraction, Hall effect, and magnetoresistance (MR), we rigorously disclose that both sign and magnitude of the MR are intrinsically determined by the electron carrier density, which could modify the s-d exchange interaction of the SnO2:Co films. Furthermore, we realized multilevel resistance states of the SnO2:Co films by combining the ferroelectric gating with ultraviolet light illumination, demonstrating that the FeFETs have potential applications in multistate resistive memories and electro-optical devices.

Understanding the Coexistence of Two Bipolar Resistive Switching Modes with Opposite Polarity in Pt/TiO2/Ti/Pt Nanosized ReRAM Devices

Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO2 cells, is still not completely understood. Here, we report on nanosized (<0.01 μm2) asymmetric memristive cells from 3 to 6 nm thick TiO2 films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO2/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO2 cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO2 Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO2/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO2 interface.

Multilevel resistive switching in Cu and Ag doped CBRAM device

A new type of memory device, based on Ag and Cu doped Ge2Se3 chalcogenide, was designed and investigated by a new method, referred to as “one pulse SET method”. The device shows multilevel resistance states. A resistance state between ON and OFF states was found with a wide range of bias. The multilevel resistance behavior could be caused by the formation and dissolution of Ag and Cu filaments. The energy-dispersive X-ray spectroscopy mapping and scanning electron microscope results prove the distribution of Cu and Ag elements in the film.

Transformations among Pure Multipartite Entangled States via Local Operations are Almost Never Possible

Local operations assisted by classical communication (LOCC) constitute the free operations in entanglement theory. Hence, the determination of LOCC transformations is crucial for the understanding of entanglement. We characterize here almost all LOCC transformations among pure multipartite multilevel states. Combined with the analogous results for qubit states shown by Gour \emph{et al.} [J. Math. Phys. 58, 092204 (2017)], this gives a characterization of almost all local transformations among multipartite pure states. We show that nontrivial LOCC transformations among generic, fully entangled, pure states are almost never possible. Thus, almost all multipartite states are isolated. They can neither be deterministically obtained from local-unitary-inequivalent (LU-inequivalent) states via local operations, nor can they be deterministically transformed to pure, fully entangled LU-inequivalent states. In order to derive this result, we prove a more general statement, namely, that, generically, a state possesses no nontrivial local symmetry. We discuss further consequences of this result for the characterization of optimal, probabilistic single copy and probabilistic multi-copy LOCC transformations and the characterization of LU-equivalence classes of multipartite pure states.

Reset Voltage-Dependent Multilevel Resistive Switching Behavior in CsPb1–xBixI3 Perovskite-Based Memory Device

All-inorganic CsPb1- xBi xI3 perovskite film was successfully fabricated by incorporating Bi3+ in CsPbI3 to stabilize the cubic lattice. Furthermore, the perovskite film was applied to manufacture a simple Ag/CsPb1- xBi xI3/indium tin oxide (ITO) memory device with a bipolar resistive switching behavior. Nonvolatile, reliable, and reproducible switching properties are demonstrated through retention and endurance test under fully open-air conditions. The memory device also presents highly uniform and long-term stable characteristics. Importantly, by modulating the reset stop voltages, multilevel high-resistance states are observed for the first time in lead halide perovskite memory device. The resistive switching behavior is proposed to explain the formation and partial rupture of conductive multifilament that are dominated by the migration of iodine ions and their corresponding vacancies in perovskite film. This study suggests Ag/CsPb1- xBi xI3/ITO device potential application for multilevel data storage in a nonvolatile memory device.

Comparative financing of megacities in multi-level BRICS states

PurposeThe BRICS states are based on multi-level government’s architecture whose megacities have an outstanding role in their respective states – not only in terms of population and in terms of economic power, but also in terms of local own-tax revenue collection, which, in turn, implies a very strong administration. At the same time, megacities are facing increasing public expenditures because of infrastructure needs, housing shortage, growth of social inequality, and environmental degradation. In order to outweigh the urban advantages associated with urbanization, reconsidering the fiscal framework is of urgent need. The paper aims to discuss these issues.Design/methodology/approachMost similar case design is applied.FindingsMegacities are at the forefront of national economic growth, that they have an above average tax base, an excellent administration, and therefore, face above average yields of property tax revenues. Rethinking the fiscal framework could considerably improve local finances.Research limitations/implicationsDue to limitations on public finance statistics, the sample draws on four megacities in BRICS, in reference to all megacities, only.Practical implicationsIn context of Habitat III, New Urban Agenda, the practical implications are manyfold: the paper focuses on megacities, its finances (financing for development), and social implications involved.Social implicationsLocal public finance, especially property taxation, has many implications on social level.Originality/valueThe paper is one in a kind.

Party support in multi-level elections: the influence of economic perceptions and vertical congruence

How does the economy influence party support in multi-level states? Using regional-level survey data from Canada, Germany and Spain, we show that there is a ‘cross-level’ effect of economic evaluations. Citizens thus take into consideration both the regional and the national economy when determining their support for incumbents at either level. However, the way in which they do so depends on whether the same party is in office at the two levels or not. If so, standard reward-punishment patterns apply. If different parties are in office at the two levels, incumbents at one level are rewarded for bad and punished for good economic outcomes at the other level. Overall, the influence of cross-level economic perceptions is about half as large as that of same-level economic perceptions. Our findings have important normative implications for the signalling function of elections.