Bistable Properties Research Articles

Finite size effects in molecular spin crossover materials

In recent years, study of nano-objects with first-order phase transitions has raised new interesting questions. Indeed, the phase stability and the transformation kinetics are highly dependent on the object size. In the case of the spin crossover phenomenon size effects may lead to a loss of the hysteresis (bistability) properties, a shift of the transition temperature or the occurrence of an incomplete transition and also reappearance of the hysteresis upon extreme size reduction (∼1–4 nm). Research in this area is growing, but the mechanisms of the spin transition at the nanoscale and in particular the size dependence of the collective behaviour are still not completely understood. The aim of this Focus paper is to point out new issues brought about by the size-reduction effects. We also review size-induced effects for a few selected phase transition phenomena, such as structural or electronic transitions, with the aim of highlighting the possible analogies with the spin crossover phenomenon.

All-Optical Stochastic Logic Gate Based on a VCSEL With Tunable Optical Injection

We study the dynamics of a vertical-cavity surface-emitting laser with continuous-wave orthogonal optical injection from a tunable laser. We use the dynamical properties of polarization bistability and the interplay with internal or external noise to demonstrate numerically a reliable logic output to two logic inputs encoded in the optical frequency of the injected light. This all-optical configuration is more than ten times faster than the electro-optical implementation and has the advantage of operating at constant injection power.

Toward a Coherent Theory of CSMA and Aloha

Aloha and Carrier Sense Multiple Access (CSMA) are two representative random-access protocols. Despite their simplicity in concept, the performance analysis of Aloha and CSMA networks has long been known as notoriously difficult. Numerous models and analytical approaches have been proposed in the past four decades. Yet how to integrate them into a coherent theory remains an open challenge. Toward this end, a unified analytical framework was recently proposed in , based on which a comprehensive study of throughput, delay and stability performance of Aloha networks was presented. In this paper, the framework is further extended to CSMA networks. The analysis shows that both CSMA and Aloha have the same bi-stable property, and the performance of both networks critically depends on the selection of backoff parameters. Different from Aloha, however, substantial gains can be achieved in CSMA networks by reducing the mini-slot length a and the collision-detection time x. The maximum throughput with CSMA is derived as an explicit function of a and x, and shown to be higher than that with Aloha if a <; e1/ϵ - 1≈0.445. With a small mini-slot length a, CSMA networks are also found to be more robust than Aloha networks thanks to larger stable regions of backoff parameters. To demonstrate how to properly tune the backoff parameters to stabilize the network, the complete stable region of the initial transmission probability q0 is characterized, and illustrated via the example of p-persistent CSMA with the cutoff phase K=0. The optimal values of q0 to maximize the network throughput and to minimize the first and second moments of access delay are also obtained, which shed important light on practical network control and optimization.

DNA looping increases the range of bistability in a stochastic model of the lac genetic switch

Conditions and parameters affecting the range of bistability of the lac genetic switch in Escherichia coli are examined for a model which includes DNA looping interactions with the lac repressor and a lactose analogue. This stochastic gene–mRNA–protein model of the lac switch describes DNA looping using a third transcriptional state. We exploit the fast bursting dynamics of mRNA by combining a novel geometric burst extension with the finite state projection method. This limits the number of protein/mRNA states, allowing for an accelerated search of the model's parameter space. We evaluate how the addition of the third state changes the bistability properties of the model and find a critical region of parameter space where the phenotypic switching occurs in a range seen in single molecule fluorescence studies. Stochastic simulations show induction in the looping model is preceded by a rare complete dissociation of the loop followed by an immediate burst of mRNA rather than a slower build up of mRNA as in the two-state model. The overall effect of the looped state is to allow for faster switching times while at the same time further differentiating the uninduced and induced phenotypes. Furthermore, the kinetic parameters are consistent with free energies derived from thermodynamic studies suggesting that this minimal model of DNA looping could have a broader range of application.

Sequential growth of bistable copper–molybdenum coordination nanolayers on inorganic surfaces

Sequential growth in solution is a powerful tool to control the growth of coordination networks on surfaces. We used this approach to prepare nanolayers of the bistable copper-molybdenum cyanide-bridged network. The nanolayers were grown on functionalized silicon and on bare platinum surfaces. The use of platinum dots organized on silicon oxide led to the growth of isolated and organized coordination objects. The bistable properties, characteristic of the bulk, have been evidenced for the nanolayers using infrared spectroscopy.

Bipolar Resistance Switching Characteristics in Zirconium Oxide

Sputtered Ti/ZrO2/Pt is investigated for its bi-stable conduction and transition properties. The transition voltage and current vary with the current compliance setting, while the conduction currents in on and off states show different dependence on the top electrode diameter. Both results provide insights into the resistive switching mechanism by the filament model. Impedance measurement in off state is made at 25°C–100°C. The extracted intercept resistance corresponds to those in low field by full-log current-voltage representation. Transport characteristics in high and low conduction states are interpreted by these temperature-dependent resistance and impedance data.

Bistability in Purkinje neurons: Ups and downs in cerebellar research

The output of cerebellar Purkinje cells has been characterized extensively and theories regarding the role of simple spike (SS) and complex spike (CS) patterns have evolved through many different studies. A bistable pattern of SS output can be observed in vitro; however, differing views exist regarding the occurrence of bistable SS output in vivo. Bistability in Purkinje cell output is characterized by abrupt transitions between tonic firing and quiescence, usually evoked by synaptic inputs to the neuron. This is in contrast to the trimodal pattern of activity which has been found in vitro and in vivo when climbing fiber input to Purkinje cells is removed. The mechanisms underlying bistable membrane properties in Purkinje cells have been determined through in vitro studies and computational analysis. In vitro studies have further established that Purkinje cells possess the ability to toggle between firing states, but in vivo studies in both awake and anesthetized animals have found conflicting results as to the presence of toggling in the intact circuit. Here, we provide an overview of the current state of research on bistability, examining the mechanisms underlying bistability and current findings from in vivo studies. We also suggest possible reasons for discrepancies between in vivo studies and propose future studies which would aid in clarifying the role of bistability in the cerebellar circuit.

Tunable photovoltaic electrochromic device and module

An innovative tunable photovoltaic electrochromic (PV–EC) device concept, in which planar distribution of silicon thin film solar cells (Si-TFSCs) structure with successively deposited electrochromic thin films (ECTFs) and electrolyte layer is demonstrated. The switching mechanism of a PV–EC device is realized by electrically separating the electrodes of the Si-TFSCs and ECTFs, and the connection of the two systems is achieved through an external switching apparatus that links each Si-TFSC and ECTF through one transparent conductive oxide (TCO) layer. According to the PV–EC device structure, Si-TFSCs arranged in stripes makes large area module fabrication feasible, while Si-TFSCs arranged in arrays allows pixel structure design possible. When illuminated by sunlight under short-circuit condition, the photopotential outputted from a single Si-TFSC drives ECTF layer to change color, simultaneously a portion of the photocurrent is stored in a capacitor. Once the ECTF is tinted, no power is needed to retain its colored state due to the bi-stable property of the electrochromic material. In order to revert the PV–EC device to its bleached state, the capacitor supplies reverse potential to cause discoloration of the ECTF. In addition, the monolithically integrated Si-TFSC module generates electricity to a connected load. In view of photoelectric conversion and optical modulation properties, the PV–EC device can both function as a Si-TFSC module and as a self powered smart glass, which has great advantages in green energy application.

Optimal homeostasis necessitates bistable control

Bistability is a fundamental phenomenon in nature. In biology, a number of fine properties of bistability have been identified. However, these properties are only consequences of bistability at the physiological level, which do not explain why it had to emerge during evolution. Using optimal homeostasis as the first principle, I find that bistability emerges as an indispensable control mechanism. It is the only solution to a dilemma in glucose homeostasis: high insulin efficiency is required to confer rapidness in plasma glucose clearance, whereas an insulin sparing state is required to guarantee the brain's safety during fasting. The optimality consideration renders a clear correspondence between the molecular and physiological levels. This new perspective can illuminate studies on the twin epidemics of obesity and diabetes and the corresponding intervening strategies. For example, overnutrition and sedentary lifestyle may represent sudden environmental changes that cause the lose of optimality, which may contribute to the marked rise of obesity and diabetes in our generation. Because this bistability result is independent of the parameters of the mathematical model (for which the result is quite general), some other biological systems may also use bistability to control homeostasis.

Reversible Switch Memory Effect in Hydrogen‐Terminated Ultrananocrystalline Diamond

AbstractInnovative memory switch devices require reliable bistable conductance properties. It would be desirable if such bistable characteristics were available in robust solid state materials, such as diamond, which benefit from outstanding physical properties. A bistable current with reversible switching effect from surface transfer doped ultrananocrystalline diamond thin films measured by electron field emission is reported. This switching is manifested by the appearance of huge jumps in the current emission, up to four orders of magnitude, that occur at specific extracting electric field values. Persistent hysteresis is exhibited whenever the field is ramped down. It is proposed that these phenomena are the result of resonant‐tunneling through a double barrier junction composed of tetrahedral amorphous carbon (ta‐C)/nanodiamond/adsorbent/vacuum. This finding may pave the way for the realization of novel types of memory switch devices with unprecedented performance.

Electrical bistable properties of copper phthalocyanine at different deposition rates

Organic bistable memory device is a next-generation of the electrical memory unit. In this paper, we report about the influence of structural properties on electrical bistable behavior of copper phthalocyanine organic memory device. Copper phthalocyanine (CuPc) layer was prepared by thermal evaporation technique at different deposition rates. When the deposition rate is increased, the film crystalline decreases and the surface morphology gradually changes from large flat grain to fine grain structure. Structural parameters such as the crystalline size of CuPc films and dislocation density can be calculated from XRD spectra. Moreover, the effect of deposition rate of CuPC layer on the bistable properties can be performed by current–voltage characteristics, retention measurement, impedance spectroscopy and temperature dependence measurement. The conduction mechanism in both ON and OFF states of the bistable device was analyzed by theoretical model, which can be proposed as a possible trap center of the carrier trapping and de-trapping process by structural defects in CuPc layer. Furthermore, the reliability issue such as cycling endurance and data retention is presented.

Elimination of bistability in constant-phase mode in atomic force microscopy

By presenting the phase properties of bistability in amplitude-modulation atomic force microscopy, we put forward a technique, the constant-phase mode, which may eliminate bistability. Using this approach, we keep the phase shift between driving and oscillation constant, slightly above −90°. In addition to the adjustment of the free amplitude, we add to amplitude-modulation atomic force microscopy another feedback so that the tip always oscillates in the high-amplitude state. A numerical simulation is carried out to demonstrate that the algorithm prevents bistability effectively.

Layer-by-layer assembled enzyme multilayers with adjustable memory performance and low power consumption via molecular-level control

Electrochemical properties of enzymes are of fundamental and practical importance in bio-electrochemical applications. These redox properties, which can cause the reversible changes in the current according to their redox reactions in solution, often depend on the chemical activity of transition metal ions as cofactors within the active sites of enzymes. Here, we demonstrate that the reversible resistance changes in enzyme-based multilayer films can be caused by the externally applied voltage as a result of charge trap/release of haem FeIII/FeII redox couples in dry form. It is also demonstrated that the electrically bistable switching properties of redox enzymes can be applied to nonvolatile memory devices requiring low power consumption. For this study, cationic poly(allylamine hydrochloride) (PAH) was alternately layer-by-layer assembled with anionic catalase enzyme onto Pt-coated substrates until the desired number of layers was deposited. A top electrode was deposited onto (PAH/catalase)n multilayer films to complete device fabrication. When an external bias was applied to the devices, a switching phenomenon depending on the voltage polarity (i.e., bipolar switching) was observed at low operating voltages (RESET at 1.8 V and SET voltage at −1.5 V), fast switching speed at the nanosecond level, and an ON/OFF current ratio of ∼102. In the case of inserting insulating layers of about 2 nm thickness between adjacent catalase (CAT) layers, these devices exhibited the higher memory performance (ON/OFF current ratio of ∼106) and the lower power consumption than those of (PAH/CAT)15 multilayer devices.

基于ZnS量子点与聚合物混合层的有机双稳态器件的记忆特性及其载流子传输机制

A write-once-read-many-times(WORM) bistable device was prepared,in which ZnS quantum dots doped poly-4-vinyl-phenol(PVP) layer was sandwiched between ITO anode and Al cathode.Current-voltage(I-V) curves showed a switching characteristic with a large ON / OFF ratio of 10 4.The electrical bistability properties and charge-transport mechanisms were discussed in detail based on I-V characteristics.The conduction mechanisms in both ON-and OFF-states were discussed in terms of different theoretical models.The data-retention characteristics of the current-time(I-t) curve exhibited permanent retention ability at ambient conditions.

Flexible Bistable Smectic-A Liquid Crystal Device Using Photolithography and Photoinduced Phase Separation

A flexible bistable smectic-A liquid crystal (SmA LC) device using pixel-isolated mode was demonstrated, in which SmA LC molecules were isolated in pixels by vertical polymer wall and horizontal polymer layer. The above microstructure was achieved by using ultraviolet (UV) photolithography and photoinduced phase separation. The polymer wall was fabricated by photolithography, and then the SmA LC was encapsulated in pixels between polymer wall through UV-induced phase separation, in which the polymer wall acts as supporting structure from mechanical pressure and maintains the cell gap from bending, and the polymer layer acts as adhesive for tight attachment of two substrates. The results demonstrated that all the intrinsic bistable properties of the SmA LC are preserved, and good electrooptical characteristics such as high contrast ratio and excellent stability of the bistable states were characterized. This kind of SmA bistable flexible display has high potential to be used as electronic paper, smart switchable reflective windows, and so forth.

Design and synthesis of conjugated polymers containing Pt(ii) complexes in the side-chain and their application in polymer memory devices

We have synthesized conjugated polymers containing Pt(II) complexes in the side-chain with different main-chains via a Suzuki coupling reaction. These polymers exhibit bistable properties and can be applied in memory devices, in which charge transfer and traps are responsible for the conductance switching behavior. The devices could be defined as resistive random-access memory (ReRAM) with a high ON/OFF current ratio, excellent stability and high read cycles (107). Furthermore, through the study of the electrochemical properties and theoretical calculations of the polymers, we investigated the significant effect of the polymer main-chain on the memory device performances. The device based on the polymer with a polycarbazole main-chain exhibited a lower threshold voltage and a higher ON/OFF current ratio than the device based on the polymer with a polyfluorene main-chain. Our preliminary results indicate that this kind of material offers promising opportunities for the development of polymer memory devices.

Electrically Bistable Properties of Layer-by-Layer Assembled Multilayers Based on Protein Nanoparticles

Electrochemical properties of redox proteins, which can cause the reversible changes in the resistance according to their redox reactions in solution, are of the fundamental and practical importance in bioelectrochemical applications. These redox properties often depend on the chemical activity of transition metal ions as cofactors within the active sites of proteins. Here, we demonstrate for the first time that the reversible resistance changes in dried protein films based on ferritin nanoparticles can be caused by the externally applied voltage as a result of charge trap/release of Fe(III)/Fe(II) redox couples. We also show that one ferritin nanoparticle of about 12 nm size can be operated as a nanoscale-memory device, and furthermore the layer-by-layer assembled protein multilayer devices can be extended to bioinspired electronics with adjustable memory performance via molecular level manipulation.

Nonlinear Dynamics of Vertical-Cavity Surface-Emitting Lasers

Nonlinear dynamics of Vertical-Cavity Surface-Emitting Lasers (VCSELs) induced by optical injection, optical feedback, current modulation and mutual coupling is reviewed. Due to the surface emission and cylindrical symmetry VCSELs lack strong polarization anisotropy and may undergo polarization switching. Furthermore, VCSELs may emit light in multiple transverse modes. These VCSEL properties provide new features to the rich nonlinear dynamics induced by an external perturbation. We demonstrate for the case of orthogonal optical injection that new Hopf bifurcation on a two-polarization-mode solution delimits the injection locking region and that polarization switching and injection locking of first-order transverse mode lead to a new resonance tongue for large positive detunings. Similarly, the underlying polarization mode competition leads to chaotic-like behavior in case of gain switching and the presence of two transverse modes additionally reduces the possibility of regular dynamics. The bistable property of VCSEL makes it possible to investigate very fundamental problems of bistable systems with time-delay, such as the coherence resonance phenomenon. We also demonstrate that the synchronization quality between unidirectionally coupled VCSELs can be significantly enhanced when the feedback-induced chaos in the master laser involves both orthogonal LP fundamental transverse modes.

Optical bistability effect in plasmonic racetrack resonator with high extinction ratio

In this paper, optical bistability effect in an ultracompact plasmonic racetrack resonator with nonlinear optical Kerr medium is investigated both analytically and numerically. The properties of optical bistability and pump threshold are studied at 1.55 µm with various detuning parameters by an analytical model. The transmission switch from the upper branch to the lower branch with a pulse is also demonstrated by a finite-difference time-domain method. An extinction ratio of 97.8% and a switching time of 0.38 ps can be achieved with proper detuning parameter. Such a plasmonic resonator design provides a promising realization for highly effective optical modulators and switch.

Bistable moving optical solitons in resonant photonic crystals

We consider some new aspects of the formation of moving optical solitons in a medium of Bragg-type resonant grating doped with two-level atoms. For generality, account is taken of the local-field effect assisted by a sufficiently high density of resonant atoms. It is established analytically that there exists a family of soliton solutions to the two-wave Maxwell-Bloch system of equations, with these solitons exhibiting bistable properties. The existence of bistable solitons and their properties are confirmed by numerical simulations.