Nanotube size-dependent melting of single crystals in carbon nanotubes

Received: 15 April 1997/Accepted: 16 April 1997     

New insights into the regulation of synaptic transmission and plasticity by the endoplasmic reticulum and its membrane contacts

Mammalian neurons are highly compartmentalized yet very large cells. To provide each compartment with its distinct properties, metabolic homeostasis and molecular composition need to be precisely coordinated in a compartment-specific manner. Despite the importance of the endoplasmic reticulum (ER) as a platform for various biochemical reactions, such as protein synthesis, protein trafficking, and intracellular calcium control, the contribution of the ER to neuronal compartment-specific functions and plasticity remains elusive. Recent advances in the development of live imaging and serial scanning electron microscopy (sSEM) analysis have revealed that the neuronal ER is a highly dynamic organelle with compartment-specific structures. sSEM studies also revealed that the ER forms contacts with other membranes, such as the mitochondria and plasma membrane, although little is known about the functions of these ER-membrane contacts. In this review, we discuss the mechanisms and physiological roles of the ER structure and ER-mitochondria contacts in synaptic transmission and plasticity, thereby highlighting a potential link between organelle ultrastructure and neuronal functions.     

Power laws in dislocation plasticity

Starting from the idea that plastic flow produces dislocation structures in a state of self-organised criticality, it is shown that one expects power-law relationships between variables. If slip bands are modelled as avalanches of shear with an ellipsoidal shape, slightly tilted from the crystallographic slip plane, limited in size by interaction with secondary slip, the observed exponents of the power laws can be rationalised. In some cases, the constant of proportionality can also be estimated, and found to agree with experiment, even though the detailed mechanism of avalanche formation is not addressed. To analyse creep data and slip-band statistics, it is further assumed that the role of cross-slip is to eliminate screw dislocation dipoles, removing them entirely at stresses found in Stage III of work-hardening. If physical constants, such as the atomic vibration frequency, play a role, the dimensionless power-law relationships do not apply. One then finds creep rates linear in stress and absolute temperature, proportional to the logarithm of time, obeying an equation of state, as observed.     

Delayed plasticity in nanoindentation of annealed crystals

Continuum constitutive relations used in the design of macro-sized components assume that the elastic limit of a crystalline solid is time independent. Recent experiments using the nanoindentation technique, however, reveal that the elastic limit of submicron-sized metallic volumes decreases as time under load increases. A submicron metallic volume can sustain a static load in the elastic regime initially, but transition to plastic deformation may occur after some waiting time. In this paper, the characteristics of this type of delayed plasticity are reviewed. The available experimental data suggest that homogeneous nucleation of the plasticity events, which was frequently discussed in the recent literature, occurs only at sufficiently high loads within a narrow range. In a lower and broader load range, the nucleation of the plasticity events occurs at a history dependent rate, thus via a damage-accumulation mechanism not compatible with the homogeneous nucleation theory. A model based on the diffusion-controlled, subcritical growth of a Frank loop just underneath the indenter is proposed in this work to explain the history dependent nucleation of instability observed at lower loads. By fitting to the available nanoindentation data in Ni₃Al, it is apparent that self-diffusion along the indenter-sample interface, rather than through the bulk, is likely to be the controlling factor for the growth of the Frank loop to a critical size to yield a dislocation avalanche.     

Crystal plasticity and multiscale modelling of superalloy creep

A crystal plasticity approach for superalloy creep has been presented which employs a finite element-based representative volume element (RVE) methodology. The γ channels are assumed to undergo crystal slip and the γ′ particles to deform elastically. A range of superalloys has been studied. Thermocalc computations provide the γ′ volume fraction and an automated scheme for generating the resulting RVE has been developed. It has been shown that primary creep response in a wide range of superalloys over high stress, low temperature regimes is represented excellently by the model, by determination of just an activation energy and an alloying element density. It has been hypothesised that the transition from primary to secondary creep results from the development of geometrically necessary dislocations within the γ channels at the γ′ interfaces. Without the need of further material parameters, it has been shown that secondary creep rates over a broad range of stress and temperature can be accurately predicted, hence supporting the hypothesis. An empirical relationship has been established between the alloying element density and the atomic weight percentages of the alloying elements, using a range of superalloy data. It is hypothesised that a role of the alloying elements within the γ channels is to act as inhibitors of ribbon dislocation motion, hence leading to the large range of macro-level primary and secondary creep responses observed in the alloys with variations in constituent alloys. The empirical relationship established, when combined with the crystal RVE methodology, then allows the prediction of superalloy creep rates from knowledge of alloying constituents.     

Serration behaviours in metallic glasses with different plasticity

We present a statistical study of serration behaviours in Pd_77.5Cu₆Si_16.5, Ti₄₁Zr₂₅Be₂₆Ag₈, Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 and Fe₅₀Ni₃₀P₁₃C₇ metallic glasses with different plasticity. The four samples show similar serration patterns in the beginning of yielding, and different patterns during later loading. These results indicate that the shear band initiation process in metallic glasses follow some similar dynamics. And the later serration process follows different dynamics and will lead to different plasticity. Here we interpret these serration behaviours from a perspective of inhomogeneity. The different serration patterns and shear band dynamics could be reasonably understood. The serration pattern of the Fe-based sample suggests that the brittleness of metallic glasses might result from a lower degree of inhomogeneity, and a less tendency of forming shear band intersections. This study might provide new experimental evidences for different micro-structures (or inhomogeneity) and dynamic behaviours in metallic glasses with different plasticity.     

High temperature deformation and extended plasticity in Mg single crystals

The high temperature deformation behavior of Mg single crystals was precisely investigated using orientation imaging microscopy. For this purpose, Mg single crystals of various orientations were tensile tested in vacuum at temperatures between 473 and 673?K. A strain rate of 4.2?×?10^?4?s^?1 was employed. The elongations to fracture depended strongly on crystal orientation, the lowest fracture strains being associated with multiple slip. Single crystals in which single slip was activated exhibited extended ductilities corresponding to more than 1.5 in true strain. The strong orientation dependence of the ductility can also be correlated with the ease of occurrence of dynamic recrystallization (DRX), which took place in the multiple-slip specimens. The role of twinning in the initiation of DRX is also discussed.     

Thermal effects and anomalies in the low-temperature plasticity of crystals

This paper discusses theoretically the mechanism that causes the temperature dependences of the yield point and yield stress, along with their rate coefficients, to deviate from behavior characteristic of thermally activated plastic strain at low temperatures (<30 K). At this time the existence of such deviations, e.g., anomalous decreases in the values of these characteristics in this temperature range, is explained by arguing that the process whereby dislocations overcome local barriers has inertia. It is shown that the observed anomalies can be caused by the development of thermal instability in the plastic strain at low temperatures. In contrast to inertia-related effects, thermal effects allow us to explain the plasticity of crystals at low temperatures without contradiction and within the framework of a single mechanism, including the unstable, discontinuous character of plastic strain that is characteristic of these temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 684–689 (April 1998)     

Long-term depression and other synaptic plasticity in the cerebellum

Cerebellar long-term depression (LTD) is a type of synaptic plasticity and has been considered as a critical cellular mechanism for motor learning. LTD occurs at excitatory synapses between parallel fibers and a Purkinje cell in the cerebellar cortex, and is expressed as reduced responsiveness to transmitter glutamate. Molecular induction mechanism of LTD has been intensively studied using culture and slice preparations, which has revealed critical roles of Ca²⁺, protein kinase C and endocytosis of AMPA-type glutamate receptors. Involvement of a large number of additional molecules has also been demonstrated, and their interactions relevant to LTD mechanisms have been studied. In vivo experiments including those on mutant mice, have reported good correlation of LTD and motor learning. However, motor learning could occur with impaired LTD. A possibility that cerebellar synaptic plasticity other than LTD compensates for the defective LTD has been proposed.     

Switching plasticity in compensated ferrimagnetic multilayers for neuromorphic computing

Current-induced multilevel magnetization switching in ferrimagnetic spintronic devices is highly pursued for the application in neuromorphic computing. In this work, we demonstrate the switching plasticity in Co/Gd ferrimagnetic multilayers where the binary states magnetization switching induced by spin-orbit toque can be tuned into a multistate one as decreasing the domain nucleation barrier. Therefore, the switching plasticity can be tuned by the perpendicular magnetic anisotropy of the multilayers and the in-plane magnetic field. Moreover, we used the switching plasticity of Co/Gd multilayers for demonstrating spike timing-dependent plasticity and sigmoid-like activation behavior. This work gives useful guidance to design multilevel spintronic devices which could be applied in high-performance neuromorphic computing.     

Magnetic isotopic effect in the plasticity of diamagnetic crystals

A magnetic isotopic effect in the plasticity of diamagnetic crystals is predicted. This effect consists in that the replacement of nonmagnetic isotopic nuclei by magnetic nuclei in ionic crystals (e.g., the introduction of ²⁵Mg²⁺ or ⁴³Ca²⁺ in place of ²⁴Mg²⁺ or ⁴⁰Ca²⁺ in NaCl) increases the plasticity of the crystals even in the absence of an external magnetic field. On the contrary, such a replacement in covalent crystals (e.g., the introduction of ²⁹Si into silicon or ¹³C into diamond) is expected to reduce the plasticity and to produce dislocation strengthening.     

Physics of the plasticity and fracture of high-strength crystals

Systematic studies of the mechanism of plastic deformation, strain hardening, and fracture of high-strength single crystals of heterophase alloys based on copper and austenitic stainless steels with nitrogen are reported. It is shown that the attainment of high resistance to the motion of dislocations results in the appearance of new mechanical behavior: strong orientation dependence of the critical shear stresses, a change in the deformation mechanism from slip to twinning, loss of mechanical flow stability at early stages in deformation, and a transition from viscous to brittle fracture.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 3–24, September, 1992.     

Defect types and room-temperature ferromagnetism in undoped rutile TiO2 single crystals

Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO2 single crystals when a magnetic field is applied parallel to the sample plane.By combining X-ray absorption near the edge structure spectrum and positron annihilation lifetime spectroscopy,Ti3+-V O defect complexes(or clusters) have been identified in annealed crystals at a high vacuum.We elucidate that the unpaired 3d electrons in Ti3+ ions provide the observed room-temperature ferromagnetism.In addition,excess oxygen ions in the TiO2 lattice could induce a number of Ti vacancies which obviously increase magnetic moments.     

Molecular dynamics simulations of the nano-scale room-temperature oxidation of aluminum single crystals

The oxidation of aluminum single crystals is studied using molecular dynamics (MD) simulations with dynamic charge transfer between atoms. The simulations are performed on three aluminum low-index surfaces ((1 0 0), (1 1 0) and (1 1 1)) at room temperature. The results show that the oxide film growth kinetics is independent of the crystallographic orientation under the present conditions. Beyond a transition regime (100 ps) the growth kinetics follow a direct logarithmic law and present a limiting thickness of ∼3 nm. The obtained amorphous structure of the oxide film has initially Al excess (compared to the composition of Al₂O₃) and evolves, during the oxidation process, to an Al percentage of 45%. We observe also the presence of an important mobile porosity in the oxide. Analysis of atomistic processes allowed us to conclude that the growth proceeds by oxygen atom migration and, to a lesser extent, by aluminum atoms migration. In both cases a layer-by-layer growth mode is observed. The results are in good agreement with both experiments and earlier MD simulations.     

Synaptic plasticity and classical conditioning mimicked in single indium-tungsten-oxide based neuromorphic transistor

Emulation of synaptic function by ionic/electronic hybrid device is crucial for brain-like computing and neuromorphic systems. Electric-double-layer(EDL) transistors with proton conducting electrolytes as the gate dielectrics provide a prospective approach for such application. Here, artificial synapses based on indium-tungsten-oxide(IWO)-based EDL transistors are proposed, and some important synaptic functions(excitatory post-synaptic current, paired-pulse facilitation,filtering) are emulated. Two types of spike-timing-dependent plasticity(Hebbian STDP and anti-Hebbian STDP) learning rules and multistore memory(sensory memory, short-term memory, and long-term memory) are also mimicked. At last, classical conditioning is successfully demonstrated. Our results indicate that IWO-based neuromorphic transistors are interesting for neuromorphic applications.     

基于抑制性突触可塑性的神经元放电率自稳态机制

神经元放电率自稳态是指大脑神经网络的放电率维持在相对稳定的状态.大量实验研究发现神经元放电率自稳态是神经电活动的重要特征,并且放电率自稳态是实现神经信息处理及维持正常脑功能的基础,因此放电率自稳态的研究是神经科学领域的重要科学问题.脑神经网络是一个高度复杂的动态系统,存在大量输入扰动信号及由于动态链接导致的参数摄动,因此如何建立并维持神经元放电率自稳态及其鲁棒性仍有待深入研究.反馈神经回路是皮层神经网络的典型连接模式,抑制性突触可塑性对神经元放电率自稳态具有重要的调控作用.本文通过构建包含抑制性突触可塑性的反馈神经回路模型对神经元放电率自稳态及其鲁棒性进行计算研究.结果表明:在抑制性突触可塑性的作用下,神经元放电率可自适应地跟踪目标放电率,从而取得放电率自稳态;在有外部输入干扰和参数摄动的情况下,神经元放电率具有良好的抗扰动性能,表明放电率自稳态具有很强的鲁棒性;理论分析揭示了抑制性突触可塑性学习规则是神经元放电率自稳态的神经机制;仿真分析进一步揭示了学习率及目标放电率对放电率自稳态建立过程具有重要影响.     

Pressure-enforced plasticity in MAX phases: from single grain to polycrystal investigation

Ti₄AlN₃, Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phases were plastically deformed at room temperature (RT) under gaseous confining pressure. Microstructures of as-grown and deformed samples are carefully analysed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). It is demonstrated that high level of plastic deformation can be reached under confining gas pressure; the later suppresses the brittle failure at RT to the profit of plasticity. Multiscale characterization techniques are shown to provide a unique insight into all the scales of the plastic deformation; in particular, the effect of the mesoscale. Indeed, grain shape and orientation relative to the compression axis are shown to play a key role in the deformation process, intergranular stresses leading to a complex stress field in the polycrystalline samples. The TEM results show that dislocation activity highly depends on the grain orientation. The observation of dislocation entanglements unambiguously demonstrates that dislocations may be organized in such a configuration so that their glide in the basal plane can be hindered when deep plastic regime is reached.     

MeV-ion-induced defects in organic crystals

We present scanning force microscope images of crater-like defects induced by MeV atomic ions incident on single-crystal L-valine surfaces. For grazing incidence ions, the craters are elongated alongthe ion azimuth of incidence and are followed by raised tails in the surface above the ion track. Craters formed by off-normal ions are wider than craters formed by nrmally incident ions. The crater volume is highly nonlinear in (dE/dx)_e. We discuss our observations qualitatively in terms of thermal-spike and pressure-pulse/shock-wave models for the sputtering of organic solids.     

Deformation behavior and enhanced plasticity of Ti-based metallic glasses with notches

The plasticity of Ti-based metallic glasses with different aspect ratios can be improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. The interaction of SBs can be ascribed to the easy initiation of SBs around the notches due to the large stress gradient, and the consequent blocking effect of notches on the propagation of shear bands. Additionally, the current findings provide a new way to understand the physical nature for the plastic deformation behavior of some brittle metallic glasses and supply an effective approach to enhance the plasticity to some extent.