Highlights d A screen in Drosophila identifies glial molecules with vital functions for neurons d Depletion of glial ferritin heavy chain results in iron-mediated axonal damage d Ferritin heavy chain is secreted by oligodendrocytes in mice

The Joule regime at large electric fields in composites is presented in the context of a conduction phase diagram in the field-concentration plane. A sample suffers breakdown when the field is too large. The resistance up to breakdown is described by a universal curve as a function of field. It is found that the ratio of the breakdown resistance to the zero-field resistance assumes a fixed value Y at breakdown. Y is found to be 1.37 in carbon high-density polyethylene composites and is independent of carbon fraction and external conditions but depends on the nature of the conductor. A quantity which is independent of conducting material is defined. Results are compared with previous data.PACS numbers: 72.80.Ng, 05.70.Jk, 72.20.Ht Application of finite force (F, mechanical or electrical) in disordered systems usually results in a nonlinear response leading to some sort of catastrophic behavior in the extreme limit (e.g., fracture in mechanical systems and dielectric breakdown or burning in electrical systems). In recent years there has been a renewed interest in the problem of catastrophic phenomena [1] although the problem of non-Ohmic electrical conductivity in the precatastrophic regime in various disordered systems has been studied for a long time [2,3]. However, there have been very few attempts so far to describe the behavior of a system over the full range of the applied force. Such a study holds the promise of unraveling many important aspects such as precursor effects, predictability, and the effect of disorder on the nature of breakdown. Yagil et al.[4] carried out somewhat limited measurements of I-V curves in thin semicontinuous metallic films of Ag and Au. Focusing on breakdown events, it was concluded that breakdown currents I b in the films scale as I b ϳ B 2x , where B is the normalized third harmonic component (see below) generated as a result of Joule heating. Breakdown was assumed to occur when the sample resistance R exhibited the first irreversible discontinuity as a function of applied current I. The exponent x was measured to be 0.48 and 0.41 in films of Ag and Au, respectively. The authors also derived theoretical bounds for x, 0.5 $ x $ 0.5͓1 2 1͞t͑2 1 w J ͔͒ so that breakdown currents were expected to lie between two bounds. Here, t is the electrical conductivity exponent and w J k͞t, k being the noise exponent [5].In this Letter, we present systematic measurements of electrical resistance, particularly in the Joule regime of a composite system of carbon high-density polyethylene (HDPE) up to breakdown. The breakdown in a sample has the nature of a first-order transition: as soon as the current from a constant current source exceeds a certain value I b , the sample resistance R starts increasing uncontrollably and becomes unsteady. Let R o R͑0͒ be the linear or zero-field resistance, R b R͑I b ͒ be the breakdown resistance and Y R b ͞R o . It is found that for p . p J , where p is the (volume) fraction of conducting component (carbon) and p J is a fraction characteristic of the system ...

Neuronal development requires proper migration, polarization and establishment of axons and dendrites. Growing evidence identifies the ubiquitin proteasome system (UPS) with its numerous components as an important regulator of various aspects of neuronal development. F-box proteins are interchangeable subunits of the Cullin-1 based E3 ubiquitin ligase, but only a few family members have been studied. Here, we report that the centrosomal E3 ligase FBXO31-SCF (Skp1/Cullin-1/F-box protein) regulates neuronal morphogenesis and axonal identity. In addition, we identified the polarity protein Par6c as a novel interaction partner and substrate targeted for proteasomal degradation in the control of axon but not dendrite growth. Finally, we ascribe a role for FBXO31 in dendrite growth and neuronal migration in the developing cerebellar cortex. Taken together, we uncovered the centrosomal E3 ligase FBXO31-SCF as a novel regulator of neuronal development.

The I-V characteristics of four conducting polymer systems like doped polypyrrole (PPy), poly 3,4 ethylene dioxythiophene (PEDOT), polydiacetylene (PDA) and polyaniline (PA) in as many physical forms have been investigated at different temperatures, quenched disorder and magnetic fields. Transport data clearly show the existence of a single electric field scale for all systems. Based upon this observation, a phenomenological scaling analysis is performed, leading to extraction of a numerical value for a nonlinearity exponent called xM which serves to characterize a set of I-V curves. The conductivity starts deviating from an Ohmic value σ0 above an onset electric field Fo which scales according to Fo ∼ σ x M 0 . The electric field-dependent data are shown to be described by Glatzman-Matveev multi-step tunneling model [JETP 67, 1276[JETP 67, (1988] in a near-perfect manner over nine orders of magnitude in conductivity and five order of magnitudes in electric field. Furthermore, xM is found to possess both positive and negative values lying between -1/2 and 3/4. There is no theory at present for this exponent. Some issues concerning applicability of the Glatzman-Matveev model are discussed.

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At a composition far above the percolation threshold, the resistance of a composite sample increases with time due to Joule heating as a constant current of a sufficiently large value is passed through the sample. If the current is less than a certain breakdown current (I(b)) the resistance eventually reaches a steady value with a characteristic relaxation time tau(h). The latter diverges with current I as tau(h) approximately (1-I(2)/I(2)(b))(-z). The value of the exponent z displays large fluctuations leading to unusual scaling of the relaxation time. It is shown that the results lead to important conclusions about the nature of breakdown phenomena.

a b s t r a c t Keywords:Zinc oxide thin film Sol-gel preparation Annealing X-ray diffraction Atomic force microscopy Optical properties This investigation deals with the effect of annealing temperature on the structural, topographical and optical properties of Zinc Oxide thin films prepared by sol-gel method. The structural properties were studied using X-ray diffraction and the recorded patterns indicated that all the films had a preferred orientation along (002) plane and the crystallinity along with the grain size were augmented with annealing temperature. The topographical modification of the films due to heat treatment was probed by atomic force microscopy which revealed that annealing roughened the surface of the film. The optical properties were examined by a UVvisible spectrophotometer which exhibited that maximum transmittance reached nearly 90% and it diminished with increasing annealing temperature.

A comparative study of the physical properties of undoped Zinc Oxide (ZnO) and Al doped Zinc Oxide (AZO) thin films were performed as a function of annealing temperature. The structural properties were analyzed using X-ray diffraction and the recorded patterns indicated that the crystallinity of the films always enhanced with increasing annealing temperature while it degrades with Al doping. The topographical modification of the films due to heat treatment was examined by atomic force microscopy which revealed that annealing roughened the surface of all the films; however the AZO films always exhibited smoother morphology than ZnO. Study of optical properties by UV-Visible spectrophotometer demonstrated that the transmittance was gradually diminished with rise in annealing temperature. In addition, a notable increase in the optical bandgap was also observed for the AZO films.