Electrical transport in ethyl cellulose-chloranil system

The charge-transport behaviour in pure and chloranil (Chl) doped ethyl cellulose (EC) system has been studied by measuring the dependence of current on field, temperature, electrode material and dopant concentration. The role of doping molecular concentration in the polymer matrix and modification in the conduction characteristics are studied. Lowering of the activation energy due to doping was observed. The current was found to increase with an increase in the chloranil concentration. An explanation for this has been attempted on the basis of formation of molecular aggregates between chloranil molecules and ethoxy groups of ethyl cellulose. It is suggested that chloranil occupies interstitial positions between the polymer chains and assists in carrier transportation by reducing the hopping barriers. The current-voltage characteristics of different samples are analyzed using space charge limited current theory and quantitative information about the transport parameters is derived. The values of effective drift mobility and trapped charge carrier concentration which result in the build up of space charge have been calculated.     

Current-carrying capacity of composite superconductors

The maximum transport current Imof the composite superconductors is investigated both theoretically and experimentally. It is shown that the high values of the transport current observed in these materials is due to the non-linear part of the current-voltage characteristic in the range of low electric fields (E lsim 10^{-6}Vcm^-1). The conductors of rather different structure with Nb-Ti superconducting filaments were tested in a wide range of the external parameters. It is shown that in the external magnetic fieldsB_{a} gsim 1 Tthe ratio Im/Ic(where Icis the critical current) is the universal function of the single dimensionless parameter which depends on the sample properties and the external conditions. The theory and experiment are in a good agreement.     

Transport properties of thermally oxidized porous silicon

The process of current transfer in thermally oxidized porous silicon has been studied. A model based on the combination of hopping and tunneling mechanisms of charge transport is proposed. The concentration of charge traps and the mobility of charge carriers are evaluated using the current-voltage characteristics measured at 100 and 300 K.     

Dissipation in current-carrying high-T c superconducting ceramics in applied magnetic field

The dissipation mechanism in high-T _csuperconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.     

Dissipation in current-carrying high-Tcsuperconducting ceramics in applied magnetic field

The dissipation mechanism in high-T _csuperconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.     

Transport properties of a heterogeneous composite: Thermally oxidized silicon micropowder

The process of current transfer in a layer of composite material—thermally oxidized silicon micropowder on a silicon substrate—has been studied. A model based on the combination of hopping and tunneling mechanisms of charge transport is proposed. The concentration of charge traps and the mobility of charge carriers are evaluated using the current-voltage characteristics measured at 100 and 300 K in the dark and under illumination. The activation energies and frequency factors of the charge traps are determined.     

Peculiarities of transport current penetration in a composite superconductor taking into account different models of flux creep

The flux-creep diffusion of transport current in superconducting composite slab and cylinder based on a hard superconductor is investigated theoretically. Power and exponential current-voltage characteristics were used to define the electric field in the flux-creep regime. Using scaling solutions, the Maxwell equations describing transport current penetration in a one-dimensional superconducting composite are solved analytically. A proposed analysis indicates the existence of special macroscopic distribution of electromagnetic field in superconductor during the flux creep. It is shown that the flux-creep electromagnetic field cannot promptly propagate into the composite and penetrates at the finite rate like in the flux-flow regime. Therefore, a moving current boundary exists in the superconducting composite. It separates the region where the current flows from free current region. Moreover, in the flux creep specific conditions are fulfilled at moving boundary in contrast to the flux-flow regime. According to these conditions the electric and magnetic field induced by current charging smoothly approach to its undisturbed values. They are independent of any parameters of current-voltage characteristics and appear due to zero value of the differential resistivity of the composite at moving boundary. To confirm these results, the numerical calculation based on more general model is executed. The equations describing the violation of stable distribution of fully penetrated current are defined.     

Periodic deviations of the field emission current of a carbon nanotube from the Fowler-Nordheim law

Carbon nanotubes (CNTs) have been studied using a field electron microscope (electron projector) and a dispersive electron energy analyzer. In the range of emission voltages studied, the field-emitted electron energy distributions showed periodic transformations. The corresponding probe current-voltage characteristics exhibited periodic deviations (reaching 20 and 50% for the two CNT samples studied) from the Fowler-Nordheim law toward lower values of the current. The observed phenomenon is explained in terms of the interference model of thermoelectron emission at high voltages.     

Peculiarities of the resistive transition in fractal superconducting structures

The influence of fractal clusters of a normal phase on the current-voltage characteristics of a percolation superconductor in the region of a resistive transition has been studied. The clusters appear as aggregates of columnar defects, the presence of which leads to a correlated microscopic disorder in the system. Dependences of the static and dynamic resistance on the transport current are determined for an arbitrary fractal dimension of the cluster boundaries. In the case under consideration, the superconducting system features a mixed state of the vortex glass type.     

High field electrical behaviour in lithium-phospho-vanadate glass system

The high field electrical switching behaviour of lithium-phospho-vanadate glasses has been studied by determining the current-voltage characteristics. The investigated glasses exhibit temperature, thickness and composition dependent trends. At low current, the I–V characteristics obey Ohm’s law followed by a negative resistance region where the bulk behaviour dominates and at higher values of current the sample goes to a low resistance state. The studied glasses exhibit memory type switching. It is suggested that electrical switching is due to the formation of conducting channels that are due to electronic origin while thermal effects dominate once the channels are formed resulting in crystallization.     

Electrical-field induced nonlinear conductive behavior in dense zirconia ceramic

The effect of the applied electric field on the conductive behavior of zirconia ceramics is studied by measuring its initial current-voltage curve at various temperatures. The results show that when the field strength is higher than the threshold for flash-sintering, the curves exhibit a nonlinear behavior by having an additional current on top of the linear current according to Ohm's law. Analyzing its transport behavior reveals that the additional current density is due to the extra oxygen vacancies induced by the electric field. The formation rate of the extra vacancies and associated current was related to the field strength.     

Depinning at the initial stage of the resistive transition in superconductors with a fractal cluster structure

The effects of the initial dissipation of the energy in a superconductor containing fractal clusters of a normal phase are considered. In the interval of currents preceding the resistive transition, an increase in the fractal dimension of clusters leads to an increase of the sample resistance and widening of the region of initial dissipation in the current-voltage characteristic. This is caused by an increase in the density of free vortices broken away from the pinning centers when the current flows. Dependences of the density of vortices on the fractal dimension of cluster boundaries are found for various values of the transport current.     

Long-range interaction of spins in an anisotropic metal with impurities

The influence of a periodic modulation of the maximum Josephson current on the current-voltage characteristic of the Josephson device is studied in a simulation experiment. It is shown that the pair current locks to the frequency of its modulation, which leads to the appearance of constant voltage steps in theI–V characteristic. The special case of square-wave modulation is treated also. A comparison between pair-current modulation and alternating current application is given.Supported by a grant of the Deutsche Forschungsgemeinschaft.     

Vertical spinal electronic device with large room temperature magnetoresistance

We report experimental transport measurements of a vertical hybrid ferromagnetic (FM)/III-V semiconductor (SC)/ferromagnetic(FM) type structure, i.e., Cr(20ML)/Co(15ML)/GaAs(50 nm, n-type)/Al/sub 0.3/Ga/sub 0.7/As(200 nm, n-type)/FeNi(30 nm). The current-voltage (I-V) characteristics reveal Schottky/tunneling type behavior in the direction of FeNi/Semiconductor/Co and observed to be dependent on external magnetic field. The magnetoresistance (MR) behavior shows a strong dependence on the measured current and field. At low fields no significant change in MR has been observed with increasing current. However, at high fields the MR initially increases with increasing current and becomes stable beyond a critical current of 10 /spl mu/A. A maximum of 12% change in the MR has been observed at room temperature, which is far larger than that of the conventional AMR effect. This property of the device could be utilized as field sensors or magnetic logic devices.     

Fractal dimension of structural inhomogeneities in granular YBCO superconductor in magnetic field

The influence of a fractal character of the structure of an YBCO type high-temperature superconductor on its current-voltage characteristic in a magnetic field has been studied. The fractal dimension and connectivity index of vortex transport channels are determined as functions of the magnetic field strength. It is established that the glassy state index corresponds to that of a vortex glass.     

Vortex dynamics at the initial stage of resistive transition in superconductors with fractal cluster structure

The effect of fractal normal-phase clusters on vortex dynamics in a percolative superconductor is considered. The superconductor contains percolative superconducting cluster carrying a transport current and clusters of a normal phase, acting as pinning centers. A prototype of such a structure is YBCO film, containing clusters of columnar defects, as well as the BSCCO/Ag sheathed tape, which is of practical interest for wire fabrication. Transition of the superconductor into a resistive state corresponds to the percolation transition from a pinned vortex state to a resistive state when the vortices are free to move. The dependencies of the free vortex density on the fractal dimension of the cluster boundary as well as the resistance on the transport current are obtained. It is revealed that a mixed state of the vortex glass type is realized in the superconducting system involved. The current-voltage characteristics of superconductors containing fractal clusters are obtained and their features are studied.     

Electrical bistability in zinc oxide nanoparticle-polymer composites

The article reports observed electrical bistability in thin films of ZnO nanoparticles embedded in an insulating polymer matrix. From the current-voltage and impedance characteristics, they studied transport mechanisms involved in the two conducting-states. The electrical bistability in such films has been associated with a memory phenomenon. The bistability, which is reversible in nature, led to read-only and random-access memory applications in the devices based on such nanoparticles.     

Switching of current channels and new mechanism of magnetoresistance in a tunneling structure

We have experimentally studied the transport properties of a planar La_0.7Sr_0.3MnO₃ (LSMO)/Mn-depleted LSMO/MnSi tunneling structure, in which the Mn-depleted LSMO layer plays the role of a potential barrier between the conducting layers of LSMO and MnSi. The measurements were performed in geometry with the current direction parallel to the planes of interfaces in the tunneling structure. It is established that the structure exhibits a nonlinear current-voltage characteristic and possesses a positive magnetoresistance, the value of which depends on the tunneling current. It is suggested that specific features of the transport properties of this structure are related to the phenomenon of current channel switching between the conducting layers. The switching mechanism is based on the dependence of the resistance of the tunneling junction between the conducting layers on the bias voltage and the applied magnetic field.     

Stepwise current oscillations in tunnel and high-frequency semiconductor diodes

The response of tunnel and high-frequency diodes to an external harmonic signal of large amplitude has been studied. Stepwise current oscillations have been found on the current-voltage characteristics measured at a direct current when an external high-frequency signal is imposed simultaneously on the sample. The frequency of these oscillations decreases with increasing frequency of the external signal. Analogs of the appearance of stepwise current oscillations in semiconductor and other systems are indicated.     

Thermal domains in inhomogeneous current-carrying superconductors. Current-voltage characteristics and dynamics of domain formation after current jumps

The static and dynamic behavior of thermal domains in inhomogeneous superconducting films, where the inhomogeneity behaves like a portion of the film with a reduced critical current, have been studied theoretically within the framework of the phenomenological approach, using the heat balance equation and the dependence of the superconductor critical current on temperature. Depending on the size of the inhomogeneity (local or extended) and on the relative values of parameters of the homogeneous and inhomogeneous regions, different types of current-voltage characteristics are obtained. The nonstationary problem of thermal domain formation near the inhomogeneity after a current jump has been solved, and the domain boundary (kink) dynamics at a distance from the inhomogeneity has been analyzed. A combination of the results allows one to describe the whole process of normal phase formation and its spread throughout the superconducting film.