Non-ohmic Regime Research Articles

Lattice Boltzmann modeling of two-phase electrohydrodynamic flows under unipolar charge injection.

In this work, a two-dimensional droplet confined between two parallel electrodes under the combined effects of a nonuniform electric field and unipolar charge injection is numerically investigated using the lattice Boltzmann method (LBM). Under the non-Ohmic regime, the interfacial tension and electric forces at the droplet surface cooperate with the volumetric Coulomb force, leading to complex deformation and motion of the droplet while at the same time inducing a bulk electroconvective flow. After we validate the model by comparing with analytical solutions at the hydrostatic state, we perform a quantitative analysis on the droplet deformation factor D and bulk flow stability criteria T_{c} under different parameters, including the electric capillary number Ca, the electric Rayleigh number T, the permittivity ratio ɛ_{r}, and the mobility ratio K_{r}. It is found that the bulk flow significantly modifies the magnitude of D, which in turn decreases T_{c} of the electroconvective flow. For a droplet repelled by the anode, ɛ_{r}>1, an interesting linear relationship can be observed in the D-Ca curves. However, for a droplet attracted to the anode, ɛ_{r}<1, the system is potentially unstable. After first evolving into a quasisteady state, the droplet successively experiences steady flow, periodic flow, second steady flow, and oscillatory flow with increasing T. Moreover, discontinuities can be observed in the D-T curves due to the transitions of bulk flow.

Generation and Demolishment Mechanisms of Vapor Bubble Around Hot Tungsten Filament in Superfluid Helium-4

The electric transport I–V characteristics of a tungsten filament immersed in superfluid helium are experimentally studied. The forward sweep I–V characteristics show an abrupt jump from the linear ohmic regime (C state) to the high-resistance non-ohmic regime (H state). In the H state, the filament is covered with a He gas bubble. In the C state, there is no gas bubble, that is, liquid He directly touches the filament surface. The transitions between these two states exhibit a well-developed hysteresis and bistability. The transition from the H state to the C state occurs at the equilibrium gas–liquid phase transition point, as reported by Date et al. (J Phys Soc Jpn 35(4):1190, 1973), whereas the C-to-H-state transition occurs in the superheat region.

Efros-Shklovskii variable range hopping and nonlinear transport in 1T/1T′−MoS2

We have studied temperature- and electric-field dependent carrier transport in single flakes of MoS$_{2}$ treated with n-butyllithium. The temperature dependence of the four-terminal resistance follows the Efros-Shklovskii variable range hopping conduction mechanism. From measurements in the Ohmic and non-Ohmic regime, we estimate the localization length and the average hopping length of the carriers, as well as the effective dielectric constant. Furthermore, comparison between two- and four-probe measurements yield a contact resistance that increases significantly with decreasing temperature.

Non-Ohmic Electrical Transport in the Vicinity of the Superconducting Transition: Implications for the Study of the Phase Diagram of the Underdoped Cuprate Superconductors

In this paper, we present a theoretical discus- sion of the non-ohmic regime of the electrical conductivity of a type-II planar superconductor near its superconduct- ing transition, focusing mainly in the region between the phase-coherence and the pair-condensation critical temper- atures, Tphase <T < T cond. For that purpose, we extend down to those temperatures existing calculations for the so- called Aslamazov-Larkin conductivity in presence of finite electric fields, and the resulting exponent α of the fluctu- ation voltage-current characteristics E∼j α . These results are then discussed by considering two possible scenarios for the phase diagram of the superconducting fluctuations in the underdoped high-temperature superconductors. Our results suggest that the strong phase fluctuations scenario (i.e., large Tcond − Tphase) leads to a plateau in the T - dependence of the exponent α near Tphase, and that the T -region over which this plateau extends becomes wider when the applied electric field is increased. These features are much smaller in the conventional fluctuations scenario (i.e., small Tcond − Tphase).

Anisotropic Josephson-Vortex Dynamics in Layered Organic Superconductor withd-Wave Pairing Symmetry

To study the anisotropic Josephson-vortex dynamics in the layered organic superconductor κ-(ET)2Cu(NCS)2, we intensively investigated the in-plane anisotropy of flux-flow resistivity in the non-Ohmic regime. Sharp peaks due to dissipative processes associated with the vortex dynamics were clearly observed in all in-plane magnetic field directions. The dependence of the flux-flow resistivity on in-plane field-orientation showed a fourfold symmetry with broad maxima along the \([0, \pm 1, \pm 1]\) directions and cusp-like minima along the \([0, \pm 1, 0]\) and \([0, 0, \pm 1]\) directions. The origin of the fourfold symmetry in terms of the interplay between the \(d\)-wave superconducting gap symmetry and the anisotropic viscous motion of the vortices is discussed.

Linear magnetoresistance in n-type silicon due to doping density fluctuations

We report the observation of a large linear magnetoresistance in the ohmic regime in commonplace commercial n-type silicon wafer with a P dopant density of (1.4±0.1) ×1015 cm–3, and report measurements of it in the temperature range 30–200 K. It arises from the deformation of current paths, which causes a part of the Hall field to be detected at the voltage probes. In short, wide samples we found linear magnetoresistance as large as 4707% in an 8 tesla field at 35 K. Sample geometry effects like these are commonplace in commercial Hall sensors. However, we found that the effect persisted in long, thin samples where the macroscopic current flow should be uniform between the voltage probes: we observed a magnetoresistance of 445% under the same conditions as above. We interpret this result as arising due to spatial fluctuations in the donor density, in the spirit of the Herring model.

Fluctuation-induced tunneling conduction through RuO2 nanowire contacts

A good understanding of the electronic conduction processes through nanocontacts is a crucial step for the implementation of functional nanoelectronic devices. We have studied the current- voltage (I-V) characteristics of nanocontacts between single metallic RuO2 nanowires and contacting Au electrodes, which were pre-patterned by simple photolithography. Both the temperature behavior of contact resistance in the low-bias voltage ohmic regime and the I-V curves in the high-bias voltage non-ohmic regime have been investigated. We found that the electronic conduction processes in the wide temperature interval 1-300 K can be well described by the fluctuation-induced tunneling (FIT) conduction theory. Taken together with our previous work [Lin et al., Nanotechnology 19, 365201 (2008)], where the nanocontacts were fabricated by delicate electron-beam lithography, our study demonstrates the general validity of the FIT model in characterizing electronic nanocontacts.

Electrodialysis with spacers: Effects of variation and correlation of boundary layer thickness

The performance of electrodialysis is strongly dependent on the boundary layer thickness near an ion-exchange membrane. While a thinner boundary layer is known to enhance the ionic separation, the effects of statistical properties of the boundary layer thickness, such as the variation of the thickness and the correlation between the two boundary layers facing across the spacer, have not been elucidated. These effects were estimated by the Monte Carlo method incorporated into an analytical model. The analytical model simulates the binary ionic transport in four distinct regions of an electrodialysis cell pair: the bulk solution, boundary layer, ion-exchange membrane, and interface between the aqueous solution and ion-exchange membrane. The model current and potential relationships found that a greater variation or more positive correlation improves the ionic separation in the non-Ohmic regime. A bench-scale electrodialyzer was operated in a batch recycle system to develop steady-state current and potential relationships. Comparison between the model and experimental results found that the mean boundary layer thickness was tens of micrometers and the standard deviation of the thickness was similar to or greater than the mean thickness with the sheet-flow type mesh spacer.

Nonlinear conductivity of electronic origin in magnetic oxides: uncommon cases at E < 1000 V/cm

We report on electronic transport measurements on polycrystalline, non-metallic samples of doped-LaMnO3, in the ohmic and non-ohmic regimes. The non-ohmic regime has been investigated using d.c. as well as pulsed measurements. Large nonlinear conductivity of electronic origin, was found with pulsed currents in (nominal) La0.8Li0.2MnO3, LaMnO3+δ and in La0.8Ca0.2Mn0.95O3 at low temperatures under electric fields E < 500 V/cm. Their thermopowers exhibit maxima associated with the onset of (partial) magnetic order. For all samples, including some with perfectly linear pulsed I-V characteristic, the d.c. characteristics show, under similar fields, large nonlinearity and hysteresis, typical of self-heating in samples with activated conductivity.

Low noise constant current source for bias dependent noise measurements

A low noise constant current source used for measuring the 1∕f noise in disordered systems in ohmic as well as nonohmic regime is described. The source can supply low noise constant current starting from as low as 1 μA to a few tens of milliampere with a high voltage compliance limit of around 20 V. The constant current source has several stages, which can work in a standalone manner or together to supply the desired value of load current. The noise contributed by the current source is very low in the entire current range. The fabrication of a low noise voltage preamplifier modified for bias dependent noise measurements and based on the existing design available in the MAT04 data sheet is also described.

Nonlinear conductivity of electronic origin in self-doped LaMnO3+δ

We report on electronic transport measurements on polycrystalline, nonmetallic samples of LaMnO3+δ, in the Ohmic and non-Ohmic regimes. The non-Ohmic regime has been investigated using single current pulses of short duration. The large nonlinearity of the pulsed I-V characteristics is probably of the same origin as that of the nonmetallicity of LaMnO3+δ, that is, disorder introduced by Mn vacancies. The dc I-V characteristics, under similar fields, show much larger nonlinearity and hysteresis, typical of self-heating in samples with activated conductivity.

Current-controlled magnetoresistance in silicon in non-Ohmic transport regimes

We show that the large positive magnetoresistance in nonmagnetic silicon devices can be controlled by a current applied in the non-Ohmic transport regime. The experimental results indicate that the carrier transport in this regime is dominated by the space-charge effect, where the magnetoresistance effect is greatly enhanced. We propose a device concept based on the space-charge-induced magnetoresistance effect in silicon that is controlled by both the current and the magnetic field, which looks similar to the characteristics of the field-effect transistors.

Tuning the correlation decay in the resistance fluctuations of multi-species networks

A new network model is proposed for describing the1/fα resistance noise in disordered materials for a wide range ofα values (0<α<2). More precisely, we have considered the resistance fluctuations of a thin resistor withgranular structure in different stationary states: from nearly equilibrium to far fromequilibrium conditions. This system has been modeled as a network made up ofdifferent species of resistors, distinguished by their resistances, by their temperaturecoefficients and by the energies associated with thermally activated processes ofbreaking and recovery. The correlation behavior of the resistance fluctuationsis analyzed as a function of the temperature and applied current, in both thefrequency and time domains. For the noise frequency exponent, the model provides0<α<1 at low currents, in theohmic regime, with α decreasing inversely with the temperature, and1<α<2 at high currents, in the non-ohmic regime. Since the threshold currentassociated with the onset of non-linearity also depends on the temperature,the proposed model qualitatively accounts for the complicated behavior ofα versus temperature and current observed in many experiments. Correspondingly, in thetime domain, the auto-correlation function of the resistance fluctuations displays a varietyof behaviors which are tuned by the external conditions.

Variable range hopping in A2MnReO6 (A=Ca, Sr, Ba)

We report on transport measurements on polycrystalline samples of A2MnReO6 (A=Ca, Sr, Ba), CMRO, SMRO, and BMRO, in the Ohmic and non-Ohmic regimes. The resistivity follows, over wide ranges of temperatures, the relation derived for variable range hopping of carriers localized in a parabolic gap, ρ=ρo exp(To/T)1/2. The values of To are unusually high for the Efros Shklovskii Coulomb gap interpretation. The thermopower is large and positive for SMRO and BMRO and even larger and negative for CMRO. The magnitude of the thermopower and its temperature dependence indicate that it may be unrelated to the hopping conductivity. The theoretical relations for hopping in the intermediate electric field regime provide reasonable approximations for the measured nonlinear conductivity of most samples (and for one sample also for the high-field regime). However, localization lengths derived from the non-Ohmic regime are much larger than those estimated from To. Clues to settle these inconsistencies are suggested.

ENHANCEMENT OF NANO-RC SWITCHING DELAY DUE TO THE RESISTANCE BLOW-UP IN InGaAs

RC and transit-time delays in a nanocircuit, where the resistor is a few nanometers in length, are evaluated taking into account the velocity and current saturation and applied to RC switching delay in InGaAs heterojunction field effect transistor (HFET). Transit time delay is the dominant factor in the ohmic regime where the applied voltage V is less than the critical voltage Vc for the onset of nonlinear nonohmic behavior. However, RC time constant is predominant in the nonohmic regime and increases linearly with the applied step voltage. The power in the nanocircuit is smaller and rises linearly in the nonohmic regime as compared to the quadratic behavior in the ohmic regime.

Hysteretic current–voltage characteristics in RF-sputtered nanocrystalline TiO 2 thin films

We have measured the current–voltage characteristics at room temperature of a nanocrystalline TiO 2 thin film fabricated by reactive RF-sputtering deposition and sandwiched between ITO (indium–tin–oxide)-buffered glass substrate and an indium top electrode. The I–V characteristics are ohmic for low voltages and become non-linear, hysteretic and asymmetric as the voltage is increased. The system is shown to be well represented by two distinct resistance states in the non-ohmic region. Current transient evolutions were also measured for constant voltage excitations. The resistance is stable in time for voltages in the ohmic regime. In contrast, for voltages in the non-ohmic regime, the resistance has a small variation for a short period of time (order of tens seconds) and then increases with time. For those transients, long characteristic times (on the order of tens of minutes up to hours) were found. The behavior of the system is discussed on the basis of experimental results reported in the literature for similar systems and existing models for electric-field induced resistive switching.

In-planeI−Vcharacteristics of interlayer Josephson vortices in cuprate high-Tcsuperconductors: Dynamical simulations

Computer simulations on Josephson vortex systems driven by in-plane currents in cuprate high-${T}_{c}$ superconductors have been performed. For high anisotropy and magnetic field, we find power-law $I\text{\ensuremath{-}}V$ characteristics with the index jumping from 1 (i.e., Ohmic) at high temperatures to $\ensuremath{\geqslant}3$ (i.e., non-Ohmic) at intermediate temperatures, which can be attributed to a Kosterlitz-Thouless (KT) transition. In the KT phase, simulations reveal a resistivity independent of the angle between the current and the magnetic field, i.e., orientation-independent dissipation, due to the intralayer quasi-long-range order and interlayer short-range order of Josephson vortices. For low anisotropy and magnetic field, an orientation-dependent non-Ohmic dissipation is observed where the three-dimensional long-range order exists. The present approach provides a unified explanation to previous experimental observations in the non-Ohmic regime.

Paraconductivity of a mesoscopic superconducting tube in the nonohmic regime

By using a generalized version of the time-dependent Ginzburg-Landau theory, we have calculated the paraconductivity, $\ensuremath{\Delta}\ensuremath{\sigma}$, of a superconducting mesoscopic tube beyond the conventional ohmic approximation. Among other dimensionality effects, the so-obtained paraconductivity expressions show that, close to ${T}_{c}$ and deep inside the nonohmic region, a dimensional crossover from the one-dimensional to the two-dimensional fluctuation regimes can be induced which, in turn, may be understood in terms of an effective shrinkage of the superconducting coherence length with the electrical field. These nonohmic effects on $\ensuremath{\Delta}\ensuremath{\sigma}$ may be observed in Al mesoscopic tubes and carbon nanotubes, thus opening new experimental possibilities to investigate the superconducting behavior of these nanostructured materials.

ELECTRO-THERMAL RESPONSE OF A VOLTAGE-BIASED HIGH-TC BOLOMETER

The characteristics of voltage biased HTc bolometers have been analysed by numerically solving the heat continuity equation of the system for ohmic and non-ohmic behaviour. The solutions show that the time constant and the linearity range of response depend strongly on biasing condition. For a bolometer in ohmic regime and operating in strong electrothermal feedback, the time constant can be reduced of more of one order of magnitude and linearity range increased of about 100 times with respect to the same device biased at low bias voltage. More limited improvements are obtained if the bolometer is in non-ohmic regime.

Instability in a non-ohmic/ohmic fluid interface under a perpendicular electric field and unipolar injection

We set the equations for the linear electrohydrodynamic instability of an interface between two fluids, subjected to a perpendicular field and a unipolar charge injection. One of the fluids is modeled as being in non-ohmic regime (insulating), whereas the other is ohmic. A new interfacial instability mechanism is described, which may account for the Rose-window instability. The equations are analytically solved in the limit of long wavelength and neglecting the fluid motion. We show that this limit applies well to the case of an air–ohmic liquid interface. The applicability to a liquid–liquid interface is also analyzed.