Ohmic Regime Research Articles

Non-local heat transport, rotation reversals and up/down impurity density asymmetries in Alcator C-Mod ohmic L-mode plasmas

Several seemingly unrelated effects in Alcator C-Mod ohmic L-mode plasmas are shown to be closely connected: non-local heat transport, core toroidal rotation reversals, energy confinement saturation and up/down impurity density asymmetries. These phenomena all abruptly transform at a critical value of the collisionality. At low densities in the linear ohmic confinement regime, with collisionality ν* ⩽ 0.35 (evaluated inside of the q = 3/2 surface), heat transport exhibits non-local behaviour, core toroidal rotation is directed co-current, edge impurity density profiles are up/down symmetric and a turbulent feature in core density fluctuations with kθ up to 15 cm−1 (kθρs ∼ 1) is present. At high density/collisionality with saturated ohmic confinement, electron thermal transport is diffusive, core rotation is in the counter-current direction, edge impurity density profiles are up/down asymmetric and the high kθ turbulent feature is absent. The rotation reversal stagnation point (just inside of the q = 3/2 surface) coincides with the non-local electron temperature profile inversion radius. All of these observations suggest a possible unification in a model with trapped electron mode prevalence at low collisionality and ion temperature gradient mode domination at high collisionality.

Transition voltage spectroscopy reveals significant solvent effects on molecular transport and settles an important issue in bipyridine-based junctions

Results of a seminal study (B. Xu and N. J. Tao, Science, 2003, 301, 1221) on the single-molecule junctions based on bipyridine placed in a solvent have been challenged recently (S. Y. Quek et al., Nat. Nano, 2009, 4, 230) by implicitly assuming a negligible solvent impact on the molecular transport and by merely considering low bias conductance data. In this paper we demonstrate that solvent effects on the molecular transport are important, and to show this we focus our attention on the energy offset ε(0) of the dominant molecular orbital (LUMO) relative to the electrode Fermi level. To estimate the energy offset ε(0)(sol) from the full I-V curves presented by Xu and Tao for wet junctions, we resort to the recently proposed transition voltage spectroscopy (TVS). TVS, which plays a key role in the present analysis, emphasizes that data beyond the ohmic conductance regime are needed to reveal the solvent impact. We show that ε(0)(sol) significantly differs from the energy offset ε(0)(0)deduced for dry junctions (J. R. Widawsky et al., Nano Lett., 2012, 12, 354). The present work demonstrates that solvent effects on molecular transport are important and can be understood quantitatively. Results of ab initio calculations with and without solvent are reported that excellently explain the difference δε(0) = ε(0)(sol)-ε(0)(0). δε(0) = ΔΔG + δΦ + δW can be disentangled in contributions with a clear physical content: solvation energies (ΔΔG), image charges (δΦ), and work functions (δW). Accurate analytical formulae for ΔΔG and δΦ are reported, which provide experimentalists with a convenient framework to quantify solvent effects obviating demanding numerical efforts.

Nonlinear-Electronic Transport in Thin-Films Grown from Grain-Oriented Iron Foils

Nonlinear-electronic transport in Fe₂O₃ thin-films grown by thermal oxidation from grain-oriented iron foils was studied by current-voltage characteristics at room temperature as a function of the oxidation temperature. Microstructure formation and its changes were investigated by atomic force microscopy and X-ray diffraction studies. X-ray diffraction has demonstrated that average grain size was weakly increased with the growth temperature. The analysis of current-voltage characteristics shows ohmic regime at low voltages and space-charge-limited regime at higher voltages. Space-charge effects resulted from the discontinuous grain growth of the foils. Further, nonlinear-electronic transport of Fe₂O₃ thin-films can be useful for the designing of adaptive oxide electronic devices.

Signature of Schwinger's pair creation rate via radiation generated in graphene by strong electric current

Electron - hole pairs are copuously created by an applied electric field near the Dirac point in graphene or similar 2D electronic systems. It was shown recently that for sufficiently large electric fields E and ballistic times the I-V characteristics become strongly nonlinear due to Schwinger's pair creation rate, proportional to E3/2. Since there is no energy gap the radiation from the pairs’ annihilation is enhanced. The spectrum of radiation is calculated and exhibits a maximum at The angular and polarization dependence of the emitted photons with respect to the graphene sheet is quite distinctive. For very large currents the recombination rate becomes so large that it leads to the second Ohmic regime due to radiation friction.

Efros-Shklovskii variable-range hopping in reduced graphene oxide sheets of varying carbonsp2fraction

We investigate the low temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon sp2 fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the temperature (T) dependent resistance (R) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with T(ES) decreasing from 30976 to 4225 K and electron localization length increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100 % sp2 fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a bandgap variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55 to 80 % which agrees remarkably well with theoretical prediction. We also show that, in the high bias regime, the hopping is field driven and the data follow R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.

Transport and turbulence studies in the linear ohmic confinement regime in Alcator C-Mod

Transport in ohmically heated plasmas in Alcator C-Mod was studied in both the linear (LOC) and saturated (SOC) ohmic L-mode confinement regimes and the importance of turbulent transport in the region r/a = 0.5–0.8 was established. After an extensive analysis with TGLF and GYRO, it is found that using an effective impurity ion species with Zi = 8, and moderately high Zeff (2.0–5.6), in the LOC regime electron transport becomes dominant due to TEM turbulence. The key ingredient in the present results is the observation that dilution of the main ion species (deuterium) by impurity species of moderate charge state reduces dominant ITG turbulence, in contrast to the SOC regime with little, if any dilution. The turbulent spectrum measured with the phase contrast imaging (PCI) diagnostic is in qualitative agreement with predictions of a synthetic PCI diagnostic adopted to Global GYRO. The toroidal rotation in the low-density LOC regime is in the co-current direction but as the density is raised in the SOC regime the rotation reverses to the counter current drive direction. The impurity content of the plasma was measured recently and an effective Zi of 9 was deduced.

All-Electrical Injection and Detection of a Spin-Polarized Current Using 1D Conductors

All-electrical control of spin transport in nanostructures has been the central interest and challenge of spin physics and spintronics. Here we demonstrate on-chip spin polarizing or filtering actions by driving the gate-defined one dimensional (1D) conductor, one of the simplest geometries for integrated quantum devices, away from the conventional Ohmic regime. Direct measurement of the spin polarization of the emitted current was performed when the momentum degeneracy was lifted, wherein both the 1D polarizer for spin injection and the analyzer for spin detection were demonstrated. The results showed that a configuration of gates and applied voltages can give rise to a tunable spin polarization, which has implications for the development of spintronic devices and future quantum information processing.

Thermoelectric efficiency in the space-charge-limited transport regime in semiconductors

The thermoelectric efficiency of semiconductors is usually considered in the ohmic electronic transport regime, which is achieved through high doping. Here we consider the opposite regime of low doping where the current-voltage characteristics are nonlinear and dominated by space-charge-limited transport. We show that in this regime, the thermoelectric efficiency can be described by a single figure of merit, in analogy with the ohmic case. Efficiencies for bulk, thin-film, and nanowire materials are discussed, and it is proposed that nanowires are the most promising to take advantage of space-charge-limited transport for thermoelectrics.

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.

Microscale electrodialysis: Concentration profiling and vortex visualization

In this paper, the microscale experimental model system for studying electrodialysis (ED) process is described, which is used for visualizing in situ fluid flow and concentration profile over a wide range of applied voltages (0–100V). On the transparent silicone rubber, polydimethylsiloxane (PDMS), microscale channels (width~1mm, height~200μm) were fabricated between ion exchange membranes, while allowing microscopic visualization of fluid flow and concentration changes within the ED unit cell. This ED platform was then operated on the commercial microscope with 10mM NaCl solution. Cationic fluorescence dyes (Rhodamine 6G) were added for flow and salt concentration tracer. Concentration and fluid flow profiles within the ED cell exhibit distinct behaviors in Ohmic, limiting, and overliming regimes, respectively. Although such dynamic behaviors have been predicted previously, to the best of our knowledge, this is the first time that they were clearly visualized in a realistic ED model. Based on visualized fluid flows and concentration profiles, one can elucidate many important factors that affect ion transport (salt removal) efficiency, such as the expansion of boundary layer and vortex instability. This model system would be instrumental to test and optimize ED system parameters, and can be applicable to the study of other desalination processes such as capacitance deionization.

Charge transport properties of dielectrics revealed by isothermal surface potential decay

Revealing the charge transport properties of space grade high insulation materials can benefit the mitigation of electrostatic discharges (ESD) on spacecraft. The charge transport properties of polyimide are investigated by an isothermal surface potential decay (ISPD) experiment under a simulated space environment chamber. After irradiated the sample by an electron gun, the 2-D surface potential distributions are measured by a non-contact potential probe. From the surface potential decay curves, we obtain current density at steady state ISPD. Analyzing the steady state current density against surface potential, we find two regimes. One is Ohmic regime and another is Space Charge Limited Current (SCLC) regime, which are separated at around -950 V with the sample thickness of 27 μm at 298 K. Ohmic resistivity and effective charge carrier mobility are calculated from these two regimes, respectively. In addition, the trap density of polyimide is derived from the SCLC theory, when the sample is charged to high initial surface potential.

Performance enhancement of phosphoric acid fuel cell by using phosphosilicate gel based electrolyte

AbstractReplacement of phosphoric acid electrolyte by phosphosilicate gel based electrolytes is proposed to enhance the performance of phosphoric acid fuel cell (PAFC). Phosphosilicate gels in paste form and in powder form are synthesized from tetraethoxysilane and orthophosphoric acid by using sol-gel method, for two different P/Si ratios of 5 and 1.5, respectively. Replacement of phosphoric acid electrolyte by phosphosilicate gel paste enhances the peak power generation of the fuel cell by 133% at a cell temperature of 120°C, increases the voltage generation in the ohmic regime and extends the maximum possible load current. Polyvinyl alcohol (PVA) is used to bind the phosphosilicate gel powder and to form the hybrid crosslinked gel-polymer electrolyte membrane. Soaking the membrane with phosphoric acid solution instead of with water improves the proton conductivity of the membrane, enhances the fuel cell voltage and power generation and extends the maximum possible operating temperature. At a low temperature of 70°C, peak power produced by phosphosilicate gel polymer electrolyte membrane fuel cell (PGMFC) is increased by 40% compared to that generated by phosphoric acid fuel cell (PAFC). However, the performance of composite membrane diminishes as the cell temperature increases. Thus the phosphosilicate gel in paste form is found to be a good alternative of phosphoric acid electrolyte at medium operating temperatures, while the phosphosilicate gel PVA composite performs better at low operating temperatures.

Alternating superconductor-insulator transport characteristics in a quantum vortex chain

Experimental studies of magnetoresistance in thin superconducting strips subject to a perpendicular magnetic field $B$ exhibit a multitude of transitions, from superconductor to insulator and vice versa. Motivated by this observation, we study a theoretical model for the transport properties of a ladderlike superconducting device close to a superconductor-insulator transition. In this regime, strong quantum fluctuations dominate the dynamics of the vortex chain forming along the device. Utilizing a mapping of the vortex system at low energies to one-dimensional fermions at a chemical potential dictated by $B$, we find that a quantum phase transition of the Ising type occurs at critical values of the vortex filling, from a superconducting phase near integer filling to an insulator near half filling. The current-voltage (I-V) characteristics of the weakly disordered device in the presence of a d.c. current bias $I$ is evaluated, and investigated as a function of $B$, $I$, the temperature $T$, and the disorder strength. In the Ohmic regime ($I/e\ensuremath{\ll}T$), the resulting magnetoresistance $R(B)$ exhibits oscillations similar to the experimental observation. More generally, we find that the I-Vcharacteristics of the system manifests a dramatically distinct behavior in the superconducting and insulating regimes.

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified, since these processes occur in a particular range of the collisionality.

Ion concentration polarization near microchannel–nanochannel interfaces: Effect of pH value

This study investigates the effect of the pH value on the ion concentration polarization phenomenon and the nonlinear current-voltage characteristics of a hybrid soda-lime glass micro/nanochannel for a constant KCl salt concentration of about 1 mM. The experimental results show that the electrical conductance of the nanochannel in the Ohmic regime and the critical threshold voltage of the limiting current are both dependent on the pH value of the salt solution when the electrical double layer thickness is considerable in the nanochannel. Specifically, the nanochannel conductance increases and the critical threshold voltage for the limiting current decreases as the pH value is increased. It also suggests that a higher pH value induces a higher surface charge density on the nanochannel walls, and therefore increases both the ionic conductance and the counter-ion flux within the nanochannel.

Electronic transport in graphene-based structures: An effective cross-section approach

We show that transport in low-dimensional carbon structures with finite concentrations of scatterers can be modeled by utilizing scaling theory and effective cross sections. Our results are based on large-scale numerical simulations of carbon nanotubes and graphene nanoribbons, using a tight-binding model with parameters obtained from first-principles electronic structure calculations. As shown by a comprehensive statistical analysis, the scattering cross sections can be used to estimate the conductance of a quasi-one-dimensional system both in the Ohmic and localized regimes. They can be computed with good accuracy from the transmission functions of single defects, greatly reducing the computational cost and paving the way toward using first-principles methods to evaluate the conductance of mesoscopic systems, consisting of millions of atoms.

ECE and ECH application for investigation of plasma self-organization at T-10 tokamak

A behaviour of the high energy electrons appointed by their ECE simultaneously on two first harmonics are used for the electron distribution function analysis. Experiments were fulfilled in Ohmic regime and with on-axis ECH. The explored spectrum of emission from the extreme column periphery comes into existence after a relaxation of the primary flow of electrons with energy more than 1 MeV on the plasma waves. Maximum energy of the generated electrons does not vary during all discharge time also as the spectrum shape of i ts emission. The form of spectrum does not depend on electron temperature and density but its character width is directly proportional to the value of magnetic field. The appointed connection of the dynamical features between the peripheral high energy ele ctrons and the periodical kinetic instability in the central plasma area (mode m/n=1/1) confirms an existence of the wave transport from the center to the plasma edge. The set of experimental data corresponds to the theory of the stationary electron distribution function formation by the potential plasma waves which apparently are the main mechanism of plasma self-organization in tokamak.

Investigation of polymer electrolyte membrane fuel cell parallel flow field with induced cross flow

We present a novel idea to utilize a parallel channel design with a pressure difference between channels in an effort to maintain a short flow path, while attaining the cross flow inherent in serpentine flow fields. For this study, a polymer electrolyte membrane (PEM) fuel cell with the ability to control the back pressure on every other flow channel (high pressure channels, HPCs), in order to induce cross flow from the high pressure to the low pressure channels (LPCs), was designed and built. Polarization curves for different back pressures on the HPCs and for different stoichiometries on the cathode were measured. Performance gains were found at the end of the ohmic and mass transport loss regimes (voltages under 0.55 V). Secondly, the current density and net power (subtracting approximate pumping work) were determined based on HPC back pressure at steady state voltages of 0.5 V, 0.3 V, and 0.1 V. The parallel flow field with induced cross flow at the optimal back pressure had up to a 24% improvement in current density and a 14% improvement in net power over the standard parallel channel design.

Low-Frequency Noise Study of p-Channel Bulk MuGFETs

The low-frequency (LF) noise in p-channel bulk Multiple-Gate Field-Effect Transistors (MuGFETs) with 2.5 nm SiON gate oxide has been investigated in the ohmic regime from weak to strong inversion. It is shown that both 1/f-like noise and Generation-Recombination (GR) noise are present, giving rise to a wide device-to-device scatter in the spectral density. The 1/f noise is dominated by number fluctuations from which an effective trap density in the range 5×1017 cm-3eV-1 has been derived. At the same time, it is demonstrated that the GR noise originates from traps in the gate oxide.

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.