Crossover Current Research Articles

Electrochemical investigation of formic acid electro-oxidation and its crossover through a Nafion ® membrane

This work studies formic acid as an anode fuel for polymer electrolyte fuel cells and investigates the electro-oxidation of formic acid on Pt/C. Fuel crossovers through the Nafion ® 115 membrane at different temperatures and concentrations are reported. A linear dependence of the crossover current on the temperature and concentration is obtained. It is found that the crossover can be reduced by five times and a higher performance can be rendered by formic acid when compared to methanol under the same conditions. Electrochemical impedance measurements are conducted to interpret the reaction mechanism of formic acid oxidation. The effect of the electrode potential on the impedance pattern is revealed and an impedance model incorporating the reaction kinetics information is developed to simulate the experimental impedance response.

Shot noise of single-electron tunneling in one-dimensional arrays

We have used numerical modeling and a semi-analytical calculation method to find the low frequency value S_{I}(0) of the spectral density of fluctuations of current through 1D arrays of small tunnel junctions, using the ``orthodox theory'' of single-electron tunneling. In all three array types studied, at low temperature (kT << eV), increasing current induces a crossover from the Schottky value S_{I}(0)=2e<I> to the ``reduced Schottky value'' S_{I}(0)=2e<I>/N (where N is the array length) at some crossover current I_{c}. In uniform arrays over a ground plane, I_{c} is proportional to exp(-\lambda N), where 1/\lambda is the single-electron soliton length. In arrays without a ground plane, I_{c} decreases slowly with both N and \lambda. Finally, we have calculated the statistics of I_{c} for ensembles of arrays with random background charges. The standard deviation of I_{c} from the ensemble average <I_{c}> is quite large, typically between 0.5 and 0.7 of <I_{c}>, while the dependence of <I_{c}> on N or \lambda is so weak that it is hidden within the random fluctuations of the crossover current.

Dynamics of the current penetration in the finite samples of flat two layer superconducting cables subjected to the time dependent magnetic field

Currents induced by the time dependent magnetic field in short samples of flat two layer superconducting cables are calculated. To simplify the problem, mutual inductances between elements of the cables were neglected, while all self-inductances were attributed to the strands constituting the cable. Evolution of screening currents in the strands and crossover currents overflowing between them is studied to estimate characteristic time constants of the process of current penetration. Dependences of the maximum characteristic time constant on the sample sizes are found.

Columnar defects and vortex fluctuations in layered superconductors.

We investigate fluctuations of Josephson-coupled pancake vortices in layered superconductors in the presence of columnar defects. We study the thermodynamics of a single pancake stack pinned by columnar defects and obtain the temperature dependence of localization length, pinning energy, and critical current. We study the creep regime and compute the crossover current between linelike and pancakelike creep motion. We find that columnar defects effectively increase interlayer Josephson coupling by suppressing thermal fluctuations of pancakes. This leads to an upward shift in the decoupling line most pronounced around the matching field.

Study of the steady state of a two-species annihilation process with separated reactants

The steady-state behaviour of the irreversible reaction A+B to C with separated reactants in one dimension is studied, when currents J of A and B particles are directed towards each other by means of an analytical approach. The interparticle distance distribution in the reaction front is derived. Fixing the reaction constant, and for large J, the process is limited by reaction and the average interparticle distance at the reaction front (lAB) behaves as lAB approximately J-1. In the limit J to 0 the process is limited by diffusion and the behaviour lAB approximately J-1/2 holds. The crossover between both regimes is well described analytically and the crossover current is found to be Jc=k2/(pi D). These results are in excellent agreement with Monte Carlo simulation results.

Percolation effects in two-component nonlinear composites: Crossover from linear to nonlinear behavior.

The nonlinear response of a two-component composite is studied. The first component is assumed to be nonlinear and obeys a current-voltage (I-V) characteristic of the form I=${\mathrm{g}}_{1}$V+${\mathrm{\ensuremath{\chi}}}_{1}$${\mathrm{V}}^{3}$, while the second component is linear with I=${\mathrm{g}}_{2}$V, where ${\mathrm{g}}_{1}$ and ${\mathrm{g}}_{2}$ are linear conductances of the constituents and ${\mathrm{\ensuremath{\chi}}}_{1}$ is the nonlinear susceptibility. The volume fractions of the two components are p and q, respectively, and p+q=1. Near the percolation threshold, we identify two important limits: (i) the conductor-insulator (N/I) limit in which ${\mathrm{g}}_{2}$=0 and (ii) the superconductor-conductor (S/N) limit in which ${\mathrm{g}}_{2}$=\ensuremath{\infty}. For the S/N case and below the percolation threshold (q${\mathrm{q}}_{\mathrm{c}}$), the crossover voltage ${\mathrm{V}}_{\mathrm{c}}$, defined as the voltage at which the linear and nonlinear response become comparable, is found to have a power-law dependence ${\mathrm{V}}_{\mathrm{c}}$\ensuremath{\approx}(${\mathrm{q}}_{\mathrm{c}}$-q${)}^{\mathrm{v}\ensuremath{'}}$ as the percolation threshold is approached from below. For the N/I limit and above the percolation threshold (p&gt;${\mathrm{p}}_{\mathrm{c}}$), the crossover current ${\mathrm{I}}_{\mathrm{c}}$ is found to have a similar dependence ${\mathrm{I}}_{\mathrm{c}}$\ensuremath{\approx}(p-${\mathrm{p}}_{\mathrm{c}}$${)}^{\mathrm{w}}$ as the percolation threshold is approached from above. By a connection between the nonlinear response of the random nonlinear composite problem and the relative conductance fluctuations of the corresponding random linear composite problem, the exponents w and v\ensuremath{'} are found to be w=(\ensuremath{\kappa}+t)/2 and v\ensuremath{'}=(\ensuremath{\kappa}\ensuremath{'}+s)/2, respectively where \ensuremath{\kappa} and \ensuremath{\kappa}\ensuremath{'} are noise exponents and t and s are conductivity exponents. Previously derived bounds and estimates on \ensuremath{\kappa} and \ensuremath{\kappa}\ensuremath{'} were used to give reasonable estimates of the exponents w and v\ensuremath{'}. For a small but finite ratio h of poor to good conductances and right at the percolation threshold, the linear and nonlinear response functions are found to cross over from the fractal (h=0) to homogeneous (h=1) behavior. The scaling functions of the crossover voltage and current are obtained within the effective-medium approximation and numerical simulations. An excellent agreement with general scaling arguments is found.

Periodicity of crossover currents in a Rutherford-type cable subjected to a time-dependent magnetic field

The behavior of Rutherford-type cables under a time-dependent magnetic field is studied. Existing models are extended describing the currents flowing through the resistive contacts at the crossovers between the cable strands by considering crossover current distributions that are not uniform along the cable axis. The generalized system of equations is applied to a few cases of practical interest and shows that, if not uniform, the crossover current distribution is periodic, with a period equal to the cable pitch length.

Ohmic-non-ohmic characteristics in YBCO single crystals

Current-voltage characteristics in YBCO single crystals are studied at temperatures near the superconducting transition. The power dependences V=Alα(T), usually observed in these cases, are preceded by linear trends with an ohmic-non ohmic crossover current which increases at lower temperatures. This effect is attributed to a Josephson coupling between superconducting layers, and a quantitative description in good agreement with the experimental results is proposed.