Electron Diffusion Constant Research Articles

Long- to short-junction crossover and field-reentrant critical current in Al/Ag-nanowires/Al Josephson junctions

We have probed the superconducting proximity effect through long high-quality monocrystalline Ag nanowires, by realizing Josephson junctions of different lengths, with different superconducting materials. Thanks to the high number of junctions probed, both the contact resistance and electron diffusion constant could be determined, enabling a comparison of the measured critical current to theoretical expectation, over the entire regime from short to long diffusive junction. Although the length dependence of the critical current is as expected, the amplitude of the $R_{N}I_c$ product is smaller than predicted by theory. We also address the magnetic field dependence of the critical current. The quasi-gaussian decay of the critical current with field expected of a long narrow junction is observed for all superconducting contacts we used except for aluminum. We present the striking non-monotonous effect of field on the critical current of junctions with aluminum contacts, and analyze it in terms of improved quasiparticle thermalization by a magnetic field.

Measurement of the drift velocity and transverse diffusion of electrons in liquid xenon with the EXO-200 detector

The EXO-200 Collaboration is searching for neutrinoless double beta decay using a liquid xenon (LXe) time projection chamber. This measurement relies on modeling the transport of charge deposits produced by interactions in the LXe to allow discrimination between signal and background events. Here we present measurements of the transverse diffusion constant and drift velocity of electrons at drift fields between 20~V/cm and 615~V/cm using EXO-200 data. At the operating field of 380~V/cm EXO-200 measures a drift velocity of 1.705$_{-0.010}^{+0.014}$~mm/$\mu$s and a transverse diffusion coefficient of 55$\pm$4~cm$^2$/s.

Formation of double-layered TiO2 structures with selectively-positioned molecular dyes for efficient flexible dye-sensitized solar cells

To fabricate flexible dye-sensitized solar cells (DSCs) utilizing full solar spectrum, the double-layered TiO2 films, selectively loading two different dyes in discrete layers, were formed on a plastic substrate by transferring the high-temperature-processed N719/TiO2 over an organic dye (TA-St-CA)-sensitized TiO2 film by a typical compression process at room temperature. It was found that interface control between two TiO2 layers is crucial for the efficient transport of photo-injected electrons from the N719/TiO2 to the TA-St-CA/TiO2 layer. Electron impedance spectra (EIS) and transient photoelectron spectroscopic analyses exhibited that introduction of a thin interfacial TiO2 layer between the two TiO2 layers remarkably decreased the resistance at the interface, while increasing the electron diffusion constant (De) by ∼10 times. As a result, the photovoltaic conversion efficiency (η) of the flexible tandem DSC was 6.64%, whereas that of the flexible cell derived from the single TA-St-CA/TiO2 layer was only 2.98%. Another organic dye (HC-acid), absorbing a short wavelength region of solar spectrum, was also applied to fabricate flexible tandem DSC. The η of the cell derived from the HC-acid/TiO2 layer was only 2.91%, but that of the tandem cell obtained by transferring the high-temperature-processed N719/TiO2 was 6.86%.

Crossover from weak localization to anti-weak localization in indium oxide systems with wide range of resistivity

We investigated the magnetoconductivity Δσ(H)≡1/ρ(H)−1/ρ(0) in a wide range of magnetic fields for three-dimensional indium oxide films doped with zinc, tin, or gallium in the range of resistivity ρ(300K) between 4.1×10−6 Ωm and 1.7×10−3 Ωm. The weak localization theory was fitted to data for Δσ(H) at various temperatures in the range 2.0 K≤T≤50 K by the use of suitable characteristics Dτin(T) and Dτso, where D, τin, and τso are the electron diffusion constant, inelastic scattering time, and spin-orbit (s-o) scattering time, respectively. It was found that (i) for films with a large value of ρ, the sign of Δσ(H) changes from positive to negative with decreasing temperature as a precursor to an anti-weak localization effect; (ii) the ratio τso/τin decreases from ≈4000 to≈4.0 with increasing ρ; (iii) the strong ρ dependence of Dτso cannot be explained by the model with a constant atomic number Z in a formula τso∝1/Z4 proposed by Abrikozov and Gorkov Zh. Eksp. Teor. Fiz. 42, 1088 (1962); [Sov. Phys. JETP 15, 752 (1962)]. As a reason for this ρ dependence, we suggest that the s-o scattering changes with increasing ρ from light oxygen atoms to heavy atoms, i.e., indium, zinc, and gallium, because of the decrease in the number of oxygen vacancies acting as s-o scattering centers.

Effects of absorbed hydrogen on the electronic properties of (Zr2Fe)1−xHx metallic glasses

The electrical conductivity (σ) of hydrogen doped (Zr2Fe)1−xHx metallic glasses has been measured in the temperature range from 290 down to 5 K. The decrease of the room temperature conductivity and the increase of its temperature coefficient are explained as consequences of increased disorder due to hydrogen doping. σ(T) for (Zr2Fe)1−xHx metallic glasses at low temperatures decreases with the increase of temperature, forming a minimum at Tmin, before it starts a monotonic increase with increasing temperature. Both the functional forms and the magnitudes of the observed σ(T) are interpreted in terms of weak localization, electron–electron interaction and spin-fluctuation effects. Our results reveal that the electron–phonon scattering rate varies with the square of temperature from low temperatures up to 100 K and changes behaviour to a linear form at higher temperatures. At low temperatures, the minimum in σ(T) is shifted to higher temperatures, which is ascribed to the increase of the screening parameter of the Coulomb interaction F* associated with the enhancement of the spin fluctuations arising from the increase of the hydrogen doping. The spin–orbit scattering rate and the electron diffusion constant are reduced by hydrogen doping.

Effect of polyelectrolyte interdiffusion on electron transport in redox-active polyelectrolyte multilayers

We report that polyelectrolyte interdiffusion has a strong influence on the speed of electron transport in redox-active polyelectrolyte multilayers. For interdiffusive polyelectrolyte multilayers, the measured electron diffusion constants were consistently two orders of magnitude larger than the ones for systems where interdiffusion was not possible. This finding was verified for different polyelectrolyte multilayer systems that either allowed or prevented polyelectrolyte interdiffusion. The electron diffusion constants were deduced from cyclic voltammograms at various scanning speeds using the Randles–Sevcik equation for irreversible redox processes. The increased speed of electron transport in interdiffusive multilayers is explained by short-range movement of the redox-sites, allowing fast electron exchange through physical contact of neighboring redox-sites.

Measuring spin diffusion of electrons in bulk n-GaAs using circularly dichromatic absorption difference spectroscopy of spin gratings

Circular dichromatic absorption difference spectroscopy is developed to measure the spin diffusion dynamics of electrons in bulk n-GaAs. This spectroscopy has higher detection sensitivity over homodyne detection of spin-grating-diffracted signal. A model to describe circular dichromatic absorption difference signal is derived and used to fit experimental signal to retrieve decaying rate of spin gratings. A spin diffusion constant of Ds=201±25 cm2/s for bulk n-GaAs has been measured at room temperature using this technique and is close to electron diffusion constant (Dc), which is much different from the case in GaAs quantum wells where Ds is markedly less than Dc.

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr 76Ni 24) 1−xH x metallic glasses

The magnetizations of Zr 76Ni 24 metallic glass and hydrogen-doped partially crystalline (Zr 76Ni 24) 1− x H x metallic glasses have been measured in the temperature range 10–300 K and magnetic fields up to 2 T for various dopant concentrations ( x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr 76Ni 24) 1− x H x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.

Erratum to Laplacian Instability of Planar Streamer Ionization Fronts—An Example of Pulled Front Analysis

Streamer ionization fronts are pulled fronts propagating into a linearly unstable state; the spatial decay of the initial condition of a planar front selects dynamically one specific long time attractor out of a continuous family. A transverse stability analysis has to take these features into account. In this paper we introduce a framework for this transverse stability analysis, involving stable and unstable manifolds in a weighted space. Within this framework, a numerical dynamical systems method for the calculation of the dispersion relation as an eigenvalue problem is defined and dispersion curves for different values of the electron diffusion constant and of the electric field ahead of the front are derived. Numerical solutions of the initial value problem confirm the eigenvalue calculations. The numerical work is complemented with analytical expressions for the dispersion relation in the limit of small and large wave numbers and with a fit formula for intermediate wave numbers. This empirical fit supports the conjecture that the smallest unstable wave length of the Laplacian instability is proportional to the diffusion length that characterizes the leading edge of the pulled ionization front.

Laplacian Instability of Planar Streamer Ionization Fronts—An Example of Pulled Front Analysis

Streamer ionization fronts are pulled fronts that propagate into a linearly unstable state; the spatial decay of the initial condition of a planar front selects dynamically one specific long-time attractor out of a continuous family. A stability analysis for perturbations in the transverse direction has to take these features into account. In this paper we show how to apply the Evans function in a weighted space for this stability analysis. Zeros of the Evans function indicate the intersection of the stable and unstable manifolds; they are used to determine the eigenvalues. Within this Evans function framework, we define a numerical dynamical systems method for the calculation of the dispersion relation as an eigenvalue problem. We also derive dispersion curves for different values of the electron diffusion constant and of the electric field ahead of the front. Numerical solutions of the initial value problem confirm the eigenvalue calculations. The numerical work is complemented with an analysis of the Evans function leading to analytical expressions for the dispersion relation in the limit of small and large wave numbers. The paper concludes with a fit formula for intermediate wave numbers. This empirical fit supports the conjecture that the smallest unstable wave length of the Laplacian instability is proportional to the diffusion length that characterizes the leading edge of the pulled ionization front.

Studies on the drift properties and spatial resolution using a microMEGAS-equipped time projection chamber

R&D studies on the performance as well as on the gas properties of the microMEGAS-based time projection chamber with standard readout were carried out in June 2005 using 4 GeV/c pion beam in a magnetic field from 0 to 1 T at the proton synchrotron beam line at KEK, Japan. Analysis of the electron drift velocity, diffusion constant and point resolution of padrow measurement for MicroMEGAS TPC filled with 95% argon and 5% isobutane gas are presented. The underlying physical mechanism which determines the optimal TPC performance are briefly discussed. Preliminary measurements of gas properties and spatial resolution in close agreement with the analytical calculation and MAGBOLTZ simulation are summarized and presented in this paper.

Low-temperature electron dephasing time in AuPd revisited

Ever since the first discoveries of the quantum-interference transport in mesoscopic systems, the electron dephasing times, τ ϕ , in the concentrated AuPd alloys have been extensively measured. The samples were made from different sources with different compositions, prepared by different deposition methods, and various geometries (1D narrow wires, 2D thin films, and 3D thick films) were studied. Surprisingly, the low-temperature behavior of τ ϕ inferred by different groups over two decades reveals a systematic correlation with the level of disorder of the sample. At low temperatures, where τ ϕ is (nearly) independent of temperature, a scaling τ ϕ max ∝ D - α is found, where τ ϕ max is the maximum value of τ ϕ measured in the experiment, D is the electron diffusion constant, and the exponent α is close to or slightly larger than 1. We address this nontrivial scaling behavior and suggest that the most possible origin for this unusual dephasing is due to dynamical structure defects, while other theoretical explanations may not be totally ruled out.

Charge Transport versus Recombination in Dye-Sensitized Solar Cells Employing Nanocrystalline TiO2 and SnO2 Films

We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs) employing nanocrystalline TiO(2) and SnO(2) films and address the impact of these dynamics upon photovoltaic device efficiency. Transient photovoltage studies of electron transport in the metal oxide film are correlated with transient absorption studies of electron recombination with both oxidized sensitizer dyes and the redox couple. For all three processes, the dynamics are observed to be 2-3 orders of magnitude faster for the SnO(2) electrode. The origins of these faster dynamics are addressed by studies correlating the electron recombination dynamics to dye cations with chronoamperometric studies of film electron density. These studies indicate that the faster recombination dynamics for the SnO(2) electrodes result both from a 100-fold higher electron diffusion constant at matched electron densities, consistent with a lower trap density for this metal oxide relative to TiO(2), and from a 300 mV positive shift of the SnO(2) conduction band/trap states density of states relative to TiO(2). The faster recombination to the redox couple results in an increased dark current for DSSCs employing SnO(2) films, limiting the device open-circuit voltage. The faster recombination dynamics to the dye cation result in a significant reduction in the efficiency of regeneration of the dye ground state by the redox couple, as confirmed by transient absorption studies of this reaction, and in a loss of device short-circuit current and fill factor. The importance of this loss pathway was confirmed by nonideal diode equation analyses of device current-voltage data. The addition of MgO blocking layers is shown to be effective at reducing recombination losses to the redox electrolyte but is found to be unable to retard recombination dynamics to the dye cation sufficiently to allow efficient dye regeneration without resulting in concomitant losses of electron injection efficiency. We conclude that such a large acceleration of electron dynamics within the metal oxide films of DSSCs may in general be detrimental to device efficiency due to the limited rate of dye regeneration by the redox couple and discuss the implications of this conclusion for strategies to optimize device performance.

Ionic liquid type dye-sensitized solar cells: increases in photovoltaic performances by adding a small amount of water

Increases in photovoltaic performances for dye-sensitized solar cells having ionic liquid type electrolytes are reported. These results are explained by diffusion coefficient for I3−, charge transfer resistances on counter electrodes, flat band potentials of TiO2, redox potentials for I−/I3−, electron diffusion constants, electron life time, and diffusion length in TiO2 layers. Methylpropylimidazolium iodide is selected because of the lowest viscosity and the highest conductivity. Increases in the photovoltaic performance are observed when a small amount of water was added into the ionic liquid consisting of both LiI and t-butylpyridine as the additives. These improvements are brought about by enhancements of all of Jsc, ff and Voc. The increases in Jsc and ff are associated with the decrease in charge transfer resistances on counter electrodes and increases in ionic conductivities. Voc may be explained by an increase in the difference between redox potentials of I−/I3− and Fermi level.

Estimation of the Electron Concentration, Diffusion Constant, and Mobility from Probe Current-Voltage Characteristics

The possibility of the simplest method of processing of the current-voltage characteristic of a Langmuir probe immersed in a non-Maxwellian plasma for finding the electron concentration, diffusion constant, and mobility is examined.

Transport properties of hydrogen‐doped (Zr803d20)1−xHx (3d = Co, Ni) metallic glasses

AbstractThe electrical resistivities of hydrogen‐doped (Zr803d20)1−xHx (3d = Ni, Co; x ≤ 0.11) metallic glasses have been measured at temperatures between 2 K and 110 K and in magnetic fields up to 1 T for various dopant concentrations. These systems have a high room‐temperature resistivity (ρ > 160 μΩ cm) and become superconducting below 4 K. The increase of the room‐temperature resistivity and its temperature coefficient with hydrogen dopant concentration is explained as due to an increase of disorder with hydrogen‐doping. The temperature and magnetic field dependence of the resistivity has been analysed using theoretical models of weak‐localisation and electron–electron interaction in disordered conductors. The hydrogen dopant is found to reduce the effective electron diffusion constant, D, the spin‐orbit scattering rate, τ−1so, the superconducting transition temperature, Tc, and broadens the superconducting transition region. The contribution of the Maki‐Thompson interaction to the magnetoresistivity is also reduced. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetoconductivity of icosahedral and amorphous Al–Pd–Re films — a comparison

The magnetoconductivity of quasicrystals is often discussed in the frame of quantum corrections, namely weak (anti-) localisation and electron–electron interaction. A premise for both effects is a strong elastic scattering of conduction electrons. Amorphous and icosahedral phases are discussed as Hume–Rothery alloys with an electronically induced structural peak at the diameter of the Fermi sphere. Therefore, both should exhibit quantum corrections. The preparation of quasicrystalline films via the amorphous route offers the possibility to compare the magnetoconductivity on samples of identical composition but different structure. We report on magnetoconductivity measurements at temperatures between 0.2 and 22K and for magnetic fields up to 16T. With the exception of the electronic diffusion constant, amorphous as well as icosahedral Al–Pd–Re films can be described by nearly the same set of parameters if the samples are well on the metallic side of the metal–insulator transition.

Upper critical field and fluctuation conductivity in Nb-doped strontium titanate thin films

We show that the transition temperature ${T}_{c}$ of Nb-doped strontium titanate thin films grown in vacuum by pulsed laser deposition is similar to bulk and that these films follow a similar dependence of ${T}_{c}$ on the carrier density as bulk. We have measured the temperature dependence of the upper critical field ${H}_{c2}$ in thin films. We fit the data to the empirical temperature dependence ${H}_{c2}\ensuremath{\propto}(1\ensuremath{-}{t}^{2}{)/(1+t}^{2}),$ where ${t=T/T}_{c},$ and determined the electronic diffusion constant from the slope of ${\mathrm{dH}}_{c2}/dT$ near ${T}_{c}.$ We further fit the temperature dependence of the fluctuation conductivity above ${T}_{c}$ successfully to the theories of Aslamazov-Larkin and Maki-Thompson.

Dephasing of conduction electrons due to zero-point fluctuation

We consider conduction-electron dephasing by the zero-point fluctuation of an electromagnetic field in a vacuum. We argue that there is an upper cutoff frequency of the zero-point fluctuation mode beyond which the zero-point fluctuation will not contribute to the dephasing of conduction electrons. The upper cutoff frequency is material-parameter dependent. We find that the dephasing time is proportional to the electron diffusion constant. This result agrees very well with existing experimental data.

Transport properties of hydrogen-doped (Zr80Fe20)1−xHx metallic glasses

The electrical resistivities of hydrogen-doped (Zr80Fe20)1−xHx metallic glasses have been measured at temperatures between 2 and 290 K and in magnetic fields to 1 T for various dopant concentrations (x=0, 0.017, 0.031, 0.071, 0.104). Zr80Fe20 metallic glass has a high room-temperature resistivity (ρ=160 μΩ cm) and becomes superconducting at 3.28 K. The temperature and magnetic field dependence of the resistivity has been analyzed using theoretical models of weak-localization and electron–electron interaction in disordered three-dimensional conductors. Doping the samples with hydrogen increases disorder. In the Zr80Fe20 system hydrogen reduces the effective electron diffusion constant, D, the electron density at the Fermi level, N(EF), and the screening of the Coulomb interaction, F, leading to a decrease in conductivity and suppression of the superconducting transition temperature. The contribution of the Maki–Thompson interaction to the magnetoresistivity is also depressed.