Steady-state Electron Transport Research Articles

High-bias quantum electron transport

High-bias, steady-state electron transport in small systems is modeled using the non-equilibrium Green's function formalism of Keldysh, Kadanoff and Baym. Using a model of local oscillators for the phonons simplifies the coupled equations for G ≶, G R, Σ ≶ and Σ R. The equations are numerically solved in real-space, r and r′ and energy, E. When an Einstein phonon spectrum is used for the oscillators, the local oscillator model corresponds precisely to dispersionless, deformation-potential, optical phonons. The electron-phonon interaction is treated in the self-consistent first- Born approximation. Example calculations are presented showing the effect of inelastic scattering on the energy dependence of the current density through a 1-D, multi-resonant-state, double-barrier structure and the occupation of an emitter quasi-bound state in a 3-D double-barrier structure.

Photoinactivation of photosystem II as studied with site-directed D2 mutants of the cyanobacterium Synechocystis sp. PCC 6803

To study the primary sites of photoinactivation in Photosystem II, electron transport stability in cells and thylakoids of wild type and four site-directed D2 mutants of the cyanobacterium Synechocystis sp. PCC 6803 with greatly increased rates of photoinactivation has been determined. In two mutants, E69Q and P161L, both impaired at or near the water-splitting complex, photoinactivation is very rapid. The cause of the high rate of photoinactivation probably is the lack of sufficient electron donation to highly oxidizing species, including Z+ and P680+. Addition of an artificial electron donor to Z+ causes a decrease in the rate of photoinactivation. Photoinactivation also is rapid in H197Q, a mutant in which one of the presumed ligands to P680 has been changed, leading to a decrease in the operating redox midpoint potential of the P680/P680+ couple, and thus to a higher steady-state radical concentration at the donor side (Z++P680+) in the light. In wild type and all mutants studied the rate of photoinactivation continued to increase upon increasing the light intensity beyond that required for maximal steady-state electron transport rates. We hypothesize that the Z+, P680+, or other radicals associated with the Photosystem II donor side are the main triggers for the photoinactivation process. To test the hypothesis of double-reduction of QA leading to loss of the quinone and to photoinactivation, the rapidly photoinactivating mutant G215W (which is mutated near the QA site) was illuminated in the presence and absence of DCMU. The rate of photoinactivation of PS II in G215W in the presence of DCMU was similar to that of wild type and of the other D2 mutants, even though in the presence of DCMU QA will tend to be more reduced than in its absence. Therefore, we do not find evidence for direct QA involvement in photoinactivation. In all of the mutants in vivo photodamage to PS II cannot be fully repaired without de novo protein synthesis.

Monte Carlo calculation of temperature dependence of the transport properties in compensated GaAs

Monte Carlo calculation has been applied to investigate the temperature and compensation dependence of steady-state electron transport in n-type GaAs over a wide range of applied field strengths. It is found that doping compensation has stronger effect on the transport properties at low temperatures than at high temperatures. Compensation-enhanced impurity scattering is responsible for the reduction not only in low-field mobilities and peak velocities but also in the negative differential mobilities and the high-field velocities. The two-maxima behavior in the velocity-field characteristics persists at low temperatures through room temperatures for high doping compensation then it starts to diminish at 450 K except for compensation ratio of 0.9. The physical origin of this unique two maxima feature in the velocity-field relation has been discussed in comparison with other compensated semiconductors.

Velocity-field characteristics for intrinsic and doped central-valley alloy semiconductor Al 0.25In 0.75As

Monte Carlo calculation on steady-state electron transport in intrinsic and doped Al 0.25In 0.75As at several lattice temperatures is reported. The small Γ valley effective mass and the large Γ to L and Γ to X valley separations result in the excellent low-field mobilities and the peak velocities in comparison to those of GaAs, InP and In 0.53Ga 0.47As at various doping levels. It is found that alloy scattering has a significant effect on electron drift velocities at high fields. Due to large electron affinity difference and large L-valley effective mass, this scattering causes rather weak temperature dependence of electron velocities at high fields and low saturation velocities. As a result, a strong Gunn effect is predicted at various temperatures as indicated by large values of the peak-to-valley ratios.

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (Φp), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (qI).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (Φexe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q N and the efficiency of excitation capture (Φexe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.

Regulation of the imbalance in light excitation between Photosystem II and Photosystem I by cations and by the energized state of the thylakoid membrane

The imbalance in photoactivity between the two photosystems in broken chloroplasts during steady-state electron transport was investigated using modulated chlorophyll a fluorimetry and oxygen evolution. No imbalance in favor of PS II (imbalance term equals zero) was found at low cation concentration (e.g., 10 mM NaCl) where the membranes are unstacked, while some imbalance in favor of PS II (imbalance term about 0.1–0.2) could be observed at ‘high’ cation medium (e.g., 100 mM for univalent, 5 mM for divalent and 100 μM for trivalent cations) where the membranes are stacked. At the high cation concentration the imbalance was particularly noticeable at a pH range 6–7.5 under conditions where the membranes were non-energized, e.g., in the presence of a range of uncouplers and ionophores (an imbalance term of between 0.44 and 1.1). In the absence of uncouplers or ionophores the imbalance term was initially high, decreasing to a low steady-state value during the light-induced energization of the membranes. The increase in the imbalance measured with the addition of gramicidin D was wavelength-dependent, implying changes in the allocation of excitation energy to the photosystems rather than any other mechanism. This effect was reversed at higher pH: At a pH higher than about 8, the imbalance in absence of uncouplers or ionophores was stronger than in their presence. The relation between the state of imbalance and the cross-membrane proton gradient (ΔpH) was not straightforward or simple, as follows: (i) Imbalance was induced by uncouplers and ionophores also at sufficiently low light intensities which produce only very small ΔpH. (ii) Valinomycin (+KCl) had the same effect as uncouplers like gramicidin D, nigericin, NH4Cl and others, although it presumably abolishes membrane potential but not ΔpH. (iii) The effect of gramicidin D and NH4Cl was close to saturation at concentrations which affect ΔpH still minutely. (iv) The same level of large imbalance could be achieved even without uncouplers or ionophores when a much higher cation concentration (approx. 10-fold) than that considered normal to achieve membrane stacking was used. It is therefore concluded that: (a) High cationic levels change the allocation of excitation energy, probably via the screening of negative charges on the thylakoid surface. (b) The effect of uncouplers and ionophores in the presence of high cationic levels most probably reflects, at least in part, the effect on external surface charges, their exposure by membrane energization and their screening by cations.

Hot phonon effects on electron high-field transport in GaAs

An original ensemble Monte Carlo technique for hot phonon problems is applied for the simulation of anisotropic nonequilibrium phonon distributions in momentum space. The dynamics of the coupled nonequilibrium electron-LO-phonon system in electric fields are simulated within the realistic three-valley model of GaAs. Hot phonon effects on the transient and steady-state high-field electron transport as well as on the relaxation of electrically heated electrons are obtained and investigated under various conditions.

High Field Balance Equations of Quantum Wire Structures11This work was supported by the National Natural Science Foundation of China.

Balance equations are established for steady-state nonlinear electronic transport in the presence of an electric field both for a single quasi 1D-heterostructure and for a multi–quantum–wire superlattice. The carrier-impurity scatterings and carrier–phonon scatterings have been taken into account. The general equations for determining the resistivities are obtained and the Ohmic mobility formula are given. The density-density correlation functions for the wire structures are also studied in some detail.

Monte Carlo studies of steady-state electronic transport in compensated In0.53Ga0.47As

The steady-state velocity-field characteristics of n-type In0.53Ga0.47As are obtained through a Monte Carlo calculation for temperatures between 77 and 400 K, and for compensation ratios Na/Nd between 0 and 0.9. Alloy scattering is found to play a significant role in the low-temperature regime. Low-field electron drift mobilities are extracted from the velocity-field curves. We conclude by suggesting empirical relationships which model the velocity-field characteristics as a function of low-field electron drift mobility.

Steady-state electron transport in short GaAs n+-n-n+ structures

The electron transport in submicrometre GaAs n+-n-n+ structures is simulated by a charge-sheet method. It is shown that the lag of inter-valley scattering between the Gamma , L and X valleys is of great importance in structures of submicrometre length. A good agreement between the experimentally measured and calculated I-V characteristic is obtained.

Inhibition of electron transfer and the electrogenic reaction in the cytochrome [formula omitted] complex by 2- n-nonyl-4-hydroxyquinoline N-oxide (NQNO) and 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB)

We investigated the interaction between the cytochrome b f complex and plastoquinone by determining the effect of the electron transport inhibitors 2- n-nonyl-4-hydroxy-quinoline N-oxide (NQNO) and 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB) on the flash-induced turnover of cytochromes b 6 and f, on the slow phase of the electrogenic reaction (515 s) and on steady-state electron transport. The experiments were performed using thylakoid membranes in the presence of duroquinol, methyl viologen, and diuron, conditions in which electron transport is driven by Photosystem I and includes the cytochrome b f complex. The data from these experiments indicate that NQNO and DBMIB inhibit at two different sites on the cytochrome b f complex. The data can be accommodated by a modified Q-cycle in which the primary site of inhibition by NQNO is the quinone reductase site (Q c) and the primary site of DBMIB inhibition is the quinol oxidase site (Q z). NQNO is envisioned to inhibit the oxidation of the cytochrome b 6-heme located nearest the outer aqueous phase, as well as to slow electron transfer between the two b-hemes. The effect of NQNO on the 515 s supports the notion that the slow electrogenic reaction is due to electron transfer between the two cytochrome b 6-hemes, followed by a reaction associated with plastoquinone reduction at the Q c-site (Jones and Whitmarsh (1985) Photobiochem. Photobiophys. 9, 119–127). The results indicate that the Q z-site and Q c-site are likely separated by 70% of the dielectrically weighted distance across the membrane. At low concentrations of DBMIB the kinetics of the 515 s and cytochrome b 6 reduction are monophasic, indicating that DBMIB moves from one inhibitory site to another within the turnover time of the cytochrome b f complex. This observation suggests that the debinding rate of DBMIB from the Q z-site is greater than 200 s −1. Lastly, comparing the effect of DBMIB on the extent of cytochrome f oxidation and the rate of cytochrome f reduction, raises the possibility that DBMIB has a second lower affinity binding site on the cytochrome b f complex, possibly interrupting electron flow between the FeS center and cytochrome f.

Transient and steady-state electron transport in GaAs/AlxGa1-xAs heterojunctions at low temperatures: The effects of electron-electron interactions.

Transient and steady-state electron transport in GaAs/AlxGa1-xAs heterojunctions at low temperatures: The effects of electron-electron interactions.

Preparation and characterization of Photosystem II particles from a thermophilic cyanobacterium

Photosystem II particles have been isolated and purified from the thermophilic cyanobacterium Mastigocladus laminosus after sulfobetaine treatment. The light-induced steady-state electron transport, flash-induced absorption changes at 830 nm and 325 nm, reflecting the turnover of P-680 and of the acceptor side, respectively, have been measured. In the presence of artificial electron acceptors the preparations showed high O 2-evolution activities (e.g., 3000–4000 μmol O 2 per mg Chl a per h with phenyl- p-benzoquinone) which were DCMU-sensitive. O 2-evolution activity was stable in the presence of various cations (10 mM) and of glycerol (25% (v/v) in the suspending medium. Furthermore, the activity was stable up to 60°C and maximum rate of the electron transport was observed at 50°C. The spectrum of the particles showed a chlorophyll a maximum at approx. 673 nm, some carotenoids were still present but phycocyanin was completely removed. After SDS-polyacrylamide gel electrophoresis the preparations contained mainly nine protein bands at 110, 105, 60, 55, 35, 33, 22, 18 and 17 kDa. Under repetitive laser flash excitation the P-680 +-reduction kinetics contained components in the nanosecond and microsecond range with an absorption rate of approx. 1:1. When PS-II particles were purified at room temperature by anion-exchange chromatography nanosecond relaxation kinetics were observed even though the particles did not evolve oxygen. The gel analysis for this preparation showed mainly four protein bands at 60, 55, 35 and 18 kDa.

Balance Equation Theory for High-Field and High-Frequency Electronic Transport in Type-II Superlattices

Balance equations are established for steady-state nonlinear electronic transport in the presence of an electric field in a type-II superlattice, which is modelled by periodically arranged electron-hole bi-layers of finite thickness. Both carrier-phonon and carrier-impurity scatterings, as well as electron-hole interaction, are taken into account. The approach is also extended to the discussion of high-frequency linear transport of the system. The general equations-for determining the current density are obtained and the explicit expressions for complex dynamical resistivity due to electron-hole coupling are given.

Evidence for slow turnover in a fraction of Photosystem II complexes in thylakoid membranes

We used two different techniques to measure the recovery time of Photosystem II following the transfer of a single electron from P-680 to Q A in thylakoid membranes isolated from spinach. Electron transfer in Photosystem II reaction centers was probed first by spectroscopic measurements of the electrochromic shift at 518 nm due to charge separation within the reaction centers. Using two short actinic flashes separated by a variable time interval we determined the time required after the first flash for the electrochromic shift at 518 nm to recover to the full extent on the second flash. In the second technique the redox state of Q A at variable times after a saturating flash was monitored by measurement of the fluorescence induction in the absence of an inhibitor and in the presence of ferricyanide. The objective was to determine the time required after the actinic flash for the fluorescence induction to recover to the value observed after a 60 s dark period. Measurements were done under conditions in which (1) the electron donor for Photosystem II was water and the acceptor was the endogenous plastoquinone pool, and (2) Q 400, the Fe 2+ near Q A, remained reduced and therefore was not a participant in the flash-induced electron-transfer reactions. The electrochromic shift at 518 nm and the fluorescence induction revealed a prominent biphasic recovery time for Photosystem II reaction centers. The majority of the Photosystem II reaction centers recovered in less than 50 ms. However, approx. one-third of the Photosystem II reaction centers required a half-time of 2–3 s to recover. Our interpretation of these data is that Photosystem II reaction centers consist of at least two distinct populations. One population, typically 68% of the total amount of Photosystem II as determined by the electrochromic shift, has a steady-state turnover rate for the electron-transfer reaction from water to the plastoquinone pool of approx. 250 e − / s, sufficiently rapid to account for measured rates of steady-state electron transport. The other population, typically 32%, has a turnover rate of approx. 0.2 e − / s. Since this turnover rate is over 1000-times slower than normally active Photosystem II complexes, we conclude that the slowly turning over Photosystem II complexes are inconsequential in contributing to energy transduction. The slowly turning over Photosystem II complexes are able to transfer an electron from P-680 to Q A rapidly, but the reoxidation of Q − A is slow ( t 1 2 = 2 s ). The fluorescence induction measurements lead us to conclude that there is significant overlap between the slowly turning over fraction of Photosystem II complexes and PS II β reaction centers. One corollary of this conclusion is that electron transfer from P-680 to Q A in PS II β reaction centers results in charge separation across the membrane and gives rise to an electrochromic shift.

Balance-equation analysis of linear and nonlinear electronic transport in quasi-two-dimensional quantum-well superlattices.

A non-Boltzmann balance-equation approach to linear and nonlinear dc steady-state electronic transport in a type-I superlattice (which is composed of infinitely many periodically arranged finite-width quantum wells) is developed in the presence of an electric field parallel to the superlattice planes. The method is based on a separation of the parallel motion of the center of mass from the relative motion of the electron system. The Coulomb interactions between intralayer and interlayer carriers are naturally built in via the electron density-density correlation function of the superlattice system. The force and energy balance equations obtained are applied to the calculation of the Ohmic mobilities limited by remote and background impurity scatterings and by acoustic and polar optical-phonon scatterings in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As superlattices. The nonlinear mobility and electron temperature are numerically calculated as functions of drift velocity, including all the above-mentioned scattering mechanisms and the full effect of carrier-carrier Coulomb interaction within the framework of the random-phase approximation. The dependence of transport on the geometrical parameters of the superlattice is discussed.

Balance equations in nonlinear electronic transport for electron-phonon-impurity systems in the presence of crossed electric and magnetic fields

A nonlinear theory for steady state electronic transport is suggested for electron-phonon-impurity systems in the presence of both an electric field and a crossed magnetic field. An electron temperature Te is introduced without preassumption of any kind of equilibrium-type distribution function. Thus the formulation based on it should be applicable for both high and low carrier density in both classical and quantum limits in a magnetic field. A force-balance equation and an energy-balance equation are derived in steady state for determining Te and current density. It is shown that the well known formula for transverse resistivity and Hall coefficient apply equally well for the nonlinear case if the electrons move in a single, isotropic and parabolic band. For moderate magnetic fields at about room lattice temperature, the longitudinal and transverse conductivity are calculated as functions of drift velocity assuming non-polar optical phonon scattering. The cooling effect of a magnetic field is also studied.

A new approach to non-linear transport for an electron-impurity system in a static electric field

A new approximate approach to steady-state non-linear electronic transport in a static electric field is developed for the system composed of interacting electrons with randomly distributed impurities. Starting from the separation of the centre-of-mass variables from the relative variables of the electrons the authors obtain a density matrix describing the relative motion of electrons to the first order of the impurity potential, but arbitrarily strong electric field. From this density matrix the force-balance equation for the steady state is derived, which can be expressed by a density-density correlation function and thus can easily be calculated diagrammatically. The formulae obtained are applied to the calculation of the non-linear impurity resistivity in the degenerate case and in the Maxwell-Boltzmann case, showing the simplicity and effectiveness of the method.

Lipid enrichment of thylakoid membranes. I. Using soybean phospholipids

(1) Using asolectin (mixed soybean phospholipids) liposomes, extra lipid, with or without additional plastoquinone, has been introduced into isolated thylakoid membranes of pea chloroplasts. (2) Evidence for this lipid enrichment was obtained from freeze-fracture which indicated that a decrease in the numbers of EF and PF particles per unit area of membrane occurred with increasing lipid incorporation. The decrease was not due to loss of integral membrane polypeptides as judged by assay of cytochrome present or SDS-polyacrylamide gel electrophoresis of lipid-enriched membrane fractions. Moreover, the enrichment procedure did not lead to extraction of low molecular weight lipophilic membrane components or of thylakoid membrane lipids. (3) The introduction of phospholipids into the membrane affected steady-state electron transport. Inhibition of electron transport was observed when either water (Photosystem (PS) II + PS I) or duroquinol (PS I) was used as electron donor with methyl viologen as electron acceptor, and the degree of inhibition increased with higher enrichment levels. Introduction of exogenous plastoquinone with the additional lipid had little effect on whole-chain electron transport, but caused an increase in the 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB)-sensitive rate of PS I electron transport. The inhibition was also detected by flash-induced oxidation-reduction changes of cytochrome f.

Transient and steady-state electron transport properties of GaAs and InP

A Monte Carlo computer technique has been used to calculate nonequilibrium electron transport properties of n-type GaAs and InP at 77 and 300 K, with and without impurity scattering. Characteristics of the electron transient, including velocity overshoot, are presented and analyzed in terms of energy and momentum relaxation times. New information about the steady-state transport properties of InP and about the electron temperatures of GaAs and InP as functions of electric field is also presented and discussed.