Electronic Transmission Coefficient Research Articles

Nonadiabaticity in the photoinduced electron transfer reactions of metal complexes

The rate of electron transfer (ET) in a variety of chemical and biological processes is influenced by factors like the free energy change (†G), the donor-acceptor electronic coupling and the medium. The effect of donor-acceptor electronic coupling on the rate of photoinduced intermolecular electron transfer is considered by taking Ru(II) and Cr(III) metal complexes in the excited state as electron acceptors and organic compounds as electron donors. The electronic coupling between the donor and acceptors depends strongly on donor-acceptor distance. The electron transfer distance is varied by introducing alkyl groups of different sizes either on the bipyridine ligand of the metal complex or on the quencher. The semiclassical theory of electron transfer expresses kET as the product of a nuclear and an electronic transmission coefficient (K n andK el respectively) and an effective nuclear-vibration frequency (v n),k ET =v nKel, Kn. The electron transfer reaction becomes nonadiabatic if the donor-acceptor distance is long. The change of electron transfer mechanism from adiabatic to nonadiabatic due to the introduction of bulky groups is explained in terms of semiclassical theory and from the temperature-dependence study of photoinduced electron transfer reactions of metal complexes.

Exchange coupling in magnetic multilayers: A quantum-size effect.

The long-wavelength oscillations observed in magnetic multilayers are explained by an indirect Ruderman-Kittel-Kasuya-Yosida--like (RKKY) exchange interaction. A perturbative theory of the RKKY-like exchange coupling between two ferromagnetic layers separated by a nonmagnetic slab is derived. The approach includes a realistic description of the multilayer one-electron states, whose wave functions satisfy matching conditions at the ferromagnetic-nonmagnetic interfaces. The quantum-size effects exhibited by the electron transmission coefficient give rise to a distinct multilayer wavelength \ensuremath{\lambda}, which provides the measured long periods.

Numerical study of electron tunneling through heterostructures

A numerical scheme based on the discretized form of the one-dimensional Schrödinger equation is presented. Using a transfer matrix method we recursively compute the transmission coefficient for electrons in arbitrary potentials. The computation time and storage are much reduced so that the code may be implemented by most programmable pocket calculators. The numerical method is used to study electron tunneling through single and double heterostructures, and the accuracy of the method is discussed.

Shot noise characteristics of a resonant tunneling diode

The shot noise characteristics of a resonant tunnelling diode are calculated from the model which includes the effects of the effective transmission coefficient of electrons in the conduction band and in the subband of the accumulation layer in the emitter. The transit time is formulated for a double barrier structure. However, the transit time for electrons at the Fermi level and the subband of the emitter are different because of the energy difference. The results show that the shot noise spectrum is independent of the frequency at low and intermediate frequencies and dependent on the frequency at high frequencies.

Transmission through abrupt heterojunction potential barriers

A new expression is derived for the transmission coefficient of incident electrons scattered by a varying potential with discontinuities. The expression is applicable above, below, and near the potential peak. As an example, electron transmission across the energy barrier in the emitter of an InP/InGaAs heterojunction bipolar transistor is calculated. Excellent agreement with a numerical calculation is obtained, even for electron energies close to the potential peak, where other expressions fail. It is shown how this new approach reduces to other expressions in limiting cases, and its validity in comparison to other approximations is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transmission coefficient of electrons through a single graded barrier.

We present a model to describe the effective mass, generalized kinetic-energy operator, and barrier potential to study the motion of an electron across a nonabrupt barrier of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs. With this model, we calculate the transmission coefficient using three different methods. The results obtained with these methods show a significant change of the transmission coefficient when compared with that of an abrupt barrier. Numerical results are obtained for different values of the interfacial width, and compositional variation of the aluminum.

Self-consistent calculations of an electron-wave Y-branch switch

A semi-classical model for studying split-gate devices in three dimensions is described. The model, useful at low bias voltages, finds the charge distribution self-consistently in a classical approximation using Poisson’s equation and a Thomas-Fermi electron distribution. This gives a potential in which the Schrödinger equation is solved for electrons near the Fermi level. The transmission coefficients for electrons then give the conductance. The model is used to evaluate the performance of an electron Y-branch switch. This device uses a control voltage to switch an incoming current between two drain leads. A single-mode switch using 80 nm gate separation, needs only 40 mV switching voltage. Multi-mode operation is also demonstrated using 200 nm wide leads. Here 1 V is needed for 10 dB current reduction in one branch.

On the differences in the magnitude of the observed solvent effect in the kinetics of simple heterogeneous electron transfer reactions

Solvent effects have been examined for 18 simple heterogeneous redox reactions involving one-electron oxidation of a molecule to form a monocation or one-electron reduction to form a monoanion. It is shown that the degree of dependence of the rate constant on the longitudinal relaxation time of the solvent decreases with the experimental value of the standard rate constant. This observation is discussed within the context of contemporary theory for the effect of the solvent on the pre-exponential factor for the rate constant. It is concluded that, as the importance of solvent fluctuations on the frequency of crossing the Gibbs activation energy barrier for electron transfer decreases, there is a corresponding decrease in the electron transmission coefficient.

A new GaAs sawtooth-doping-superlatticed switching device

A GaAs switching device with a double sawtooth-doping-superlattice (SDS) structure has been fabricated and demonstrated. By using the Kronig–Penny model, the dispersion relations of the electrons and holes in the zigzag quantum well were investigated. Two different delta-doping sheets, 1 × 1012 and 1 × 1013 cm−2, were used in the double SDS structure. The period length of the SDS was 600 Å. Thus, due to the sufficient barrier height and width, the calculated transmission coefficient of electrons was negligible when the studied structure was at thermal equilibrium or under a small-biased condition. However, when a larger bias was applied, the studied structure exhibited an interesting negative-differential-resistance (NDR) performance in the experimental current-voltage characteristics. This was caused by an avalanche multiplication process and the potential redistribution in SDS periods. Particularly, an interesting intermediate state between the initial off state and final on state was observed at 77 K. Consequently, with a suitable adjustment of structural parameters, e.g., the delta-doping density and the period number of the SDS, the proposed device provides good potential for switching circuit applications.

Effects of Tyr-83 nitromodification and Phe-82 phenyl group rotation on electronic transmission coefficients at the remote electron-transfer site of higher plant plastocyanins

The blue single-copper plant protein plastocyanin (PC) has two electron-transfer (ET) paths, one via the exposed His-87 ligand, the other one through the protein and involving sites close to Tyr-83. We have investigated features of this pattern using ET theory and extended Huckel calculations. Prompted by previous observations that through-space Cu/Met-92/Phe-14/Phe-82 ET involving aromatic groups is unfavorable, the dynamics of this route was investigated. The Phe-82 ring was rotated from equilibrium almost perpendicular to the Phe-14 phenyl ring, to an almost parallel orientation

Gate-controlled conductance oscillations in small one-dimensional electron cavities

The low-temperature, low-bias regime of electron transport through narrow channels in semiconductor heterostructures interrupted by a finite number of potential barriers at zero magnetic field, is investigated with a simple model of resonant tunneling of noninteracting electrons in a single one-dimensional subband. By using a generalized wave impedance concept, we calculate the transmission coefficient for electrons passing through such devices, thus providing a satisfactory explanation for the periodic conductance oscillations observed in recent experiments. We also show that the oscillatory behaviour in the current of arrays of submicrometer normal-metal tunnel junctions may be understood as the formation of miniband structures in such coupled electron systems. Our results call into question the dominance of charging effects in small electron resonator devices.

Effects of NO2-modification of Tyr83 on the reactivity of spinach plastocyanin with cytochrome f

We have investigated the electron transfer (ET) reactions between turnip cytochrome f , and the native and NO 2 -Tyr83-modified forms of spinach plastocyanin (PCu) at 10.0°C and ionic strength 0.200 M(NaCl), in both directions as a functions of pH. The PCu(II)/cytochrome f (II) rate constants in the pH-range 4–6.8 reflect active and remote binding site protonation. At higher pH, NO 2 -Tyr83 and positively charged residues on cytochrome f are depronanated, and both native and NO 2 -modified PCu exhibit a composite rate constant variation in this pH range. When framed by ET theory this pattern is fully understandable in terms of variations in reduction potentials and electrostatic interactions ccaused by the protonation equilibria. The rate constant ratio k nitro / k native is, however, only 1.04 for the PCu(II)/cytochrome f (II) reactions in spite of a 18 mV higher reduction potential for NO 2 -Tyr83-modified PCu. This is much lower than the value of 1.42 expected from ET thlely on the basis of such a reduction potential effect. A similar effect is seen for PCu(I)/cytochrome ratio is 0.51. Notable but smaller effects are also observed for the small reaction partners [Fe(CN) 6 ] 3 t-4− and [Co(phen) 3 ] 3++ 2 + . The effect of NO 2 -modification in addition to the reduction potential effect an be resolved into a small reorganization energy increase around the copper atom and a smaller electronic transmission coefficient for ET through the Cu/Cys84/Tyr83 sequence. The former effect dominates in the reactions with the small reaction partners, while the electronic effects contribute significantly for PCu/cytochrome f , supporting the concept that the PCu/cytochrome f ET is at the remote PCu binding site.

Effects of NO2-modification of Tyr83 on the reactivity of spinach plastocyanin with cytochrome f

We have investigated the electron transfer (ET) reactions between turnip cytochrome f , and the native and NO 2 -Tyr83-modified forms of spinach plastocyanin (PCu) at 10.0°C and ionic strength 0.200 M(NaCl), in both directions as a functions of pH. The PCu(II)/cytochrome f (II) rate constants in the pH-range 4–6.8 reflect active and remote binding site protonation. At higher pH, NO 2 -Tyr83 and positively charged residues on cytochrome f are depronanated, and both native and NO 2 -modified PCu exhibit a composite rate constant variation in this pH range. When framed by ET theory this pattern is fully understandable in terms of variations in reduction potentials and electrostatic interactions ccaused by the protonation equilibria. The rate constant ratio k nitro / k native is, however, only 1.04 for the PCu(II)/cytochrome f (II) reactions in spite of a 18 mV higher reduction potential for NO 2 -Tyr83-modified PCu. This is much lower than the value of 1.42 expected from ET thlely on the basis of such a reduction potential effect. A similar effect is seen for PCu(I)/cytochrome ratio is 0.51. Notable but smaller effects are also observed for the small reaction partners [Fe(CN) 6 ] 3 t-4− and [Co(phen) 3 ] 3++ 2 + . The effect of NO 2 -modification in addition to the reduction potential effect an be resolved into a small reorganization energy increase around the copper atom and a smaller electronic transmission coefficient for ET through the Cu/Cys84/Tyr83 sequence. The former effect dominates in the reactions with the small reaction partners, while the electronic effects contribute significantly for PCu/cytochrome f , supporting the concept that the PCu/cytochrome f ET is at the remote PCu binding site.

On Modeling of Plasma Edge Conditions at Divertors with Solid Metal Neutralizers

AbstractIt is widely recognized that results from divertor models can be quite sensitive to the boundary conditions that are assumed at the divertor neutralizer plate. However, some past models assumed electron and ion heat transmission coefficients with little justification. In fact, energy and momentum fluxes from backscattered neutral deuterium and tritium atoms can significantly contribute to the energy and momentum balance of the divertor plasma and consequently affect the estimates of steady-state plasma conditions. In illustration of this point, a two-point model similar to that of Galambos and Peng is rederived, including momentum and energy sources from charge-exchange and a self-consistent fluid treatment of the sheath heat transmission coefficients. Divertor conditions associated with the International Tokamak Reactor (INTOR) and International Thermonuclear Experimental Reactor (ITER)-like fusion reactors are estimated, and the effects of including the backscattered fluxes are discussed.

Effects of NO2-modification of Tyr83 on the reactivity of spinach plastocyanin with cytochrome f

We have investigated the electron transfer (ET) reactions between turnip cytochrome f, and the native and NO2-Tyr83-modified forms of spinach plastocyanin (PCu) at 10.0°C and ionic strength 0.200 M(NaCl), in both directions as a functions of pH. The PCu(II)/cytochrome f(II) rate constants in the pH-range 4–6.8 reflect active and remote binding site protonation. At higher pH, NO2-Tyr83 and positively charged residues on cytochrome f are depronanated, and both native and NO2-modified PCu exhibit a composite rate constant variation in this pH range. When framed by ET theory this pattern is fully understandable in terms of variations in reduction potentials and electrostatic interactions ccaused by the protonation equilibria. The rate constant ratio knitro/knative is, however, only 1.04 for the PCu(II)/cytochrome f(II) reactions in spite of a 18 mV higher reduction potential for NO2-Tyr83-modified PCu. This is much lower than the value of 1.42 expected from ET thlely on the basis of such a reduction potential effect. A similar effect is seen for PCu(I)/cytochrome ratio is 0.51. Notable but smaller effects are also observed for the small reaction partners [Fe(CN)6]3t-4− and [Co(phen)3]3++ 2 +. The effect of NO2-modification in addition to the reduction potential effect an be resolved into a small reorganization energy increase around the copper atom and a smaller electronic transmission coefficient for ET through the Cu/Cys84/Tyr83 sequence. The former effect dominates in the reactions with the small reaction partners, while the electronic effects contribute significantly for PCu/cytochrome f, supporting the concept that the PCu/cytochrome f ET is at the remote PCu binding site.

Vertical transport, transmission coefficients, and dwell time in asymmetric quantum well structures

The transport of hot electrons in the Al xGa 1−xAs barriers above the wells in a multiple quantum well (MQW) structure is investigated. The structures that are studied are asymmetric quantum well infrared detectors. The transport of the hot electrons normal to the layers is strongly dependent on both voltage and well shape. It is suggested that the key parameter which affects the transport properties is the dwell time of the electrons in the continuum, above the well region. This is most readily seen in asymmetric MQW structures, in which the dwell time under an applied bias depends very strongly on bias polarity. Calculations of electron transmission coefficient and dwell time show that the electron mean free path in asymmetric wells is much larger in positive bias than in a negative one. Employing this model, we achieve a very good fit to experimental data.

Resonant transmission in the base/collector junction of a bipolar quantum-well resonant-tunneling transistor

A new transistor effect is demonstrated in a 120 nm base, bipolar quantum-well, resonant-tunneling transistor (BiQuaRTT). In this BiQuaRTT, a strong, multiple negative differential resistance (NDR) characteristic is obtained at room temperature with high-current gain (≳50). The effect is shown to be the consequence of an asymmetric, quantum-well-base heterostructure whose shape is controlled by the base/collector bias. Changes in the quantum-well shape lead to large modulations of the transmission coefficient for quasi-thermalized minority electrons crossing the quantum-well base. In this letter, we describe the transport characteristics of these transistors, including also temperature and magnetic field dependence.

Electron transmission in one-dimensional crystals with interfaces

An analysis is made of the transmission coefficient for electrons at a sharp interface separating two periodic one-dimensional potentials. Numerical results are presented for the case that the cell potentials are square wells. The transmission coefficient is sensitive to the position of the interface relative to the cells in the two crystals. Results are also presented for two interfaces (a barrier).

Transport through one-dimensional channels.

The oscillatory behavior in the conductance of narrow channels in semiconductor heterostructures interrupted by a finite number of potential barriers can be explained by resonant tunneling of noninteracting electrons in a single one-dimensional subband. A simple and powerful method based on a generalized wave-impedance concept is used to calculate the transmission coefficient for electrons passing through such devices. The results show intriguing similarities with recent experiments.

Differences in the multiple scattering of positrons and electrons

After a brief review of differences between the scattering cross sections for electrons and positrons, results are presented on the transmission and reflection of these particles by foils. These results were obtained with the Monte Carlo code ETRAN, which takes into account positron-electron differences of stopping powers and elastic-scattering cross sections. The principal output of the calculations consists of number and energy transmission and reflection coefficients for electrons and positrons. A few results are also shown pertaining to differences of energy spectra and angular distributions of positrons and electrons reflected from thick foils. The calculated transmission coefficients for positrons are greater, and the reflection coefficients smaller, than those for electrons of the same energy. These results are in qualitative agreement with experimental data, but the quantitative agreement is not very close.