DC voltage profile of a 1D pumped wire with two dynamical and one static impurities

In this work we study the behavior of the voltage profile of a 1D quantum wire with an impurity when transport is induced by two ac voltages that oscillating with a phase lag define a quantum pump. The voltage profile sensed along the wire by the voltage probe, that we assume weakly coupled to the system, exhibits a Friedel's oscillations structure inside the region delimited by the position of the two ac voltages that induce transport. On the other hand, outside this region the oscillations are suppressed. Using perturbation theory in the coupling constant of the voltage probe we derived analytical expressions for the DC current valid for the adiabatic regime. We also compare our analytical results with the exact numerical calculations using Keldysh non-equilibrium Green's functions formalism.     

The Transport Properties of the Cell Membrane Ion Channels in Electric Fields: Bethe Lattice Treatment

The interactive two-state model of cell membrane ion channels in an electric field is formulated on the Bethe lattice by means of the exact recursion relations. The probability of channel opening or maximum fractions of open potassium and sodium channels are obtained by solving a non-linear algebraic equation. Using known parameters for the conventional mean-field theory the model gives a good agreement with the experiment both at low and high trans-membrane potential values. For intermediate voltages, the numerical results imply that collective effects are introduced by trans-membrane voltage.     

Electrical tree initiation in silicone rubber under DC and polarity reversal voltages

With the fast development of high voltage DC (HVDC) cable, cable insulation under DC conditions has got more attention. In this paper, tests were conducted to study the electrical tree initiation in silicone rubber (SIR) under DC and polarity reversal voltages. It is found that electrical tree initiation has significant polarity effects under both DC and polarity reversal voltages. There are only single-branch-like trees and branch-like trees under DC and polarity reversal voltages. As for pre-stressing effects under polarity reversal, the pre-stressing voltage has positive effects to electrical tree initiation, while the pre-stressing time has little influence. Space charge distribution of SIR under high electric field was studied with flat plate pulsed electro-acoustic (PEA) system, and their characteristics were discussed to explain this phenomenon. Moreover, different treeing breakdown phenomenon is found under polarity reversal voltage which differs from that under DC voltage. The existence of fast charges and slow charges gives reasonable explanation to it. Special attention should be paid to the transient situation like polarity reversal which would result in irreversible effects more easily, and affect true length of life for HVDC cables.     

Nonadiabatic Geometric Quantum Computation with Asymmetric Superconducting Quantum Interference Device

We propose a method of controlling the dc-SQUID (superconducting quantum interference device) systemby changing the gate voltages, which controls the amplitude of the fictitious magnetic fields Bz, and the externallyapplied current that produces the piercing magnetic fiux φx for the dc-SQUID system. We have also introduced aphysical model for the dc-SQUID system. Using this physical model, one can obtain the non-adiabatic geometric phasegate for the single qubit and the non-adiabatic conditional geometric phase gate (controlled NOT gate) for the twoqubits. It is shown that when the gate voltage and the externally applied current of the dc-SQUID system satisfies anappropriate constraint condition, the charge state evolution can be controlled exactly on a dynamic phase free path. Thenon-adiabatic evolution of the charge states is given as well.     

Quantum dynamics studies on the non-adiabatic effects of H + LiD reaction

After the Big Bang, chemical reactions of hydrogen with LiH and its isotopic variants played an important role in the late stage of recombination. Moreover, these reactions have attracted the attention of experts in the field of molecular dynamics because of its simple structure. Electronically non-adiabatic effects play a key role in many chemical reactions, while the related studies in LiH₂ reactive system and its isotopic variants are not enough, so the microscopic mechanism of this system has not been fully explored. In this work, the microscopic mechanism of H + LiD reaction are performed by comparing both the adiabatic and non-adiabatic results to study the non-adiabatic effects. The reactivity of R1 (H + LiD → Li + HD) channel is inhibited, while that of R2 (H + LiD → D + LiH) channel is enhanced when the non-adiabatic couplings are considered. For R1 channel, a direct stripping process dominates this channel and the main reaction mechanism is not influenced by the non-adiabatic effects. For R2 channel, at relatively low collision energy, the dominance changes from a rebound process to the complex-forming mechanism when the non-adiabatic effects are considered, whereas the rebound collision approach still dominates the reaction at relatively high collision energy in both calculations. The presented results provide a basis for further detailed study on this importantly astrophysical reaction system.     

Scattering mechanism of integer quantum Hall conductance in a model system

We consider a large class of two-dimensional systems of charged particles in crossed electric and strong magnetic fields in the presence of a static substrate potentialV(x,y), which contains disorder. The time dependence of the single-particle Schroedinger functions are investigated. It is found on general grounds, that adiabatic states are dc-insulating and, that the macroscopic Hall current is carried by the non-adiabatic states. Hall conductance plateaus correspond to spectral domains, which contain only adiabatic states. The plateaus disappear, when these states become non-adiabatic at sufficiently high electric fields, i.e., at sufficiently large Hall currents. An explicit model system is investigated with a substrate potential composed of disorder and of a sequence of smooth barriers and wells. Here the non-adiabatic states can be calculated in a weak-disorder approximation, whenever the electric field (and hence the Hall current) is sufficiently low. In this case the quantum mechanical scattering process leading, to the quantisation of the Hall conductance can be understood in detail. Non-classical particle propagation plays a crucial role in this process. Our analysis opens new perspectives for the theory of the integer quantum Hall effect.     

Analysis of the photovoltaic efficiency of a molecular solar cell based on a two-level system

A molecular solar cell is modelled as a two-level system connected to electrodes by chains of electron-transporting and hole-transporting orbitals. Light absorption and emission are simulated using the generalised Planck equation and intermolecular charge transfer using non-adiabatic Marcus theory. Quantum efficiency–voltage characteristics, open-circuit voltage and monochromatic power-conversion efficiency are calculated as a function of the following parameters: charge-separation rate, interfacial recombination rate, charge mobility, light intensity and built-in bias. We find that slow charge separation, fast recombination and low mobility all contribute to a decrease in efficiency compared to the ideal (detailed balance) limit. When charge-separation and interfacial recombination rates are related through the intermolecular coupling, maximum efficiency is achieved at some optimum, but not the maximum, charge-separation rate. Two regimes are distinguished for the open-circuit voltage: when interfacial recombination is important, V_oc varies approximately linearly with the donor–acceptor energy gap; but when recombination is insignificant, V_oc is determined by the optical gap. Including exciton binding energy in the driving force for charge separation reduces V_oc. In systems with significant recombination, V_oc first increases and then saturates with increasing light intensity. Low mobility and interfacial recombination are the main avoidable sources of loss when realistic parameters are used, but the effects of low mobility can be partly compensated by applying a built-in bias between the electrodes. PACS 72.40.+w; 73.40.Lq; 72.80.Le     

AC modulation of the recombination electroluminescence in anthracene single crystal

The recombination electroluminescence (REL) in an anthracene single crystal has been modulated by ac voltage at different dc biasing voltages. The frequency and ac voltage dependences observed are interpreted in terms of the presented model of sinusoidal current wave. The dc-voltage-induced changes in the modulation are explained by the voltage variation of the triplet exciton lifetime and wavelength of the current wave based on concomitant evolution of the triplet-to-singlet exciton concentration ratio responsible for the contribution of delayed and prompt components of the REL.     

一种静电驱动微机械变形反射镜

 以ANSYS软件的有限元分析为依据对变形反射镜结构模型进行研究,探讨各主要结构参数及驱动电压对变形位移的影响。考虑静电驱动力、张力和空气阻尼衰减对变形镜回复过程的影响,建立一种复合参数模型来预测变形镜的瞬态行为。结果表明,驱动器的弹性薄板厚度、跨度和电极间距明显影响变形位移;驱动器的变形位移随驱动电压的增加出现稳定的非线性增加和不稳定的“拉入”现象;驱动电压撤除后镜面回复时间短,重复性好。     

Discrete gauge symmetry anomalies

It has been recently argued that quantum gravity effects strongly violate all non-gauge symmetries. This would suggest that all low energy discrete symmetries should be gauge symmetries, either continuous or discrete. Acceptable continuous gauge symmetries are constrained by the condition they should be anomaly free. We show here that any discrete gauge symmetry should also obey certain “discrete anomaly cancellation” conditions. These conditions strongly constrains the massles fermion content of the theory and follow from the “parent” cancellation of the usual continuous gauge anomalies. They have interesting applications in model building. As an example we consider the constraints on the Z_N “generalized matter parities” of the supersymmetric standard model. We show that only a few (including the standard R-parity) are “discrete anomaly free” unless the fermion content of the minimal supersymmetric standard model is enlarged.     

Analysis of the electrical properties of p–n GaAs homojunction under dc and ac fields

In this work, the n-type GaAs films were grown on p-type GaAs single crystalline substrate by metal organic chemical vapor deposition (MOCVD). The temperature dependence of the current density–voltage (J–V) characteristics of n-GaAs/p-GaAs homojunction contacts were measured in the temperature range 293–413 K. These characteristics showed a rectifying behavior consistent with a potential barrier formed at the interface. The forward current density–voltage characteristics under low voltage biasing were explained on the basis of thermionic emission mechanism. The high values of ideality factor (n) may be ascribed to the presence of an interfacial layer. Analysis of the experimental data under the reverse voltage biasing suggests a dominant mechanism was found to be a Schottky effect. The impedance properties and the alternating current (ac) conductivity of n-GaAs/p-GaAs homojunction were investigated as a function of frequency and temperature. The ac conductivity was found to obey the universal power law. The variation of the exponent s with the temperature suggested that the conduction mechanism is an overlapping large-polaron tunneling (OLPT) model associated with correlated barrier hopping (CBH) model at the higher temperature.     

Investigation of electrical properties (ac and dc) of organic zinc phthalocyanine,ZnPc, semiconductor thin films

We report measurements on the electrical properties of thermally evaporated zinc phthalocyanine, ZnPc, semiconductor thin films. Aluminum and gold metal electrodes were used and both proved to act as ohmic contacts. A relative permittivity, ε_r, of 1.56 was estimated from the dependence of capacitance on film thickness. The room temperature current density–voltage measurements indicated an ohmic conduction at low voltages, while a space–charge-limited conduction at higher voltages. An average value of a thermally generated hole concentration of the order 10¹³ m⁻³ was estimated at room temperature.The ac conductivity, capacitance and loss tangent were measured over a wide range of temperature (from 170 to 430 K) and frequency (between 0.1 and 20 kHz). The ac conductivity of ZnPc films was observed to be proportional to ω^s, where ω is the angular frequency, and the index s is a temperature and frequency-dependent constant. At low temperatures and for higher frequencies the ac conduction was due to hopping. The capacitance, as well as the loss tangent, was found to be dependent on both temperature and frequency, but was constant for all frequencies at low temperatures. Such dependences were accounted for the equivalent-circuit model consisting of inherent capacitance in parallel with a temperature dependent resistive element.     

Voltage rectification by hard superconductors

The Bean critical state model is used to calculate voltages and voltage waveforms in hard superconductors carrying ac and dc transport currents. A comparison with the experiment shows that these characteristics are accurately described by this model. The voltage rectification effect by current-carrying superconductors is explained. Zh. Tekh. Fiz. 69, 125–127 (July 1999)     

Improving the resolution and the uniformity of AFM tip induced oxide patterns with pulsed voltages

Oxide line patterns were fabricated on the surface of titanium (Ti) film using atomic force microscopy (AFM) tip induced local oxidation technique. The growth behavior of the oxide under static voltages was studied. It was found the lateral growth of the oxide experienced two stages and the growth rate at the initial stage was very high. Pulsed voltages were employed and their effects on the controlling of the oxidation dynamics were examined. The results indicated that the high lateral and vertical growth rates of oxide at the initial stage could be suppressed with pulsed voltages. A minimum line width of 8 nm and highly uniform patterns were obtained with optimized voltage pulses. These results indicated that applying pulsed voltage is an effective method for improving both the resolution and the uniformity of the fabricated structures with scanning probe microscopy (SPM) tip induced local oxidation technique.     

On the high voltage behaviour of STM distance-voltage characteristics

We investigate the recent claim that a linear extrapolation from high bias voltages of a “blunt tip” STM distance-voltage characteristic should pass through the origin. We find both analytically and numerically that this is not true in general, even in the absence of three dimensional effects. This is unfortunate because it would have provided a simple and direct method for determining absolute vacuum gaps. However, our calculations indicate that within one dimensional tunnelling theory linear extrapolation from high voltages gives a voltage intercept at zero vacuum gap that differs by twice the difference in electrode work functions from the corresponding intercept obtained upon bias reversal. Hence, if the distance-voltage characteristics for both positive and negative bias have well defined linear regions at high voltages, free of significant contributions from Gundlach oscillations and three dimensional effects, it is still possible to obtain a good estimate of the absolute vacuum gap by extrapolation, provided the work functions are known and the bias can be reversed without the STM tip losing station.     

Electronic model for VCSELs: Switching mode, control of threshold current and saturation

An electronic model of VCSELs based on the mathematical rate equations of Danckaert et al. [J. Danckaert, B. Nagler, J. Albert, K.K. Panajotov, I. Veretennicoff, T. Erneux, Opt. Commun. 201 (2002) 129] is built. The polarization switching (PS) phenomenon is generated. The PS phenomenon here is analyzed using two methods. In the first method, the behavior of the polarization modes is investigated in terms of the injection voltage parameter. A threshold voltage value and a switching value are obtained as prescribed by the theoretical model. In the second method, the ratio of the voltages (modes) outputs is plotted in terms of the voltage parameter. It is found that the ratio reaches a maximum at a critical value of the voltage parameter. Controlling the threshold current, it is found that it can be reduced voluntarily by the use of an external voltage supplier (denoted E₂). When E₂ increases, the threshold current decreases and the maximum reached by the polarization mode increases. When the control voltage is high and below a second critical value, the polarization modes saturate to the same value. But above this critical value, they saturate to opposite values.     

Characteristics of p-ZnO/n-GaN heterojunction photodetector

Thin film heterojuction of the type p-ZnO/n-GaN was prepared by spray pyrolysis and electron beam evaporation technique, respectively. Hall measurements demonstrate the firm p-type conductivity of the p-doped ZnO film. The structural and electrical properties of the p-ZnO/n-GaN heterojunction are investigated by X-ray diffraction (XRD) and current-voltage (I-V) measurements. The XRD shows that the p-ZnO/n-GaN heterojunction is highly crystalline in nature with preferred orientation along the [0001] direction. The current-voltage curve of the heterojunction demonstrates obvious rectifying diode behavior in the dark and under illumination conditions. The ideality factor of the detector was determined in case of forward bias at low voltages and it was found to be 13.35. The turn-on voltage appears at about 1V under forward-biased voltage, and the reverse breakdown voltage is about 4V. It was found that the current of the illumination increases with the increase of bias voltages.     

Studies of electron screening effects on the electron mobility in silicon surface inversion layers

Theoretical calculations of the effects of electron screening of the randomly distributed oxide charges on the electron mobility in surface inversion layers at 4.2 K show that screening substantially enhances the mobility even in weakly inverted surface layer. Surface conductances are measured at 4.2 and 77 K on n-channel MOS transistors with 4 × 10¹¹ ions/cm². Threshold voltages are obtained by matching the inversion layer conductance versus gate voltage data to the theory. The observed mobility magnitude at 4.2 K is about 2000 cm²/V-s which is in excellent agreement with the theory including screening but without adjustable parameters. A conductance tail of several volts wide below the theoretical threshold voltage and a large positive threshold voltage shift are observed at low temperatures which are attributed to an areal inhomogeneous distribution of fast surface states near the silicon conduction band edge.     

On the hydrodynamic theory of a magnetic liquid I. General description

Using Zubarev's method of nonequilibrium statistical operator, the generalized hydrodynamic equations are obtained for a model of magnetic liquid in an inhomogeneous external field. In this model the “liquid” subsystem is treated as a classical one and the “magnetic” subsystem is described by quantum mechanical methods. The properties of the transport equations are analysed in the case of a weak nonequilibrium. The equations for time correlation functions and collective mode spectrum are also found in the same manner. It is shown that the generalized hydrodynamic equations reduce to the well-known results in the limiting cases when the dynamic variables of one subsystem are formally neglected. As an illustration, a simple model of spin relaxation is considered, and the frequency matrix and the matrix of memory functions are calculated. A comparison with previous works is made.