Quantum transport of slow charge carriers in quasicrystals and correlated systems

We show that the semiclassical model of conduction breaks down if the mean free path of charge carriers is smaller than a typical extension of their wave function. This situation is realized for sufficiently slow charge carriers and leads to a transition from a metalliclike to an insulatinglike regime when scattering by defects increases. This explains the unconventional conduction properties of quasicrystals and related alloys. The conduction properties of some heavy fermions or polaronic systems, where charge carriers are also slow, present a deep analogy.     

Generation of excess minority carriers in mos devices due to gamma irradiation

The effect of gamma irradiation on MOS devices prepared under different oxidation conditions Is investigated. The C-V characteristics of the devices are studied before and after exposing the latter with gamma radiations of CO⁶⁰ (1.17 and 1.33 MeV gamma rays). For MOS transistor (n-channel depletion type devices) the C-V characteristics change slightly towards the negative voltage axis and the C_min also decreases after Irradiation. For MOS capacitor (wet oxide) there is a change from high frequency C-V characteristics to low frequency C-V characteristics. In the case of a MOS capacitor (HCl grown) breakdown occurs relatively at lower voltage.     

In-crystal and surface charge transport of electric-field-induced carriers in organic single-crystal semiconductors

Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity sigma(square) and Hall coefficient R(H). The Hall mobility mu(H) (identical with sigma(square)R(H)) reaches nearly 10 cm(2)/V s when relatively low-density carriers (<10(11) cm(-2)) distribute into the crystal. mu(H) rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.     

Effect of electron-electron interaction on spin relaxation of charge carriers in semiconductors

An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting. It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas. The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed. Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures. The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire.     

Effect of gamma irradiation on internal friction in lithium tetraborate

The effect of gamma irradiation on the mechanical properties of lithium tetraborate Li₂B₄O₇ in the single-crystal and vitreous states is investigated. It is found that, after irradiation of the Li₂B₄O₇ single crystal, the temperature range of the dissipative process initially occurring at 380–420 K becomes broader and the fine structure of the peak in the temperature dependence of the internal friction Q^?1(T) undergoes a substantial trans-formation. After irradiation of the vitreous Li₂B₄O₇ sample, the increase in the internal friction, which is characteristic of the onset of the α relaxation in this material, is not observed in the dependence Q^?1(T) up to a temperature of 570 K. It is shown that the mechanical properties of the irradiated samples are almost completely recovered after annealing at 570 K for 1 h.     

Effect of surface states on electron transport in individual ZnO nanowires

The current–voltage (I–V$I\text{–}V$) characteristics of single ZnO nanowires have been studied in the humid air, dry air, vacuum, and under ultraviolet (UV) irradiation. A model of a single ZnO nanowire connected with two opposite diodes was proposed to calculate the observed I–V$I\text{–}V$ behaviors. The results show that the electrical characteristics are dominated by the reverse barrier height, and the barrier height can be adjusted by surface adsorption, which is ascribed to the effect of surface states on surface band bending and Fermi level pining. Furthermore, the nanowire exhibited an enormous increase of conductance upon UV irradiation and a considerable persistent photocurrent after withdraw of the UV excitation, which further confirms the surface states have a pronounced effect on the electronic transport in single ZnO nanowires.     

Drift mobility and kinetics of transport of excess charge carriers in glassy CdGexAs2

Conclusions We tried to measure transient conductivity response to pulse strongly absorbed excitation (light, accelerated electrons) in sandwich type samples of glassy CdGe _x As₂ compounds. We observed the signal due to transport of free excess carriers. From analysis of experimental results we conclude that in our materials strong trapping effects are present, so the range of excited carriers is very short (10^–4-10^–3 cm) even in the highest electrical fields used (to 10⁴ V. cm^–1). Estimates of upper limit of drift mobility give the values 10^–1- 1 cm² V^–1 sec^–1. We did not succeed in determining the type of carriers which are responsible for the observed effects.     

Concentration and lifetime of nonequilibrium charge carriers in CsI and NaCl subjected to x-ray irradiation

The temperature dependence of radiation-induced conductivity was studied in the range of 80–300 K in alkali halide CsI and NaCl crystals subjected to pulsed x-ray irradiation. It is shown that an increase in electrical conductivity with increasing temperature is satisfactorily accounted for by the thermal separation of electrons and holes with common origins. The concentration and lifetime of conducting electrons, as well as the spatial distribution and the probability of thermal separation of nonequilibrium charge carriers in the common-origin electron-hole pairs after thermalization were estimated. The possible effect of the two commonorigin holes generated in the Auger process on the enhancement of recombination rate of electrons is discussed.     

伽马辐照对掺镱光纤材料特性影响的研究

掺镱光纤是高功率激光器的核心材料,但在高能射线辐照后其应用性能会显著下降,因此有必要对掺镱光纤材料在辐照环境下的性能变化进行深入研究。采用改进型化学气相沉积法结合稀土螯合物掺杂制备了系列光纤预制棒及光纤,测试了光纤在不同剂量下射线辐照前后的高功率输出性能,以及光纤预制棒辐照前后的吸收光谱及镱离子荧光寿命。结果表明：小剂量辐照后掺镱光纤的高功率输出显著下降,通过预制棒吸收光谱可看出主要是因为伽马辐照后使掺镱光纤材料中Al的相关缺陷浓度增多,在可见光区域吸收损耗增加。Ce离子的掺杂通过缓减辐致铝氧空位中心（Al-OHC）色心缺陷的增加,减少Yb离子荧光寿命的下降,可在一定程度上抑制高功率掺镱光纤的辐致暗化。     

Effect of gamma irradiation on hydrothermally synthesized barium titanate nanoparticles

The effect of gamma irradiation on hydrothermally synthesized BaTiO₃ nanoparticles has been investigated. Gamma irradiation was carried out at room temperature from 0, 50, 100, 150, 200?kGy to a maximum dose up to 250?kGy, source being ⁶⁰Co gamma radiations. The structure, size and chemical changes of the BaTiO₃ were studied using X-ray diffraction, Fourier-transform infrared spectrophotometry (FTIR) techniques and scanning electron microscopy (SEM). The optical band gap has been computed by UV–Visible spectroscopy data. From the results obtained, it is evident that the gamma irradiation increases the crystallinity, whereas the particle size of BaTiO₃ nanoparticles is altered. UV–Visible spectroscopy shows a noticeable change in the energy band gap due to gamma irradiation. Significant changes in anharmonicity constant computed using FTIR data due to irradiation has been observed. SEM shows the size and deviation from uniformity of particles.     

Effect of gamma irradiation on HPMC/ZnO nanocomposite films

The present work looks into the structural, chemical, mechanical, optical and thermal modification in ZnO nanoparticle incorporated hydroxypropyl methylcellulose (HPMC) polymer films, induced by gamma irradiation. The irradiation process was performed in a gamma chamber at room temperature using Cobalt-60 source (average energy of 1.25?MeV) at different doses: 0, 50, 100, 150 and 200?kGy. The modifications in structural, chemical, mechanical, optical and thermal properties, due to gamma irradiation in HPMC/ZnO nanocomposite films, have been studied using wide angle X-ray scattering (XRD), Fourier transform infrared spectroscopy, universal testing machine, ultraviolet–visible spectrophotometry and thermogravimetric analysis. It is found that gamma irradiation improves the mechanical and thermal properties of nanocomposite films.     

Effect of gamma irradiation on chromate sorption over magnetite surface

Aqueous chromate sorption on suspended magnetite in the presence of gamma irradiation has been evaluated. Kinetics of chromate removal was evaluated using Lagergren's absorption model. Chromate removal with respect to the accumulated dose followed a Lagergren's pseudo-first-order kinetic model. A comparison of kinetics of chromate removal with respect to total accumulated dose for gamma irradiation experiment vis-à-vis with respect to time of treatment for different isothermal interactions has been undertaken. Rate constants indicate that the chromate removed per minute in isothermal equilibration at 80 °C is comparable to the chromate removed per kilogrey of gamma irradiation received. There is a redox interaction between the chromate and the ferrous of the suspended magnetite which was confirmed by X ray photo electron spectroscopy (XPS) analysis. This process reaches a saturation much before the consumption of entire ferrous ions of magnetite, indicating a passivating nature of the product. The effect of radiation on both chromate solution and dispersed magnetite to alter the redox process could be ascertained. Gaussian–Lorenzian peak fittings to the XPS data have been carried out to evaluate the chemical composition of the deposited chromium and the resultant change in the chemical composition of the iron in the oxide lattice. This indicated that the magnetite equilibrated with chromate under gamma irradiation resulted in a different surface composition as compared with the one obtained in absence of gamma irradiation. XPS data indicated the presence of hydroxyl group and oxide group attached to both iron and chromium moieties in case of irradiation, whereas only oxide group was seen with only temperature treatment.     

Individual charge traps in silicon nanowires

We study anomalies in the Coulomb blockade spectrum of a quantum dot formed in a silicon nanowire. These anomalies are attributed to electrostatic interaction with charge traps in the device. A simple model reproduces these anomalies accurately and we show how the capacitance matrices of the traps can be obtained from the shape of the anomalies. From these capacitance matrices we deduce that the traps are located near or inside the wire. Based on the occurrence of the anomalies in wires with different doping levels we infer that most of the traps are arsenic dopant states. In some cases the anomalies are accompanied by a random telegraph signal which allows time resolved monitoring of the occupation of the trap. The spin of the trap states is determined via the Zeeman shift.     

Effect of adsorption on the surface mobility of charge carriers in a semiconductor substrate

The effect of adsorption on the carrier mobility in the near-surface region of a semiconductor substrate has been investigated within the framework of the Schrieffer model. The dependence of the carrier surface mobility on the concentration of adatoms has been determined. The systems chosen for the study are the gases adsorbed on the surface of semiconductor oxides. Empirical estimates of the surface mobility, which are based on modification of conventional volume scattering mechanisms, have been proposed.     

金属/掺杂聚合物/金属结构中载流子的注入与输运

基于紧束缚的Su-Schrieffer-Heeger模型，利用非绝热的动力学方法，研究了金属/聚合物/金属三明治结构中载流子注入与输运的动力学过程，其中，聚合物中含有一个掺杂离子. 电子波函数随时间的演化满足含时Schrdinger方程，晶格运动满足经典的牛顿运动方程. 研究发现，掺杂离子对注入到聚合物中的载流子的作用类似一个控制阀门，阀门的状态与电场的强度和偏压的大小密切相关.     

金属/掺杂聚合物/金属结构中载流子的注入与输运

基于紧束缚的Su-Schrieffer-Heeger模型,利用非绝热的动力学方法,研究了金属/聚合物/金属三明治结构中载流子注入与输运的动力学过程,其中,聚合物中含有一个掺杂离子. 电子波函数随时间的演化满足含时Schrdinger方程,晶格运动满足经典的牛顿运动方程. 研究发现,掺杂离子对注入到聚合物中的载流子的作用类似一个控制阀门,阀门的状态与电场的强度和偏压的大小密切相关. 关键词： 载流子输运 波包 非绝热动力学     

Effect of gamma-ray irradiation on surface charge behavior of polybutylene naphthalate

Polymer insulating materials are widely used in radioactive environment. Surface charge accumulated on the material has been considered as an important factor for inducing discharge which may cause serious damage to electrical devices. Thus, from the safety point of view, it is necessary to investigate the charge behavior on polymer surface after the irradiation. In this paper, polybutylene naphthalate (PBN) was employed as test sample which was previously irradiated in air up to 100 kGy and then up to 1000 kGy with dosage rate of 10 kGy/h by using a ⁶⁰Co gamma-source. The experiment was carried out under a negative dc stress between two parallel electrodes. An electrostatic probe was designed to measure the charge density. Obtained results show that with the increase of the total dose of gamma-ray irradiation, the charge density and the storing capacity decrease. The charge accumulation depends upon the density of localized surface states, which is varied by the radiation induced cross-linking and degradation reactions of the molecule structure. The charge decay mainly relies on the recombination of surface charge with ions of opposite sign in surrounding air.     

Effect of interface recombination of charge carriers on the photoconductivity of variable-band structures

The effect of interface recombination of charge carriers on photoconductivity of variable-band structures with extrinsic conductivity type and a linear coordinate dependence of the forbidden band width has been studied theoretically. It has been shown that under conditions of strong light absorption the interface recombination of charge carriers leads to the formation of a minimum, in the photoconduction spectrum for a photon energy equal to the forbidden band width at the interface. The details of the spectral dependence of the photoconductivity of a variable-band structure in the neighborhood of the photoconductivity minimum are analyzed. Institute of Applied Physics, Iv. Franko L’vov State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 94–98, July, 1997.     

Effect of gamma irradiation on high temperature hardness of low-density polyethylene

Gamma irradiation can cause the change of microstructure and molecular structure of polymer, resulting in the change of mechanical properties of polymers. Using the hardness measurement, the effect of gamma irradiation on the high temperature hardness of low-density polyethylene (LDPE) was investigated. The gamma irradiation caused the increase in the melting point, the enthalpy of fusion, and the portion of crystallinity of LDPE. The Vickers hardness of the irradiated LDPE increases with increasing the irradiation dose, annealing temperature, and annealing time. The activation energy for the rate process controlling the reaction between defects linearly decreases with the irradiation dose. The process controlling the hardness evolution in LDPE is endothermic because LDPE is semi-crystalline.     

Low-temperature thermal transport in nanowires

We propose a theory of low temperature thermal transport in nanowires in a regime in which competition between a phonon and flexural modes governs the relaxation processes. Starting with the standard kinetic equations for two different types of quasiparticles, we derive a general expression for the coefficient of thermal conductivity. The underlying physics of thermal conductance is completely determined by the corresponding relaxation times, which can be calculated directly for any dispersion of quasiparticles, depending on the size of a system. We show that, if the considered relaxation mechanism is dominant, then at small wire diameters the temperature dependence of thermal conductivity experiences a crossover from T^1/2 to T³-dependence. Quantitative analysis shows reasonable agreement with resent experimental results.