Influence of nanocrystalline structure of surface on boron gettering from silicon

The boron gettering from nanocrystallites of porous silicon with rate significantly greater in comparison with planar structure has been for the first time observed. As a probe of gettering process, the EPR signal intensity of P_b center was used. It was established that strain effect during the surface oxidation stimulates strongly the boron gettering process from nanocrystallites bulk to SiO₂/Si interface of porous layer.Phenomenon of strong absorption of microwave radiation by porous layer after the sample was annealed in air at 400–500 °C was also detected at 37 GHz. This absorption is equivalent to high conductivity of layer (ρ = 10^− 2–10^− 3 Ω cm) and it is much higher (at least in 10⁴ times) than of c-Si substrate. Removal of porous layer (thickness of ∼ 20 μm) from 400-μm substrate leads to where Q-factor of cavity is restored completely. The high conductivity of the porous layer comes mainly from conduction along the nanocrystalline grains, while SiO₂ layer essentially insulates their interface. One possible effect explanation is that high concentration of positive charge states formed in SiO₂ layer can lead to the creation of high density of free electrons in the SiO₂/Si interface with formation of conductive channels along each nanocrystallite. On the other hand, appreciable microwave absorption is not detected on 9.4 GHz (X-band); therefore, we cannot exclude the existence of quasi-resonant microwave absorption that is related with nanocrystallites sizes, i.e. quantum confinement and electrons tunneling between grains.     

Melting of Two-Dimensional Electron Crystal on Liquid 3He Induced by Resonance Microwave Absorption

Microwave-resonance induced increase of magneto-resistivity σ _xx ⁻¹ of the surface-state electrons (SSE) on liquid ³He is observed at low temperatures at which SSE crystallize into the Wigner solid (WS). At the resonance, the relative change of σ _xx ⁻¹ of SSE in the solid phase is found to be several times of the corresponding change in the liquid phase measured under similar experimental conditions. We attribute this effect to melting of WS induced by electron heating which accompanies the microwave absorption. σ _xx ⁻¹ is found to increase gradually with microwave power, which is rather different from the sharp resistivity increase induced by direct heating of the substrate in the absence of microwaves. The effect of microwave resonance-induced heating on the sliding of WS from the periodic surface deformation of liquid is also investigated.      

Effects of a highly dispersive atomic medium inside an optical ring cavity

Atomic media inside an optical cavity can significantly alter the spectral response of the cavity. Both theoretical and experimental examinations are made of the cavity transmission with a highly dispersive intracavity multilevel atomic medium. It is found, owing to the reduced absorption and steep dispersion change accompanying electromagnetically induced transparency in such a multi-level atomic medium, that the cavity linewidth can be made much narrower than the empty cavity linewidth. Cavity linewidth narrowing is measured as a function of both the coupling beam power and the atomic density. These experimental results are in good agreement with the theoretical predictions.     

Microwave dielectric properties of flexible silicone rubber – Ba(Zn1/3Ta2/3)O3 composite substrates

Silicone rubber composites filled with Ba(Zn_1/3Ta_2/3)O₃ (BZT) were prepared by hot pressing and the effect of filler content on the microwave dielectric, mechanical and thermal properties as well as on moisture absorption were investigated. The observed relative permittivity (ɛ_r) was compared with different theoretical models. Among the different theoretical models Jayasundere Smith and Modified Lichtenecker were in good agreement with experimental values of ɛ_r. The study of the mechanical property showed that the silicone rubber – BZT composites were flexible and stretchable. The coefficient of thermal expansion and specific heat capacity decreased whereas thermal conductivity, thermal diffusivity and the moisture absorption increased with increase in filler loading.     

Microwave-Resonance Induced Change in Magneto-Resistivity: Hot Surface Electrons on Liquid 3He

Behaviour of the linear magneto-resistivity σ _xx ⁻¹ of the surface-state electrons (SSE) exposed to the microwave radiation is studied in a wide temperature range which covers both vapor-atom and ripplon scattering regimes. The microwaves induce the resonance transitions of SSE between two lowest Rydberg states, and σ _xx ⁻¹ is observed to vary in the vicinity of the resonance. In the vapor-atom scattering regime (T≳0.5 K), magneto-resistivity decreases at the resonance, while in ripplon scattering regime (T≲0.3 K) it rapidly increases with the microwave power in the given range of the pressing electric field. The observed variation of σ _xx ⁻¹, which under the conditions of our experiment is inversely proportional to the scattering rate, is attributed to the strong heating of SSE. The heating results in the increase of electron temperature well above ambient temperature and in significant thermal population of higher excited states. The theoretical predictions are found to be in good agreement with our experimental results in the whole temperature range.      

Influence of chromium concentration and particle size on the ESR linewidth of Al2O3: Cr3+ powders

The linewidth of electron spin resonance (ESR) absorption in polycrystalline samples of chromium-doped alumina (Al₂O₃: Cr³⁺) has been investigated as a function of chromium concentration and particle size. The linewidth is found to be directly proportional to chromium concentration and inversely proportional to particle size. The experimental results are consistent with the mechanism of dipolar broadening in diluted solid solutions; the influence of particle size may be attributed to size-dependent fluctuations of the zero-field parameter, D.     

Unexpected Giant Microwave Conductivity in a Nominally Silent BiFeO3 Domain Wall

Nanoelectronic devices based on ferroelectric domain walls (DWs), such as memories, transistors, and rectifiers, have been demonstrated in recent years. Practical high-speed electronics, on the other hand, usually demand operation frequencies in the gigahertz (GHz) regime, where the effect of dipolar oscillation is important. Herein, an unexpected giant GHz conductivity on the order of 10³ S m⁻¹ is observed in certain BiFeO₃ DWs, which is about 100 000 times greater than the carrier-induced direct current (dc) conductivity of the same walls. Surprisingly, the nominal configuration of the DWs precludes the alternating current (ac) conduction under an excitation electric field perpendicular to the surface. Theoretical analysis shows that the inclined DWs are stressed asymmetrically near the film surface, whereas the vertical walls in a control sample are not. The resultant imbalanced polarization profile can then couple to the out-of-plane microwave fields and induce power dissipation, which is confirmed by the phase-field modeling. Since the contributions from mobile-carrier conduction and bound-charge oscillation to the ac conductivity are equivalent in a microwave circuit, the research on local structural dynamics may open a new avenue to implement DW nano-devices for radio-frequency applications.     

Some physical properties of Mg-Zn ferrites

Abstract

Electric, dielectric, and structural studies were carried out at room temperature for a series of Mg_xZn_l-xFe₂0₄ ferrite samples prepared by the usual ceramic technique. The experimental results showed that the real dielectric constant ε and dielectric loss factor tan δ decrease whereas the ac electrical conductivity σ₂ (ω, T) increases on increasing the angular frequency ω. The dc and ac electrical conductivities, real dielectric constant, and dielectric loss factor were found to decrease with increasing Mg ion substitution. The parameters nand B in the power law σ₂ (ω, T) = Bωⁿ for the electrical conductivity σ (ω, T) were found to be composition dependent, with both nand B decreasing as the Mg ion addition increased. According to the experimental results, empirical formulae are suggested for the composition dependences of the electrical conductivity σ and the parameters nand B. The analysis of X-ray diffraction patterns showed that the samples investigated have a single phase cubic spinel structure. The bulk and theoretical density, lattice constant, and radius of the tetrahedral ion decrease whereas porosity increases as Mg ion substitution increases. The infrared absorption spectra showed three of the characteristic bands for ferrites with intensities and positions (wavenumbers) that are composition dependent. The fourth band of the infrared spectra V4, related to the lattice vibrations, was too weak to be observed at room temperature.     

The thermoelectric power and the temperature coefficient of resistance of thin polycrystalline antimony films

Starting from the Mayadas-Shatzkes equations for the conductivity of a metal film, the temperature coefficient of resistivity and the thermoelectric power for polycrystalline semi-metal films were calculated.From the data obtained for various grain sizes D in the range 200–2000 Å, the concentrations n of electrons and p of holes, as well as the respective mobilities μ_n and μ_p, were determined without assuming n = p. Good agreement was found between experimental results and the theoretical equations we proposed.     

钇铁石榴石薄膜材料的光吸收谱及Bi,Al掺入对谱的影响研究

采用单离子晶场跃迁模型，拟合出钇铁石榴石（ＹＩＧ）薄膜材料的光吸收谱，与实验谱吻合得较好。从实验和理论上分析了（ＢＩＡＩ）ＹＩＧ 薄膜材料的光吸收谱，结果表明，Ａｌ３＋离子的作用如同稀释剂一样，减小了光吸收，Ｂｉ３＋离子由于其强的自旋轨道耦合作用，增大了跃迁振子强度和跃迁线宽，从而增大了光吸收损耗，在此基础上所作的（ＢｉＡｌ）ＹＩＧ的理论谱与实验谱符合得较好。     

Anomalous pulsed conductivity of silver bromide

The first experimental results on the room-temperature pulsed conductivity of silver bromide excited in a pulsed mode by the X-ray bremsstrahlung radiation are reported. The data indicate that photosensitive materials are probably featuring the process of charge carrier multiplication during a time on the order of 10^?9 s. The concentration of electrons in the conduction band of AgBr is estimated. The experimental concentration of the conduction electrons is about ten times greater than the concentration of electron-hole pairs created by the exciting radiation pulse.     

Effect of Isotopic Composition on the Electron Paramagnetic Resonance in Silicon1

The effect of isotopic composition on the electron paramagnetic resonance of dangling bonds was studied in single-crystal and polycrystalline silicon. The results demonstrate that the resonance linewidth in monoisotopic ²⁸Si is smaller than that in Si with the natural isotopic composition because of the smaller contribution of superhyperfine interaction of electrons with the nuclear spins of ²⁹Si. The relaxation contribution to the linewidth is larger in monoisotopic ²⁸Si because of the changes in the scattering of phonons responsible for the spin relaxation of paramagnetic dangling bonds.     

Electrical conductivity of metallic hydrogen as a ternary system

The electrical conductivity of metallic hydrogen is calculated at a temperature of 3000 K and density of 0.3 mol/cm³, which correspond to the conditions of its preparation under terrestrial conditions. Hydrogen is treated as a ternary system. One subsystem is provided by protons, and the second one—by neutral atoms of hydrogen. For the third, electron, subsystem, the model of nearly free electrons is used, and for inverse relaxation time for electrical conductivity—the second order of the perturbation theory with respect to electron-proton and electron-atom interactions. The Coulomb electron-electron interaction is included in the random-phase approximation, and the exchange interaction and correlations of conduction electrons—in the local-field approximation. The hard-sphere model is used for the proton and atomic subsystem. The electrostatic interaction alone is included as electron-atom interaction. The concentration of electroneutral atomic component is determined from the condition of agreement between the experimentally obtained and theoretical values of electrical resistance at temperature and density corresponding to the experimental conditions for preparation of metallic hydrogen. This concentration turns out to be 66%.     

Features of accelerated electron beam formation in LHCD experiments on FT-2 tokamak

In experiments with lower hybrid current drive (LHCD) on the FT-2 tokamak, lower hybrid (LH) waves have been successfully used for the first time to ensure effective additional heating of plasma electrons from 450 to 600 eV (I _Pl = 32 kA, Δt _RF = 14 ms, P _RF = 100 kW, F = 920 MHz). Several factors influencing the efficiency of plasma heating have been discovered. In particular, significant growth of radiation losses in the LHCD regime has been found, which is probably related to an increase in the intensity of synchrotron radiation from accelerated electrons. The increase in this intensity in the 53–156 GHz frequency range was accompanied by short spikes of microwave radiation, which were observed only in a narrower frequency range (53–78 GHz) and apparently resulted from interaction of a runaway electron beam with significant local mirrors of toroidal magnetic field. A model of the additional heating of plasma electrons due to absorption of the microwave radiation generated by a beam of accelerated electrons is proposed.     

Electromagnetic interference shielding of carbon nanotube/ethylene vinyl acetate composites

Single-walled carbon nanotube (SWCNT) and ethylene vinyl acetate (EVA) composites were synthesized in an internal mixer by melt mixing. The electrical conductivity as well as electromagnetic interference (EMI) shielding effectiveness (SE) over the X-band (8–12 GHz) and microwave (200–2,000 MHz) frequency ranges of these composites were investigated. It was observed that the electrical conductivity of composites increases with increasing SWCNT loading. A percolation threshold of about 3.5 wt.% was obtained and the electrical conductivity of EVA was increased by ten orders of magnitude, from 10⁻¹⁴ to 10⁻⁴ Ω⁻¹ cm⁻¹. The effect of sample thickness on SE was investigated. The correlation between SE and conductivity of the composites is discussed. The experimental data showed that the SE of the composites containing higher carbon nanotube loadings (above 10 wt.%) could be used as an EMI shielding material and lower SWCNT loadings could be used for the dissipation of electrostatic charge.     

Effect of high-energy electron beam irradiation on the properties of AZO thin films prepared by rf magnetron sputtering

We investigated that high-energy electron beam irradiation (HEEBI) performed in air at room temperature affected remarkably the properties of Al-doped ZnO (AZO) films grown on SiO₂ substrates by radio frequency magnetron sputtering techniques. Hall and photoluminescence measurements revealed that the n-type conductivity was preserved in HEEBI treated films with low dose up to 10¹⁵ electrons/cm² and converted to p-type conductivity with further increase in the amount of dose. X-ray photoelectron spectroscopy confirmed the conversion of conductivity by showing that in-diffusion of O₂ from the ambient as well as out-diffusion of Zn from the films took place as a result of HEEBI treatment at high dose of 10¹⁶ electrons/cm². X-ray diffraction analysis indicated that all as-grown films were found to have compressive stress, which was enhanced by HEEBI treatment with the increase of doses. It was also found that worse crystallinity with a smaller grain size was observed in HEEBI treated films with a higher dose, which was correlated with rougher surface morphologies of films observed by an atomic force microscope.     

Investigation of high temperature superconductivity through microwave absorption method

The field dependence and near zero magnetic field microwave absorption as a function of rf power in YBa₂Cu₃O_7−δ and Bi-Ca-Sr-Cu-O has been studied using a varian ESR spectrometer. A model of microwave absorption built on diamagnetic tensor susceptibility has been proposed which explains the observed results satisfactorily.     

Observations of absorptive photon switching and suppression of two-photon absorption in cold atoms

Atomic coherence and interference in an atomic medium exhibiting electromagnetically induced transparency may lead to enhancement or suppression of nonlinear susceptibilities. Absorptive photon switching has been observed by constructive quantum interference, which is based on the enhanced third-order, nonlinear absorption in a four-level system. In a different four-level system, suppression of the two-photon absorption by destructive quantum interference has been observed. Experiments were carried out on ⁸⁷Rb atoms cooled and confined in a magneto-optical trap and the experimental results agree with theoretical calculations of simple four-level model systems.     

Control of the electron temperature by varying the resonance zone width in ECR plasma

The electron temperature (T_e) in an electron cyclotron resonance plasma is clarified to depend on the spatial profiles of the microwave-power absorption by both the electromagnetic-waves measurement and the simulation of microwave power absorption. It is found that T_e is controlled by varying the magnetic field configuration and/or the microwave frequency since the power absorption profile is influenced by the effective resonance width. In fact, T_e is observed to decrease with decreasing the magnetic field gradient at the resonance point for N₂, Ar and O₂/Ar plasma.     

Effect of Rough Walls on Transport in Mesoscopic 3He Films

The interplay of bulk and boundary scattering is explored in a regime where quantum size effects modify mesoscopic transport in a degenerate Fermi liquid film of ³He on a rough surface. We discuss mass transport and the momentum relaxation time of the film in a torsional oscillator geometry within the framework of a quasiclassical theory that includes the experimentally determined power spectrum of the rough surface. The theory explains the anomalous temperature dependence of the relaxation rate observed experimentally. We model further studies on ³He confined in nanofluidic sample chambers with lithographically defined surface roughness. The improved understanding of surface roughness scattering can be extended to the analogous system of electrons in metals and suggests routes to improve the conductivity of thin metallic films.