对超热电子诱生的磁场分布的估算

利用简化模型估算了电荷分离场及由超热电子逃逸在等离子体表面产生的自生磁场的大小和空间分布.受电荷分离场的影响以及超热电子逃逸数的限制,超热电子产生的环形磁场主要分布于等离子体表面附近的焦斑半径内,仅当超热电子束流很强时(在1μm半径截面内达到10³A量级),环形磁场才可以达到10²T量级.一般情况下,由超热电子产生的磁场极小. 关键词： 磁场 超热电子     

Temperature dependence of the critical magnetic field of the structural transition in MBBA-based ferronematics

We studied the structural transitions in ferronematics based on the thermotropic nematic liquid crystal MBBA (4^′ -methoxybenzylidene-4-n-butylaniline) having a nematic-to-isotropic transition temperature T _N–I?=?48.0^○C and in MBBA-based ferronematics doped with a magnetic suspension consisting of Fe₃O₄ particles (10?nm in diameter) coated with oleic acid as a surfactant. The ferronematic samples were prepared with different volume concentrations of magnetic particles φ?=,1× 10^?4, 2× 10^?4 and 5×10^?4. The temperature dependences of the critical magnetic fields in a bias electric field under strong applied magnetic fields are presented. We calculated the surface density of anchoring energy W at the nematic–magnetic particle boundary. Scaling of the structural transition in the MBBA and MBBA-based ferronematics with the temperature of the nematic-to-isotropic transition was observed.     

Surface magnetism of β-FeO(OH) by Mössbauer spectroscopy

Fine particles of -FeO(OH) were prepared using the⁵⁶Fe isotope and the surfaces of the particles were coated with extremely thin⁵⁷Fe(III) layers. Mössbauer results show that the Fe(III) ions in the top surface layer are involved in the magnetic order and occupy two kinds of surface sites. Both of the two exchange field at surface sites, estimated from the temperature dependences of the hyperfine fields, are smaller than the bulk value. The decrease of exchange fields at the surface sites corresponds to the reduced number of neighboring magnetic ions at each site.     

Upper limit for the electric charge of the photons from the millisecond pulsar 1937+21 observations

Radio signals of different frequencies emitted simultaneously by a pulsar 16000 ly away, arrive on the Earth within less than 2 μs, notwithstanding the galactic magnetic fields encountered along their path. It follows that photons cannot have an electric charge larger than about 10⁻³² times the electron charge.     

Cationic starch as a precursor to prepare porous activated carbon for application in supercapacitor electrodes

Three activated carbons (ACs) for the electrodes of supercapacitor were prepared from cationic starch using KOH, ZnCl₂ and ZnCl₂/CO₂ activation. The BET surface area, pore volume and pore size distribution of the ACs were evaluated using density functional theory method, based on N₂ adsorption isotherms at 77 K. The surface morphology was characterized with SEM. Their electrochemical performance in prototype capacitors was determined by galvanostatic charge/discharge characteristics and cyclic voltammetry, and compared with that of a commercial AC, which was especially prepared for use in supercapacitors. The KOH-activated starch AC presented higher BET surface area (3332 m² g⁻¹) and larger pore volume (1.585 cm³ g⁻¹) than those of the others, and had a different surface morphology. When used for the electrodes of supercapacitors, it exhibited excellent capacitance characteristics in 30 wt% KOH aqueous electrolytes and showed a high specific capacitance of 238 F g⁻¹ at 370 mA g⁻¹, which was nearly twice that of the commercial AC.     

Magnetic field-induced cluster formation and variation of magneto-optical signals in zinc-substituted ferrofluids

Fine magnetic particles (size≅100 Å) belonging to the series Zn_xFe_1−xFe₂O₄ were synthesized by cold co-precipitation methods and their structural properties were evaluated using X-ray diffraction. Magnetization studies have been carried out using vibrating sample magnetometry (VSM) showing near-zero loss loop characteristics. Ferrofluids were then prepared employing these fine magnetic powders using oleic acid as surfactant and kerosene as carrier liquid by modifying the usually reported synthesis technique in order to induce anisotropy and enhance the magneto-optical signals. Liquid thin films of these fluids were prepared and field-induced laser transmission through these films was studied. The transmitted light intensity decreases at the centre with applied magnetic field in a linear fashion when subjected to low magnetic fields and saturate at higher fields. This is in accordance with the saturation in cluster formation. The pattern exhibited by these films in the presence of different magnetic fields was observed with the help of a CCD camera and was recorded photographically.     

Calculation of electric and magnetic induced fields in humans subjected to electric power lines

In this work, analysis of the human body exposed to high voltage electric and magnetic fields is presented. The distribution of the electric field is obtained by using Laplace's equation. This relates the surface charge induced on the body to the potential in a reciprocal Laplace problem, which is then calculated by charge simulation method coupled with genetic algorithms to determine the appropriate arrangement of simulating charges inside the human body. The magnetic field intensity along the vertical center line of the human is calculated. Exposure to external electric and magnetic fields at power frequency induces electric field, magnetic field and currents inside the human body. The presented model for simulating electric and magnetic fields are a three dimensional field problem and introduced different types of charges to simulate the different elementary geometrical shapes of human body. The particular strength of the charge simulation method in this application is its ability to allow a detailed representation of the shape and posture of the human body. The results have been assessed through comparison induced current, electric field, magnetic field and there distribution over the body surface, as estimated in other experimental and computational work.     

Long-range potentials and the electromagnetic polarizabilities

The long-range spin and velocity independent forces of electromagnetic origin which act between any two systems are studied for those cases in which no forces of this type exist to order e². It is shown that they are uniquely determined by the charge, magnetic moment, and polarizabilities of both systems, not only to the dominant order r^?n, but also to the next one r^?(n+1). These potentials provide the link between Compton scattering polarizabilities (response to real photons) and classically defined polarizabilities (response to static electromagnetic field). The two definitions are shown to be equivalent for neutral spinless systems; the problems arising for a neutral particle with magnetic moment are studied in detail. The r^?(n+1) terms have no classical counterpart, since they are due to the relativistic quantum propagation of the system which carries charge or magnetic moment. The results are of general validity with analyticity, crossing, unitarity, and gauge invariance as only inputs. The most general conclusion is that the polarizabilities represent electromagnetic properties of a system at order e², as the charge and magnetic moment do at order e. Thus they give the strength of the response to electric and magnetic fields, independently of the specific characteristics of the electromagnetic agent.     

Spin thermoelectricity in dualhydrogenated zigzag silicene nanoribbons with surface adsorptions

The buckled structure of silicene provides a feasible pathway to influence its electric and magnetic properties via surface adsorptions. Here, we investigate the magnetic and spin thermoelectric transport properties of dual-hydrogenated zigzag silicene nanoribbons (ZSiNRs) without/with the hydrogen adsorption. The band gaps for two spin channels in ZSiNRs under the hydrogen adsorption are shifted near the Fermi level, leading to the appearance of spin Seebeck effect. Using a temperature difference, one can derive the carriers with the different spin index to flow in the opposite direction. Moreover, a large rectification ratio close to 10⁵ at room temperature is achieved for the spin current, and the charge current exhibits a remarkable negative differential thermoelectric resistance (NDTR) behavior. The results presented here are fascinating potential applications in the fields of silicon-based spin caloritronic devices.     

混合电解质溶液中考虑介电饱和度的表面电位评估原理

基于非线性泊松-玻尔兹曼方程，推导了混合电解质溶液中考虑介电饱和度的表面电位的解析表达式. 近似解析解和精确数值解计算出的表面电位在很大范围的电荷密度和离子强度条件下均具有很好的一致性. 当表面电荷密度大于0.30 C/m^{2 时，介电饱和度对表面电位的影响变得尤为重要；当表面电荷密度小于0.30 C/m2时，可忽略介电饱和度的影响，即基于经典泊松-玻尔兹曼方程可获得有效的表面电位解析模型. 因此，0.3 C/m2可作为是否考虑介电饱和度的颗粒临界表面电荷密度值. 在低表面电荷密度时，考虑介质饱和度的表面电位解析模型可自然回归到经典泊松-玻尔兹曼理论的结果，得到的表面电位可以正确地预测一价和二价反离子之间的吸附选择性.}     

Non-Ohmic conduction in polydiacetylene thin films

We have studied the current–voltage (I–V) characteristics of polydiacetylene (PDA) thin films in the temperature region 300–1.7 K. It was found that at electric fields higher than 2 × 10⁴ V/cm, the I–V characteristics are strongly super-linear with negative temperature coefficient of starting voltage. Negative gate voltage increases the source-drain current (the effect is more pronounced at low temperature), whereas the magnetic field up to 7 T does not affect it. The results demonstrate that at low temperature the charge transport is mainly supported due to a charge injection and tunneling from the metallic banks, whereas at higher temperatures the activation energy is related to the band gap mismatch between the different polymer chains or granules.     

Magnetic-field-induced retardation of the recombination of nonequilibrium charge carriers in semiconductors

The retardation of the recombination of electrons and holes in semiconductors in an applied uniform magnetic field has been predicted. It has been shown that the recombination time in germanium in the temperature range of T = 1–10 K at charge carrier densities of n _e = 10¹⁰−10¹⁴ cm⁻³ in magnetic fields of B = 3 × 10²−3 × 10⁴ G can be more than two orders of magnitude larger than that at zero magnetic field. This means that, after creation of nonequilibrium charge carriers by their injection at the p-n junction owing to some radiation sources or fast electron irradiation, the semiconductor retains its conductivity for a much longer time at nonzero applied magnetic field. The effect under study can be used, for example, to detect radiation sources.     

Photovoltaic properties of a heterojunction based on copper phthalocyanine films on the surface of polycrystalline cadmium sulfide

The photovoltaic effect has been detected and studied in thin-film structures based on thermally deposited 200-nm-thick copper phthalocyanine (CuPc) films on the surface of polycrystalline CdS. The structures under study demonstrate the linear current-voltage characteristics at external electric fields to 3.5 × 10⁴ V/cm. Two components of the photovoltage of different signs have been revealed when the sample is illuminated in the wavelength range from 350 to 700 nm. The first component has the positive sign on the CuPc film side and is observed when using the radiation with a wavelength lesser than 500 nm, i.e., in conditions of predominant absorption of the radiation in the CdS layer. The second component has the negative sign on the CuPc film side and is observed when using the radiation with a wavelength in the range from 500 to 570 nm, corresponding to the spectral region of the absorption edge of the CuPc films. The dependences of the photovoltage on the radiation intensity studied in the range from 5 × 10¹² to 10¹⁴ photons cm^?2 s^?1 are different in the cases of the two detected components. Mechanisms of generation of the photovoltage components associated with a change in the band bending during photogeneration of charge carriers in the region of space charge in CdS and a change in conditions of the charge transfer in the interfacial CuPc/CdS region during the radiation absorption in the CuPc film have been proposed.     

Surface structure of quark stars with magnetic fields

We investigate the impact of magnetic fields on the electron distribution of the electrosphere of quark stars. For moderately strong magnetic fields of B ∼ 10¹³ G, quantization effects are generally weak due to the large number density of electrons at surface, but can nevertheless affect the photon emission properties of quark stars. We outline the main observational characteristics of quark stars as determined by their surface emission, and briefly discuss their formation in explosive events termed as quark-novae, which may be connected to the r-process.     

Nuclear magnetic resonance and transferred hyperfine interaction study of the crystallographically inequivalent Cs(I) and Cs(II) in a Cs2CuCl4 single crystal

¹³³Cs (I=7/2) nuclear magnetic resonance in a Cs₂CuCl₄ single crystal grown by using the slow evaporation method was measured in its three mutually perpendicular crystal planes. The ¹³³Cs resonances of two different groups with two crystallographically inequivalent cesium nuclei, Cs(I) and Cs(II), in the unit cell were recorded. The transferred hyperfine fields for Cs(I) and Cs(II) calculated from the paramagnetic shift and the molecular susceptibility measurements could be expressed by the linear equation H_hf=AT+B. The angular dependence of the ¹³³Cs nuclear magnetic resonance spectra showed that the Cs(I) and the Cs(II) nuclei had different values for the quadrupole coupling constant. The electric field gradient tensors of Cs(I) and Cs(II) were symmetric, and the orientations of their principal axes did not coincide. The Cs(I) ion surrounded by 11 chlorine ions had a small quadrupole parameter, a smaller charge distribution, and a small value for the transferred hyperfine field. However, the Cs(II) ion surrounded by nine chlorine ions had a larger quadrupole parameter, a larger charge distribution, and a larger value for the transferred hyperfine field.     

Electrical conduction in coevaporated antimony trisulphide films

Antimony trisulphide films were prepared by the three temperature method. The temperature dependence of conductivity for films prepared at different substrate temperatures were measured. The I–V characteristics of the Sb-Sb₂S₃-Sb systems studied showed a space charge limited conduction indicating a I ∝ V² /d³ dependence. The trap density and the trap energy level were determined.     

Intricacies of strain and magnetic field induced charge order melting in Pr0.5Ca0.5MnO3 thin films

Thin films of the half-doped manganite Pr_0.5Ca_0.5MnO₃ were grown on (1 0 0) oriented MgO substrates by pulsed laser deposition technique. In order to study the effect of strain on the magnetic field induced charge order melting, films of different thicknesses were prepared and their properties were studied by X-ray diffraction, electrical resistivity and magnetoresistance measurements. A field induced charge order melting is observed for films with very small thicknesses. The charge order transition temperature and the magnetic field induced charge order melting are observed to depend on the nature of strain that is experienced by the film.     

Use of mucolytics to enhance magnetic particle retention at a model airway surface

A previous study has shown that retention of magnetic particles at a model airway surface requires prohibitively strong magnetic fields. As mucus viscoelasticity is the most significant factor contributing to clearance of magnetic particles from the airway surface, mucolytics are considered in this study to reduce mucus viscoelasticity and enable particle retention with moderate strength magnetic fields. The excised frog palate model was used to simulate the airway surface. Two mucolytics, N-acetylcysteine (NAC) and dextran sulfate (DS) were tested. NAC was found to enable retention at moderate field values (148 mT with a gradient of 10.2 T/m), whereas DS was found to be effective only for sufficiently large particle concentrations at the airway surface. The possible mechanisms for the observed behavior with different mucolytics are also discussed based on aggregate formation and the loading of cilia.     

The electron spin-density space distribution in ferromagnetic Heusler alloys

The hyperfine magnetic fields on Mn⁵⁵, Co⁵⁹, Al²⁷ and Si²⁹ were measured by the spinecho technique in several systems of Heusler alloys. The concentration dependence of local magnetic moments of Mn and Co ions in different lattice positions was determined and the space distribution of delocalized electron spin-density was analyzed. The hyperfine fields on spelements were calculated using the main theoretical models.