Calculation of electric fields induced near metal implants by magnetic resonance imaging switched-gradient magnetic fields

Electric (E) fields induced near metal implants by MRI switched-gradient magnetic fields are calculated by a new equivalent-circuit numerical technique. Induced E-field results are found for a metallic spinal-fusion implant consisting of two thin wires connected to the metallic case of a current generator as well as for its subsections: a bare U-shaped wire, an insulated U-shaped wire, a cut insulated wire, and a generator. The presence of the metallic implants perturbs the E field significantly. Near the ends of the bare U-shaped wire, the E field is 89.7 times larger than in the absence of the wire. The greatest E field concentration occurs near the ends of the cut insulated wire, where the E field is 196.7 times greater than in the absence of the wire. In all cases, the perturbation of the induced E field by the implanted wire is highly localized within a few diameters of the wire.     

Cosmological magnetic fields

Magnetic fields are observed not only in stars, but in galaxies, clusters, and even high redshift Lyman-α systems. In principle, these fields could play an important role in structure formation and also affect the anisotropies in the cosmic microwave background radiation (CMB). The study of cosmological magnetic fields aims not only to quantify these effects on large-scale structure and the CMB, but also to answer one of the outstanding puzzles of modern cosmology: when and how do magnetic fields originate? They are either primoridial, i.e. created before the onset of structure formation, or they are generated during the process of structure formation itself.     

Single-dot spectroscopy in high magnetic fields

We report on photoluminescence measurements from a single InAs/GaAs quantum dot in magnetic fields up to 28 T. Mesa-patterned structure has been used to limit the number of investigated dots. Three pairs of Zeeman-split emission lines with the same effective g*-factor and diamagnetic shift have been observed. The attribution of the lines to recombination of a neutral exciton, a biexciton, and a charged exciton is discussed.     

Optical properties of semiconductor quantum dots in magnetic fields

Optical properties of semiconductor quantum dots in magnetic fields are reviewed. A theory is described based on a multi-band effective-mass approximation with a nonparabolic conduction electron dispersion, the direct Coulomb interaction, and the electron-hole exchange interaction taken into account. The transition from the quantum-confined Zeeman effect for a weak magnetic field to the quantum-confined Paschen-Back effect to a strong magnetic field is discussed in comparison with atomic spectra in magnetic fields. Experimental results of the optical properties of isolated CuCl, CdSSe, and Si quantum dots in magnetic fields are also discussed in conjunction with the theoretical results.     

周期会切磁场聚焦强流相对论带状电子束的理论分析和粒子模拟

对低幅值、短周期、有偏置磁极的周期会切磁场中300 keV,3 kA带状电子束的传输进行了理论分析和粒子模拟。给出了将波导宽度考虑在内的轴向均匀带状电子束的空间电荷场和作用在有限厚度的带状电子束的短边和宽边上的聚焦力的表达式,并利用束匹配的方法得到了磁场的幅值和周期,以及电子束通道的宽度和高度等参数。最后根据理论计算的结果进行了３维粒子模拟验证,结果表明：束宽边上聚焦良好,而在短边上带状电子束的轴向有限长效应使得聚焦力与散焦力沿轴向不能完全匹配,在束包络上产生了Betatron振荡,但在300 mm的距离上传输效率仍能达到98%以上,说明有偏置磁极的周期会切磁场聚焦强流带状电子束在理论上是可行的。     

Nonthermal effect of dilatonic black holes

The quantum nonthermal effect of the spherically symmetric and rotating dilatonic black holes is studied. A crossing of the positive and negative Dirac energy of particles occurs near dilatonic black holes. We find that the dilaton coupling parameter α affects the energy of spontaneous radiant particles. The energy of particles decreases when the coupling parameter α increases.     

Stationary electromagnetic fields around black holes. II. General solutions and the fields of some special sources near a Kerr black hole

The electromagnetic field of a general stationary source, occurring in the vicinity of a rotating (Kerr) black hole, is obtained by solving the Maxwell and Teukolsky equations. The field is expressed both outside and inside the radius at which the source is located. As examples the fields of point charges, charged rings, current loops, and magnetic dipoles not necessarily located in axisymmetric positions are calculated. The electromagnetic field occurring when a Kerr black hole is placed in an originally uniform magnetic field is derived without assuming the alignment of the direction of the magnetic field and the axis of symmetry of the black hole.We have recently learned that A. King from the University of Hamburg has independently found the vacuum solutions given in Section 2 of the present paper. His work will be published inMathematics Proceedings of the Cambridge Philosophical Society. In another paper (to be published inLettere al Nuovo Cimento) he gives the fields of a stationary point charge and a stationary magnetic monopole.     

Tidal dynamics of relativistic flows near black holes

We point out novel consequences of general relativity involving tidal dynamics of ultrarelativistic relative motion. Specifically, we use the generalized Jacobi equation and its extension to study the force‐free dynamics of relativistic flows near a massive rotating source. We show that along the rotation axis of the gravitational source, relativistic tidal effects strongly decelerate an initially ultrarelativistic flow with respect to the ambient medium, contrary to Newtonian expectations. Moreover, an initially ultrarelativistic flow perpendicular to the axis of rotation is strongly accelerated by the relativistic tidal forces. The astrophysical implications of these results for jets and ultrahigh energy cosmic rays are briefly mentioned.     

Magnetic fields associated with a nuclear magnetic resonance medical imaging system

Measurements were made of magnetic and electric field levels in and around a nuclear magnetic resonance imaging system undergoing a clinical trial. Magnetic field levels ranged from 0.04 tesla (T) in the imaging volume down to about 0.0006 T at the end of the patient table. The peak radio-frequency magnetic field level was 15 amperes per meter (A/m) in the imaging volume, while the rms value was 4.6 A/m. The specific absorption rate resulting from the radio-frequency magnetic field was calculated to be no more than 0.017 watts per kilogram (W/kg). The radio-frequency electric field was detectable only within a few centimeters of the coil assembly, and does not significantly contribute to the specific absorption rate. These exposure levels were much lower than existing guidelines for clinical NMR procedures.     

Biological effects of magnetic fields: chronic exposure of the nematode Panagrellus redivivus

The Panagrellus redivivus bioassay, an established monitor of adverse toxic effects of different environments, has been used to study the biological effects of exposure to static and time-varying uniform and gradient magnetic fields, and to time-varying magnetic field gradients superimposed on a static uniform magnetic field of 2.35 Tesla. Temporally stationary magnetic fields have no effect on the fitness of the test animals. Time-varying magnetic fields cause some inhibition of growth and maturation in the test populations. The combination of pulsed magnetic field gradients in a static uniform magnetic field also has a small detrimental effect on the fitness of the test animals.     

现代数字物理教学连载②——关于电流磁场的数字教学研究

以电磁场的教学为例,介绍了数字物理教学的重要作用,并借助现代数字技术,解决了长期以来基础物理教学未能处理好的环形电流及螺线管磁场和交变电磁场的问题,丰富了电磁学的教学内容,提高了学生处理实际问题的能力．     

Gibbons-Maeda黑洞附近中微子场的熵

从Gibbons-Maeda(G-M)时空背景下的线元出发,利用WKB近似,由自旋为1/2的中微子场方程求得径向波数pkr,在此基础上利用brick-wall方法计算了G-M黑洞附近中微子场的自由能和熵,并与标量场的熵作了比较,发现中微子场的主项熵是标量场的主项熵的7/8倍.     

Ionization energies of beryllium in strong magnetic fields

We have develop an effective frozen core approximation to calculate energy levels and ionization enegies of the beryllium atom in magnetic field strengths up to 2.35×105T. Systematic improvement over the Hartree-Fock results for the beryllium low-lying states has been accomplished.     

Direct coupling of magnetic fields to tunneling systems in glasses

We report on investigations of spontaneous polarization echoes in the nonmagnetic multicomponent glass BaO-Al2O3-SiO2 in static magnetic fields. While the echo decay is only marginally influenced, the echo amplitude depends strongly on magnetic fields. It seems that the intrinsic magnetic moment of tunneling systems causes dephasing effects which are detected in our echo experiments. In addition we find a strong increase of the echo amplitude with magnetic fields. This result shows that the coupling of the tunneling systems to magnetic fields is surprisingly strong and cannot be understood on the basis of current theories.     

Strategies for discovery and optimization of thermoelectric materials: Role of real objects and local fields

Thermoelectric materials provide a renewable and eco-friendly solution to mitigate energy shortages and to reduce environmental pollution via direct heat-to-electricity conversion. Discovery of the novel thermoelectric materials and optimization of the state-of-the-art material systems lie at the core of the thermoelectric society, the basic concept behind these being comprehension and manipulation of the physical principles and transport properties regarding thermoelectric materials. In this mini-review, certain examples for designing high-performance bulk thermoelectric materials are presented from the perspectives of both real objects and local fields. The highlights of this topic involve the Rashba effect, Peierls distortion, local magnetic field, and local stress field, which cover several aspects in the field of thermoelectric research. We conclude with an overview of future developments in thermoelectricity.     

圆形面偶极层与圆电流的电磁场

通过解电场的轴对称边值问题求出了圆形面偶极层的电势分布，使用其结果和类比方法找到了圆电流的磁场分布。     

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.     

On standardization of low symmetry crystal fields

Standardization methods of low symmetry – orthorhombic, monoclinic and triclinic – crystal fields are formulated and discussed. Two alternative approaches are presented, the conventional one, based on the second-rank parameters and the standardization based on the fourth-rank parameters. Mainly f-electron systems are considered but some guidelines for d-electron systems and the spin Hamiltonian describing the zero-field splitting are given. The discussion focuses on premises for choosing the most suitable method, in particular on inadequacy of the conventional one. Few examples from the literature illustrate this situation.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

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.