Mixing of singlet and triplet exciton states in highly excited CdS

Fluorescence spectra due to the free excitons have been studied in CdS at 4.2 K under various excitation levels. It has been found that the triplet-exciton emission intensity relative to the singlet line is enhanced remarkably with the excitation power density. This effect is reasonably explained by the mixing of the singlet with the triplet through the many body interaction. The density of optically generated excitons is determined from the magnetic field dependence of the triplet emission intensity.     

Influence of the magnetic field strength and excitation intensity on the shape of microphotoluminescence spectra of quantum-well structures based on GaN/InGaN doped with Sm and Eu + Sm

This paper presents the results of complex measurements of the microphotoluminescence spectra of quantum-well structures based on InGaN/GaN〈Sm〉 and the determination of the concentration and charge state of the Sm dopant. It has been shown that an increase in the magnetic field strength and the excitation intensity of the microphotoluminescence spectra leads to an increase in the luminescence intensity and a shift in the position of the maximum of the emission wavelength toward the short-wavelength region of the spectrum. Measurements of the microphotoluminescence spectra with variations in the external magnetic field strength, as well as with the introduction of paramagnetic and magnetic impurities, provide additional information on the mechanisms of formation of luminescence spectra in the quantum-well structures InGaN/GaN〈Sm〉,〈Eu + Sm〉. In the long-wavelength region, the influence of the magnetic field on the shape of the microphotoluminescence spectra of the InGaN/GaN structures doped with Sm and Sm + Eu is less pronounced than that in the short-wavelength region.     

Influence of the vacuum-lunar regolith interface on the generation of radio emission by a cascade shower from an ultrahigh-energy particle

We present the results of our calculation of the radio emission field produced by an electron-photon shower from an ultrahigh-energy particle under the surface of the Moon. We consider two cases of cascade propagation: the first in the lunar regolith-vacuum direction at small angles to the interface; and the second in the opposite direction when the particle generates a cascade almost immediately after it has crossed the vacuum-regolith interface. To calculate the relative energy density of the emission emerged at the surface, we have used the method of decomposing spherical waves into plane ones. The intensity of the refracted waves for high frequencies has been found to depend strongly on the shower inclination angle.     

Influence of the magnetic field and measurement temperature on the shape of microphotoluminescence spectra of Eu-Doped InGaN/GaN quantum-well structures

Based on the results of complex investigations of the influence of the magnetic field strength and measurement temperature on the shape of microphotoluminescence (micro-PL) spectra of Eu-doped InGaN/GaN quantum-well structures, the determination of the charge state of Eu impurity ions, and the change in the concentration of dopant ions, it has been shown that an increase in the magnetic field strength (0–5 T) leads to a more significant decrease in the luminescence intensity as compared to the undoped structures. An increase in the measurement temperature from 4.2 to 78 K results in an enhancement of the effect of the magnetic field on the shape of the micro-PL spectra of Eu-doped InGaN/GaN structures. It has been demonstrated that the doping of InGaN/GaN quantum-well structures with europium at a high excitation level leads to a shift in the maximum of the luminescence intensity toward the long-wavelength range of the spectrum.     

Magnetic field induced orbital polarization in cubic YbInNi4: determining the quartet ground state using x-ray linear dichroism

We have been able to induce a linear dichroic signal in the Yb M(5) x-ray absorption white line of cubic YbInNi(4) by the application of a magnetic field. The nonzero integrated intensity of the magnetic field induced dichroic spectrum indicates a net noncubic 4f orbital polarization. A quantitative analysis of the temperature and field strength dependence establishes that the crystal-field ground state is a Γ(8) quartet. The results demonstrate the potential of magnetic field induced linear dichroism as a new powerful approach for the investigation of the degeneracy and orbital degrees of freedom of cubic heavy-fermion and Kondo systems.     

Dynamic magnetic susceptibility of a two-layer film in a strong magnetic field

A theoretical study is reported of the effect of interlayer exchange coupling on the resonance properties of a two-layer magnetic film with “easy-axis” and “easy-plane” anisotropic layers in a strong tilted magnetic field. The dependence of the resonance fields on the tilting angle of the external magnetic field to the film has been obtained, the tensor of integrated high-frequency film susceptibility has been found, and its dependence on the strength and orientation of the external field, as well as on layer thickness, has been analyzed. The results obtained agree with the available experimental data.     

Density oscillations in laser produced plasma decelerated by external magnetic field

This paper presents the dynamics as well as the stability of laser produced plasma expanding across the magnetic field. Observation of some high frequency fluctuations superimposed on ion saturation current along with structuring in the pin hole images of x-ray emitting plasma plume indicate the presence of instability in the plasma. Two type of slope in the variation of x-ray emission with laser intensity in the absence and presence of magnetic field shows appearance of different threshold intensity of laser corresponding to each magnetic field at which this instability or density fluctuation sets on. This instability has been identified as a large Larmor radius instability instead of classical Rayleigh-Taylor (R-T) instability.     

Frequency difference lock-in phenomenon's weakening by transverse magnetic field in Y-shaped cavity dual-frequency laser

We report experimental progress in weakening the frequency difference lock-in phenomenon in a Y-shaped cavity dual-frequency laser. A cube coil pair is chosen to provide a uniform magnetic field for tunability and uniformity of magnetic field strength. When the transverse magnetic field intensity is 9 m T, the frequency difference lock-in phenomenon is evidently weakened and the frequency difference can be continuously tuned in the range of0.12 MHz to 1.15 GHz. Moreover, the relationship between the minimal frequency difference and magnetic field intensity are investigated and discussed. Then a Y-shaped cavity dual-frequency laser is expected to be utilized as an optimum light source for heterodyne interferometric sensing and precise laser measurement.     

Laser-induced fluorescent measurements of magnetic field contoursin a low-pressure discharge

Faraday rotation of a laser beam and emission spectroscopy to resolve Zeeman splitting provide information about the plasma magnetic field, integrated along the line of sight. Information about the local magnetic field strength can be obtained using a dye laser tuned off the center of an atomic or ionic transition by an amount δλ_z. If the absorption linewidth of the transition probed is less than the Zeeman splitting, only those atoms/ions residing in a magnetic field where the Zeeman splitting is δλ_z will resonantly absorb energy from the laser and fluoresce. The feasibility of this magnetic field contour technique was studied in a low-pressure neon discharge. A conductor insulated from the discharge generated a large magnetic field in the discharge free of the Stark broadening effects associated with large plasma currents. The laser-induced fluorescence (LIF) intensity profile measured along the laser beams had peaks at those spatial locations where local magnetic fields, inferred from the conductor current, agreed with the Zeeman shifted wavelength of the laser     

Arterial spin tagging perfusion imaging of rat brain: dependency on magnetic field strength

Perfusion-weighted imaging (PWI), using the method of arterial spin tagging, is strongly T(1)-dependent. This translates into a high field dependency of the perfusion signal intensity. In order to determine the expected signal improvement at higher magnetic fields we compared perfusion-weighted images in rat brain at 4.7 T and 7 T. Application of PWI to focal ischemia and functional activation of the brain and the use of two different anesthetics allowed the observation of a wide range of flow values. For all these (patho-)physiological conditions switching from 4.7 T to 7 T resulted in a significant increase of mean perfusion signal intensity by a factor of 2.96. The ratio of signal intensities of homotopic regions in the ipsi- and contralateral hemisphere was field-independent. The relative contribution of a) T(1) relaxation time, b) net magnetization, c) the Q-value of the receiver coils and d) the degree of adiabatic inversion to the signal improvement at higher field strength were discussed. It was shown that the main parameters contributing to the higher signal intensity are the lengthening of T(1) and the higher magnetization at the higher magnetic field.     

等离子体中相对论电子的同步曲率辐射特性研究

根据同步曲率辐射理论推导了在等离子体环境中,不同磁场条件下的相对论性电子的吸收系数和发射系数表达式，计算了电子的发射强度，并且在此基础上研究了同步曲率辐射机制的脉泽效应.研究了两种磁场位型，第一种是强度均匀但弯曲的磁场，第二种是偶极磁场，结果发现了一些偶极磁场下特有的辐射特性.考虑到在天体的环境下电子具有的不同的能谱分布，分别选用了三种典型的能谱分布(幂率分布，高斯分布，热分布)进行了研究，通过计算负吸收和脉泽放大效应在发射强度上的表现后， 发现在某些天体物理环境中，同步曲率辐射在等离子体中的确存在脉泽放大效应.这些研究结果对太阳系中行星外层辐射的研究和宇宙中的射电高亮温度等问题的研究可能提供有益的帮助. 关键词： 同步曲率辐射 负吸收 脉泽效应     

横向静磁场对电磁悬浮液滴稳定性的影响

在利用电磁悬浮技术实现液滴悬浮的过程中,液滴内部往往存在剧烈对流、外部伴随快速旋转和质心的水平位移等不稳定因素;因此,实现液滴的稳定悬浮是完善电磁悬浮技术的关键.本文采用实验观测的方法,通过U形静磁场组件对液滴所在空间施加横向静磁场,利用高速相机记录了不同磁场强度下纯铜熔融液滴的振荡变形过程;分析了横向静磁场对悬浮铜液滴振荡频率、振幅以及旋转的影响.实验发现:对于熔融前的固态铜颗粒,若静磁场强度超过0.3 T,铜颗粒几乎以静止状态悬浮.熔融后,当施加0.15 T的静磁场,与未加静磁场时相比,液滴俯视图轮廓线拟合出的椭圆分别与x轴和y轴的交点坐标之差R-、椭圆面积A和椭圆长轴长度Dmax的振幅分别减小了25%,76%和60%;随着磁场强度的继续增加,振幅和频率继续减小,但在静磁场强度为0.3 T时,相比静磁场强度为0.2 T,频率增加了1 Hz.横向静磁场还抑制了悬浮铜液滴的旋转,当磁场强度增加到0.53 T时,悬浮液滴只在10?的角度范围内摆动.这些结果表明,施加横向静磁场能够有效提高悬浮液滴的稳定性.     

Influence of magnetic field on exciton luminescence

The magnetic field dependence of the exciton emission intensity I_ex(H) has been investigated in Ge crystals stressed along the direction near 〈100〉. In the low field limit the magnetic field correction has been evaluated to the wave functions of the ground and some excited states of an isotropic exciton. The calculated dependence I_ex(H) in the case of Ge is in a good agreement with the experimental one at H ? 0.5 T.     

金属有机化学气相沉积生长Zn1-xMnxSe薄膜的发光和磁光性质

利用金属有机化学气相沉积(MOCVD)方法在GaAs衬底上生长了不同组分的Zn_1-xMn_xSe薄膜。X射线衍射和X射线摇摆曲线证明样品具有较好的结晶质量。在低温、强磁场下对样品的发光进行了研究,在带边附近观察到两个发光峰的相对强度随着磁场增强发生了变化。通过变温光谱探讨了这两个发光峰的来源,并被分别归因于自由激子跃迁和与Mn有关的束缚态激子跃迁。同时随着磁场的增强,ZnMnSe带隙发光红移是由于类S带和类P带电子与Mn离子的3d⁵电子的自旋交换作用。     

Terahertz emission from quantum cascade lasers in the quantum Hall regime: evidence for many body resonances and localization effects

A terahertz quantum cascade laser, operating at lambda=159 microm and exploiting the in-plane confinement arising from perpendicular magnetic field, is used to investigate the physics of electrons confined on excited subbands in the regime of a large ratio of the magnetic field confinement energy to the photon energy. As the magnetic field is increased above about 6 T, and the temperature lowered below 20 K, the devices are characterized by a very low threshold current density, with values as low as J(th)=1A/cm(2), and an increase of gain by five times the low field value. We show that, as with the quantum Hall effect, the key physical process is the localization of the carriers. Evidences for resonant electron-electron scattering processes are directly obtained from light intensity and transport measurements.     

Analytical Ground-State of an Exciton Trapped in a Circular Parabolic Quantum Dot in the Presence of a Magnetic Field

The quasi-exact properties of an exciton are investigated theoretically in the presence of an external magnetic field using the effective-mass approach in GaAs parabolic quantum dot. The energy spectrum is obtained analytically as a function of the dot radius, interaction strength and magnetic field. It is established that, a steady bound state of an exciton in the ground state exists under the effect of a strong magnetic field; also I noticed that the exciton binding energy decreases by increasing both the radius of the dot and the magnetic field strength and the reduction becomes pronounced for larger dots. As expected, it has been found that the exciton total energy decreases with increasing the size of the dot and it enhances by increasing the magnetic field. It appears that the exciton total energy strongly depends on the magnetic field for dots with big size. The magnetic field effect on the exciton size also has been studied. It is shown that the increase in the magnetic field leads to a reduction in the exciton size; due to magnetic field confinement, while the size of an exciton reach its bulk limit as the dot size increases. Moreover, it is shown that, if the dot radius is sufficiently large the oscillator strength saturates and it becomes insensitive to the magnetic field while the increase in the magnetic field gradually weakened the oscillator strength. I have calculated the ground-state distribution for both the electron and the hole. It is found that the localization of the electron/hole increases in the presence of a magnetic field. Moreover, the ground-state optical-absorption intensity is investigated. Finally, the dependence of the lowest five states of an exciton on both the dot radius and the magnetic field are discussed.

     

The magnetic field dependence of the breathing artifact

Breathing causes ghost artifacts in magnetic resonance (MR) images. These ghosts are more conspicuous at high magnetic field strength due to (i) operational factors that affect the relative intensity of the individual ghosts and (ii) factors that affect the image intensity of the moving structures producing the ghosts. Both types of factors are identified and illustrated with images of a human subject. A brief theoretical analysis of the noiseless MR image of a point object in sinusoidal motion shows that the intensity structure of each ghost depends on the number of pixels over which the object moves. Generally, images with narrower pixels have ghosts with greater relative intensity. However, the breathing artifact is more apparent at high magnetic field strength primarily because the signal-to-noise ratio and the cotnrast between fat and muscle tissues are increased.     

Polarized nonequilibrium Bose-Einstein condensates of spinor exciton polaritons in a magnetic field

The effect of a magnetic field on a spinor exciton-polariton condensate has been investigated. A quenching of a polariton Zeeman splitting and an elliptical polarization of the condensate have been observed at low magnetic fields B<2 T. The effects are attributed to a competition between the magnetic field induced circular polarization buildup and the spin-anisotropic polariton-polariton interaction which favors a linear polarization. The sign of the circular polarization of the condensate emission at B<3 T is negative, suggesting that a dynamic condensation in the excited spin state rather than the ground spin state takes place in this magnetic field range. From about 2T on, the Zeeman splitting opens and from then on the slope of the circular polarization degree changes its sign. For magnetic fields larger than the 3 T, the upper spin state occupation is energetically suppressed and circularly polarized condensation takes place in the ground state.     

Measurement of interchain and intrachain exciton hopping barriers in luminescent polymer

The integrated photoluminescence intensity in thin films of 'Super Yellow' copolymer has been analyzed using a Mott-like temperature dependence. This has enabled us to observe contributions from two emission channels, indicative of exciton recombination proceeding from two distinct origins. At high temperature, interchain thermally activated exciton energy transfer and migration dominates, resulting in large scale quenching of the integrated emission intensity and hence the photoluminescence quantum yield. However, at relatively low temperature, an additional increase of the integrated emission intensity occurs. This new channel of emission has been attributed to recombination from excitons where intrachain exciton energy transfer between adjacent subunits of the copolymer backbone becomes hindered. The activation energy barriers that control both of these emission channels have been obtained and are correlated with chain backbone degrees of freedom.     

An improved approach to calibrating high magnetic field gradients for pulsed field gradient experiments

Probes capable of generating short high intensity pulsed magnetic field gradients are commonly used in diffusion studies of systems with very short T(2). Traditional methods of calibrating magnetic field gradients present unique challenges at ultrahigh field strengths and are often inapplicable. Currently the most accurate method of determining magnetic gradient strength is to use the known diffusion coefficient of a standard sample and determine gradient strength from the echo attenuation plot of a diffusion experiment, however, there are problems with finding suitable standards for high intensity gradients. Here, we show that molecules containing at least two receptive nuclei (i.e. one with high and one with low gyromagnetic ratios) are excellent systems for calibrating high intensity gradients.