The cumulative resonance in atomic ground state induced by modulated light field

The interaction of an atomic system with a light beam, intensity-modulated at the ground-state Zeeman frequency is analysed by the dressed-atom method. When the light field is resonant, this interaction produces a new type of degereracy of dressed-atom energy levels which is not of level crossing type. This gives rise to the cumulative resonance with a width depending on the light intensity.     

Magnetooptical force in the resonance field formed by elliptically polarized light waves

The screening of the Coulomb interaction is studied with regard to Friedel oscillations in multicomponent electron plasma structure. A double quantum well (QW) and a superlattice are considered. The groundstate energy of a donor (exciton) in a double quantum well is calculated by a variational method as a function of the population of subbands.     

Magnetic field induced incommensurate resonance in cuprate superconductors

The influence of a uniform external magnetic field on the dynamical spin response of cuprate superconductors in the superconducting state is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic scattering around low and intermediate energies is dramatically changed with a modest external magnetic field. With increasing the external magnetic field, although the incommensurate magnetic scattering from both low and high energies is rather robust, the commensurate magnetic resonance scattering peak is broadened. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime. The theory also predicts that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying an external magnetic field large enough.     

磁共振扩散张量成像中扩散敏感梯度磁场方向分布方案的研究进展

磁共振扩散张量成像可以定量无创研究人体内水分子在三维空间中的各向异性扩散规律,进而获取重要的病理及生理信息.为了得到水分子各向异性扩散信息,需要按照一定的方案依次施加不同方向的扩散敏感梯度磁场,测量水分子在这些方向上的扩散系数用以估算扩散张量.扩散张量成像测量结果的准确程度受梯度磁场方向分布方案的影响,本文对扩散敏感梯度磁场方向分布方案进行综述,包括完全随机方案、启发式方案、规则多面体式方案和数值优化方案等,分析这些方案的优势与局限性,并提出需进一步研究的问题.     

Anisotropic diffusion of metabolites in peripheral nerve using diffusion weighted magnetic resonance spectroscopy at ultra-high field

Although the diffusivity and anisotropy of water has been investigated thoroughly in ordered axonal systems (i.e., nervous tissue), there have been very few studies on the directional dependence of diffusion of metabolites. In this study, the mean apparent diffusion coefficient (Trace/3 ADC) and fractional anisotropy (FA) values of the intracellular metabolites N-acetyl aspartate (NAA), creatine and phosphocreatine (tCr), choline (Cho), taurine (Tau), and glutamate and glutamine (Glx) were measured parallel and perpendicular to the length of excised frog sciatic nerve using a water suppressed, diffusion-weighted, spin-echo pulse sequence at 18.8T. The degree of anisotropy (FA) of NAA (0.41+/-0.09) was determined to be less than tCr (0.59+/-0.07) and Cho (0.61+/-0.11), which is consistent with previously reported human studies of white matter. In contrast, Glx diffusion was found to be almost isotropic with an FA value of 0.20+/-0.06. The differences of FA between the metabolites is most likely due to their differing micro-environments and could be beneficial as an indicator of compartment specific changes with disease, information not readily available with water diffusion.     

Determination of diffusion length in photovoltaic crystalline silicon by modelisation of light beam induced current

The diffusion length of minority carriers is one of the most important electrical parameters to qualify silicon for photovoltaic applications. One way to evaluate this parameter is to analyse the decay of the current induced when a focused beam is scanned away from the collector using Light Beam Induced Current (LBIC) technique. The LBIC signal was numerically calculated with 2D-DESSIS software under different boundary conditions, as a function of material thickness and surface recombination velocity in order to verify the limitations of analytical models and to fit the LBIC signal measured in thin silicon samples. Samples with thickness ranging from 55 μm to 2500 μm were evaluated with diffusion length values ranging from 70 μm to 2.5 mm. Analytical expressions of the Internal Quantum Efficiency (IQE) were also used to extract the minority carrier bulk and effective diffusion lengths from surface averaged spectral response and reflectivity data in thick solar cells.     

Fictitious magnetic resonance by quasielectrostatic field

We propose a new kind of spin manipulation method using a fictitious magnetic field generated by a quasielectrostatic field. The method can be applicable to every atom with electron spins and has distinct advantages of small photon scattering rate and local addressability. By using a CO₂ laser as a quasielectrostatic field, we have experimentally demonstrated the proposed method by observing the Rabi oscillation of the ground state hyperfine spin F=1 of the cold ⁸⁷Rb atoms and the Bose–Einstein condensate.     

Electric field induced nuclear spin resonance mediated by oscillating electron spin domains in GaAs-based semiconductors

We demonstrate an alternative nuclear spin resonance using a radio frequency (rf) electric field [nuclear electric resonance (NER)] instead of a magnetic field. The NER is based on the electronic control of electron spins forming a domain structure. The rf electric field applied to a gate excites spatial oscillations of the domain walls and thus temporal oscillations of the hyperfine field to nuclear spins. The rf power and burst duration dependence of the NER spectrum provides insight into the interplay between nuclear spins and the oscillating domain walls.     

Fluorescence spectra of uranium induced by a linearly polarized laser light in a magnetic field

In this paper we reported the fluorescence spectra of U~(238) induced by a linearly polarized laser field in a magnetic field and offered a theoretical analysis. The consistence between the theoretical prediction and experimental results was satisfactory.     

Coherence relaxation during the diffusion of resonance radiation in media with a magnetic field

The kinetic Green's function method is used to obtain equations which describe the transport of resonance radiation in magnetic fields for arbitrary ratios between the natural widthγ, the Doppler width Δω _D, and the Zeeman splitting of the excited atomic levels. It is shown that as ΔωD/γ→0 the equations become very much simpler and in particular cases admit an exact solution. In particular, the decay of coherence in an infinite homogeneous space is characterized by five relaxation times, which are defined by a system of algebraic equations.     

Multiple-quantum vector field imaging by magnetic resonance

We introduce a method for non-invasively mapping fiber orientation in materials and biological tissues using intermolecular multiple-quantum coherences. The nuclear magnetic dipole field of water molecules is configured by a CRAZED sequence to encode spatial distributions of material heterogeneities. At any given point r in space, we obtain the spherical coordinates of fiber orientation (theta,phi) with respect to the external field by comparing three signals ||G(X)||, ||(Y)||, and ||G(Z)|| (modulus), acquired with linear gradients applied along the X, Y, and Z axes, respectively. For homogeneous isotropic materials, a subtraction ||G(Z)|| - ||G(X)|| - ||G(Y)|| gives zero. With anisotropic materials, we find an empirical relationship relating ||G(Z)|| - ||G(X)|| - ||G(Y)||/(||G(X)|| + ||G(Y)|| + ||G(Z)||) to the polar angle theta, while ||G(X|| - ||G(Y)||/(||G(X)|| + ||G(Y)|| + ||G(Z)||) is related to the azimuthal angle phi. Experiments in structured media confirm the structural sensitivity. This technique can probe length scales not accessible by conventional MRI and diffusion tensor imaging.     

Quantum diffusion of light interstitials

The diffusion rate of light interstitials is calculated avoiding the usual adiabatic and Condon approximations for the phonons. The method employed is a generalization of the standard small polaron theory taking explicitely account of the strongly coupled interstitial-host vibrations.     

Ballistic diffusion induced by a thermal broadband noise

We present a thermal broadband noise from the difference between two Ornstein-Uhlenbeck noises, which can induce a ballistic diffusion, i.e., long-time mean square displacement of a free particle driven by this noise reads proportional to t(2). We apply this noise to a flashing ratchet and the mean velocity of the particle is calculated via Langevin simulation. The results show that a double peak of the mean velocity and flux reversal appears for the ratchet with large and small asymmetries, respectively; the inertia effect induces a large mean velocity and multireversal of flux. These rich and interesting phenomena are explained.     

Silica luminescence induced by fast light ions

Silica luminescence induced by hydrogen and helium ions of 200 keV–2.4 MeV was experimentally studied in the near ultraviolet and visible wavelength ranges. The luminescence spectrum consists of three asymmetric wide bands with maxima at wavelengths of 282, 456 and 644–648 nm. For nonirradiated silica, it was shown that the spectral shape depends on observation angle, ion species and energy. The changes in the spectra and relations between light intensity of luminescence and both absorption dose and observation angle were determined. Differences in ionoluminescence spectra measured for molecular and atomic hydrogen ions were observed, i.e. the so-called “molecular effect” was found. The influence of different beam parameters (incidence angle, ion species and energy) on indicatrix at a wavelength of 456 nm was found for nonirradiated silica. The applications of ionoluminescence measurements for distant monitoring of silica irradiation processes are discussed.     

Drift of gases induced by nonmonochromatic light

The phenomenon of light-induced drift of gases influenced by a nonmonochromatic light with different spectral properties is under consideration. The dependence of the drift velocity on the spectrum shape, bandwidth, detunings and light intensity is studied. The largest drift velocity is found to be obtained at the bandwidth of the spectrum being of the order of the Doppler atomic transition width, and when the wings of the spectrum drop rapidly. It is shown that nonmonochromatic light of the same intensity as a monochromatic one allows to reach a much larger drift velocity. Principal possibilities to obtain supersonic light-induced drift velocities are shown.     

Photoelectron spectra induced by broad-band chaotic light

We consider a model for laser-induced autoionisation in which the laser light is treated as a chaotic white noise. We solve a set of coupled stochastic integro-differential equations, which are based on the Fano model for autoionisation. For the case of symmetric Fano profile we determine the exact photoelectron spectrum and compare it with the results of previous models including different (diagonal and off-diagonal) relaxation mechanisms, and also spontaneous emission. It is interesting to note that equations of our model are of great similarity to those in the model with radiation damping.     

Local field corrections for light absorption by fullerenes

Semi-empirical atom-atom potential energy calculations based on pairwise additive interactions are performed and, after applying the Born-Oppenheimer approximation to separate high frequency vibrational modes from low frequency orientational and translational modes, the infrared vibrational spectra of CO₂ and N₂O monomers trapped in an argon matrix at a temperature of 5 K are determined. It is shown that only a double substitutional site in argon can accommodate N₂O, whereas CO₂ is trapped in two distinct sites, of single and double substitutional types. The model shows that splitting of the degenerate mode occurs for both molecules in the double site. In the ground electronic state, the vibrational frequency shifts due to the matrix and the vibrational transition moments for low-lying levels are determined using the contact transformation method, as used for gas phase calculations. Calculated energy levels compare well with observed ones and the theory also predicts some unobserved levels. Moreover, calculations show no significant changes in the dipole moments of both CO₂ and N₂O trapped molecules. Received 22 March 2000 and Received in final form 10 May 2000