13C- and 1H-NMR Relaxation Investigation of a Polyciclic Nitrogen Compound: 3,3-Dimethyl-1,5-diphenyl-9-hydroxy-bispidinium Iodide

The rigid polycyclic nitrogen compound was considered as a test for the reliability of internuclear distances calculated by ¹H-NMR spin-lattice relaxation rates. The ‘isotropic’ motional correlation time was calculated from ¹³C relaxation rates (τ_C = 0.11 ns at 298 K). Dipolar cross-relaxation rates were calculated by measuring non-, mono- and double-selective proton spin-lattice relaxation rates. All the experimental relaxation rates were thoroughly accounted for by dipolar pairwise interactions. Only at high temperatures a certain contribution from the spin rotational mechanism was apparent.     

与Ising链耦合的中心双量子比特系统的量子关联

通过对双量子比特系统分别独自与Ising链耦合情形下的关联问题的研究, 推导出了量子失协和量子关联几何度量的演化规律. 在弱耦合相互作用情况下Ising链的临界点附近, 量子关联存在突变. 此外本文发现在某段时间内的演化过程中几何量子关联度保持不变. 关键词： 量子关联 量子失协 量子关联几何度量     

Coherent cross-polarization theory for a spin-12 coupled to a general object

This work describes a segmented radial turbo-spin-echo technique (DW-rTSE) for high-resolution multislice diffusion-weighted imaging and quantitative ADC mapping. Diffusion-weighted images with an in-plane resolution of 700 microm and almost free of bulk motion can be obtained in vivo without cardiac gating. However, eddy currents and pulsatile brain motion cause severe artifacts when strong diffusion weighting is applied. This work explains in detail the artifacts in projection reconstruction (PR) imaging arising from eddy currents and describes an effective eddy current compensation based on the adjustment of gradient timing. Application of the diffusion gradients in all three orthogonal directions is possible without degradation of the images due to eddy current artifacts, allowing studies of the diffusional anisotropy. Finally, a self-navigation approach is proposed to reduce residual nonrigid body motion artifacts. Five healthy volunteers were examined to show the feasibility of this method.     

Particle injection into a chain: decoherence versus relaxation for Hermitian and non‐Hermitian dynamics

A model system for the injection of fermionic particles from filled source sites into an empty chain is investigated. The ensuing dynamics for Hermitian as well as for non‐Hermitian time evolution, where the particles cannot return to the bath sites (quantum ratchet), is studied. A non‐homogeneous hybridization between bath and chain sites permits transient currents in the chain. Non‐interacting particles show decoherence in the thermodynamic limit: the average particle number and the average current density in the chain become stationary for long times, whereas the single‐particle density matrix displays large fluctuations around its mean value. Using the numerical time‐dependent density‐matrix renormalization group (t‐DMRG) method it is demonstrated, on the other hand, that sizable density‐density interactions between the particles introduce relaxation which is by orders of magnitudes faster than the decoherence processes.     

Anomalous viscosity exponent in a discretized model for a chain diffusion in one dimension

We present a one-dimensional Monte Carlo simulation for the diffusion motion of a chain of N beads. We found that the scaling exponent for the viscosity can be smaller or greater than 3. This anomalous behavior cannot be attributed to the diffusivity scaling or the length fluctuations but is due to the chain dynamics details during diffusion in which the end beads play the key role. The viscosity exponent 3 and its expected relation with the diffusivity exponent are recovered in the asymptotic regime (N ↦∞). Received 24 September 2001 and Received in final form 28 January 2002     

Travelling Waves for a Density Dependent Diffusion Nagumo Equation over the Real Line

We consider the density dependent diffusion Nagumo equation, where the diffusion coefficient is a simple power function. This equation is used in modelling electrical pulse propagation in nerve axons and in population genetics (amongst other areas). In the present paper, the δ-expansion method is applied to a travelling wave reduction of the problem, so that we may obtain globally valid perturbation solutions (in the sense that the perturbation solutions are valid over the entire infinite domain, not just locally; hence the results are a generalization of the local solutions considered recently in the literature). The resulting boundary value problem is solved on the real line subject to conditions at z → ±∞. Whenever a perturbative method is applied, it is important to discuss the accuracy and convergence properties of the resulting perturbation expansions. We compare our results with those of two different numerical methods (designed for initial and boundary value problems, respectively) and deduce that the perturbation expansions agree with the numerical results after a reasonable number of iterations. Finally, we are able to discuss the influence of the wave speed c and the asymptotic concentration value α on the obtained solutions. Upon recasting the density dependent diffusion Nagumo equation as a two-dimensional dynamical system, we are also able to discuss the influence of the nonlinear density dependence (which is governed by a power-law parameter m) on oscillations of the travelling wave solutions.     

MSE-MRI sequence optimisation for measurement of bi- and tri-exponential T2 relaxation in a phantom and fruit

The transverse relaxation signal from vegetal cells can be described by multi-exponential behaviour, reflecting different water compartments. This multi-exponential relaxation is rarely measured by conventional MRI imaging protocols; mono-exponential relaxation times are measured instead, thus limiting information about of the microstructure and water status in vegetal cells. In this study, an optimised multiple spin echo (MSE) MRI sequence was evaluated for assessment of multi-exponential transverse relaxation in fruit tissues. The sequence was designed for the acquisition of a maximum of 512 echoes. Non-selective refocusing RF pulses were used in combination with balanced crusher gradients for elimination of spurious echoes. The study was performed on a bi-compartmental phantom with known T₂ values and on apple and tomato fruit. T₂ decays measured in the phantom and fruit were analysed using bi- and tri-exponential fits, respectively. The MRI results were compared with low field non-spatially resolved NMR measurements performed on the same samples.     

Microwave specific loss power of magnetic fluids subjected to a static magnetic field

This paper reports on the frequency dependence of the magnetic and electric power dissipation in a magnetic fluid sample, in the microwave frequency range (0.5 to 8GHz), at various values of the static magnetic field (0 to 167.8kA/m). The computation of the power dissipation relies on the experimental values measured for the complex dielectric permittivity, ɛ = ɛ′ - iɛ″, and the complex magnetic permeability, μ = μ′ - iμ″, over the same frequency range. The results show that the magnetic power dissipation is much larger than the electric one for the investigated sample. At a specific frequency, f (Hz) , the power dissipation, p, depends on the external magnetic field, and exhibits a maximum. The result obtained suggests the possibility of controlling the energy absorption in the microwave range by means of the application of an external magnetic field.     

A performance study of a logical gate using PPM optical pulse modulation for TDM systems

In this paper we propose the operation of an all-optical logical gate based in an acousto optical tunable filter (AOTF) operating with a pulse position modulation (PPM). The performance of an AOTF realizing two-input AND/OR logical functions, which can be applied in transmission and processing of signals in all-optical form in TDM systems, was examined. This novel integrated acousto optical logical gate operates with two ultrashort soliton light pulses (2 ps), which are modulated in agreement with the technique of pulse position modulation (PPM). Initially, we evaluated the effect resulting of an increment in the acousto optical interaction length, for the temporal position of the output pulse, considering the anomalous group velocity dispersion (GVD), nonlinear self phase modulation (SPM) and without loss propagation regime of TE and TM input pulses not modulated. We have analyzed the four possible situations for the two-input logical gate, modulating the TE and TM input pulses through temporal displacement and allowing a variation in the coding parameter offset (ε). We can conclude that is possible to get AND/OR logical operations for the modes TM or TE, without to insert PPM error, since a phase control (Δ?) exists (applied on the TM input pulse) in agreement with each situation in the truth table. Finally we defined the truth table, considering the adequate phase difference and coding parameter offset for the operation of the AND/OR logical gate based in the AOTF.     

Equations for the Magnetization Modes in a System Having a Quadruple Nucleus with the Spin S = 1

Within the framework of the method of a density matrix, differential equations are obtained that describe the evolution of the modes of magnetization <Î₊>, <Î_{+ z} >, and <Î_{+ z} ²> for a spin system containing a nucleus with a 1/2 spin connected scalarly with the quadruple nucleus of the spin S = 1 in the presence of crosscorrelation between the dipole ISinteractions, anisotropy of the chemical shift of the I and S nuclei, and quadruple interaction of the S nucleus. The equations are used for analysis of the evolution of the intensities of each line in the triplet of the Raman spectrum of the 1/2 spin. Expressions for the times of crossrelaxation and the times of transverse relaxation of each of the lines of the triplet are obtained.