Influence of Spin-Orbit Couplings to Nuclear Spin-Lattice Relaxation Rates in Sr-Doped La2CuO4

The electronic structure of Sr-doped La₂CuO₄ has been investigated by means of cluster calculations. The results indicate changes in the energies of the molecular orbitals when a copper atom is in the neighborhood of a Sr; in particular, the spin-orbit coupling is altered. It is shown that the Cu nuclear spin-lattice relaxation time depends crucially on this coupling when measured along the direction perpendicular to the CuO₂ planes. It is argued that the results presented here are compatible with recent experimental observations.     

NMR Evidence for Spatial Modulations in the Cuprates

Nuclear magnetic resonance (NMR) data on Cu, apical and planar O in La_1.85Sr_0.15CuO₄ are presented. Spin echo double resonance shows that the large Cu magnetic shift distribution is of short-length scale. Analysis of the O data reveals static modulations of the spin susceptibility with a spin–spin correlation function near zero. The Cu shift distribution is found to be of orbital origin. The full planar oxygen spectra show a correlated modulation of the electric field gradient with the spin susceptibility. Similar results on other cuprates indicate universality of these phenomena.     

核磁共振功能成像技术

本文介绍了可直接窥视大脑内部活动过程的核磁共振功能成像技术的基本原理、技术要点、临床应用举例及其目前尚存在的问题。     

NMR study of the slow dynamics and local structure in lyotropic micellar liquid crystals

The first experimental evidence of director fluctuations in a micellar lyotropic liquid crystal, studied by ¹H spin-lattice relaxation rate, is reported. The system is constituted by the ternary mixture: potassium laurate/1-decanol/water. With the aim to test the slow dynamics, the experiment has been performed over a broad range of Larmor frequencies (2 × 10³− 6.6 × 10⁶ Hz), using fast field-cycling NMR relaxometry. The results evidence that in the nematic and isotropic mesophases and the poliphasyc region, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range (<10⁵ Hz). By increasing the intermicellar water content, a crossover was found between a quasi-isotropic 3D director fluctuations behavior to a 2D pseudo-lamellar undulation fluctuations. In order to explain the higher frequency range (>10⁵ Hz), two relaxation mechanisms are assigned: i) molecular reorientation by translational diffusion on the micellar surface, and ii) molecular exchange between the micelle and the bulk. Received: 30 March 2000     

Structural nature of Li and B sites in the nonlinear optical material LiB3O5 using static NMR and MAS NMR

The structural nature of the nonlinear optical properties of LiB₃O₅ is analyzed using single-crystal nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR. The 3-coordinated trigonal [B(1) and B(2)] and 4-coordinated tetragonal [B(3)] sites are distinguished using the spectrum and the spin-lattice relaxation time in rotating frame T_1ρ, which was obtained from the ¹¹B MAS NMR. Moreover, the T₁ and T_1ρ values for ⁷Li and ¹¹B are compared, and the activation energies were obtained. The T_1ρ values of the boron nuclei in LiB₃O₅ show no significant changes. These results may be closely related to the largest second-order nonlinear optical coefficient.     

Charging Effects on Nuclear Spin Relaxation in Quantum Dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with 2D gas. We show that the nuclear spin relaxation rate T ₁ ^–1 is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin–orbit (SO) coupling the relaxation rate is maximal in the CB valleys whereas for the weak SO coupling the maximum of 1/T ₁ is near the CB peaks. The physical mechanism of nuclear spin relaxation rate at strong SO coupling is identified as Debye–Mandelstam–Leontovich–Pollak–Geballe relaxational mechanism.     

柠檬酸修饰的磁性Fe_3O_4纳米颗粒制备及其弛豫性能

采用共沉淀法制备了柠檬酸修饰的磁性Fe3O4纳米颗粒。通过改变反应过程中柠檬酸的量,研究柠檬酸加入量对纳米颗粒粒径的影响。通过透射电子显微镜、红外、热重、震动样品磁强计等表征手段对制备的磁性纳米颗粒进行表征。结果显示,随着柠檬酸加入量的增加,纳米颗粒的粒径减小,单位质量的颗粒表面包覆的柠檬酸量增大,比饱和磁化强度降低。最后研究了它们的磁共振弛豫性能,发现随着纳米颗粒粒径的逐渐减小,纵向弛豫率和横向弛豫率都显著减小,表明柠檬酸小分子可以成为磁性纳米颗粒造影剂合成有效的调控分子。     

Pinning Energies and Relaxation Rates in Laser Ablated Al-Doped YBCO Superconducting Thin Films

Systematic magnetization measurements were performed on laser ablated Al-doped YBCO thin films in the temperature range from 4 to 80 K and fields up to 12 T. The magnetization as a function of logarithmic time in the temperature range from 10 to 80 K generally showed a linear behavior. This type of performance allows us to determine the temperature and field dependence of the apparent pinning potential on the basis of the Kim–Anderson model for thermal activation of flux motion. The relaxation rate values are calculated from the time-dependent magnetization curves. The relaxation rate dependence on temperature, magnetic field, and doping concentrations has been investigated for different Al concentrations and applied fields up to 5 T. A strong nonlinearity was observed for the relaxation rate dependence on both temperature and applied fields. The apparent pinning potential values, U _o, are estimated from the dependence of relaxation rates temperature curves using U _o=k _B T/S expression. The dependence of U _o on both temperature and fields is elaborated and discussed in detail. The pinning potential, U _o, increases with temperature up to 40 K, however, above 40 K, a rapid decrease of U _o has been observed. Besides, a noticeable decrease in U _o as the applied field increases up to 0.1 T for temperatures below 40 K, and it is almost field independent for temperatures exceeding 40 K.     

Spin Relaxation in a Two-Dimensional Electron Gas in Si Quantum Wells

Electron spin resonance of two-dimensional (2D) electron gas in Si/SiGe quantum wells allows to evaluate both the longitudinal and the dephasing spin relaxation time. Diakonov–Perel (DP) relaxation, caused by Bychkov–Rashba (BR) spin orbit coupling, occurs to be the dominant mechanism in high mobility samples. For low mobility the Elliott–Yaffet mechanism dominates the longitudinal spin relaxation. When the BR effect is small, inhomogeneous broadening caused by potential fluctuations is seen. We compare spin relaxation of the 2D electron gas in Si and in GaAs quantum wells with respect to applications of these materials in spintronics.     

Intrinsic Spin Relaxation Processes in Metallic Magnetic Multilayers

Spin relaxation processes in metallic magnetic nanostructures are reviewed. First a brief review of the phenomenology of magnetic damping is presented using the Landau Lifshitz Gilbert (LLG) equations of motion. It is shown that the Gilbert damping in bulk metallic layers is caused by the spin orbit interaction and itinerant character of 3d and 4s-p electrons. Spin dynamics in magnetic nanostructures acquires an additional nonlocal damping. This means that a part of the magnetic damping is not given by the local Gilbert damping but arises from the proximity to other layers. Spin pumping and spin sink concepts will be introduced and used to describe the interface nonlocal Gilbert damping in magnetic multilayers. The modified LLG equation of motion in magnetic multilayers will be introduced and tested against the ferromagnetic resonance (FMR) data around the accidental crossover of FMR fields. The spin pumping theory will be compared to the early theories introduced in the 1970s for the interpretation of transmission electron spin resonance (TESR) measurements across ferromagnet/normal metal sandwiches.     

真空冷却豆制品水分布状态的核磁共振分析

利用低场核磁共振及其成像技术,研究真空冷却豆制品中水分分布状态.实验测得熟食豆制品真空冷却后其中的结合水、吸附水和自由水三种状态水的迁移过程.反映结合水的T21处于相对稳定的状态,经真空冷却后,发生变化的主要是T22(吸附水)和T23(自由水),吸附水和自由水明显减少;经真空冷却后,样品水分含量减少,体现在核磁共振成像图上,就是质子的流动性降低,使弛豫信号减弱,图像变暗.     

低场核磁共振技术在水泥基材料研究中的应用及展望

阐述了低场核磁共振技术在水泥基材料研究中的应用现状,认为现有的研究主要集中于水泥水化进程和水在硬化浆体中的扩散特征,也包括对硬化水泥浆体孔结构和比表面积的测试。分析了低场核磁共振技术在实际应用中面临的挑战,展望了该技术在新拌水泥浆体结构性能研究中的应用前景。     

Hydrophilic and hydrophobic interactions in cross-linked chitosan membranes

Chitosan membranes with different cross-linking density were prepared by modifying cross-linking time. Sodium tripolyphosphate was the cross-linking agent. A pulsed nuclear magnetic resonance study was performed on uncross-linked and cross-linked membranes. Different fraction of water molecules were identified in different zones within the membranes. The ratio of water molecules per chitosan repeating unit were calculated. A maximum of twelve water molecules were tightly coordinated to the chitosan repeating unit. Also, a very small water molecule fraction was identified but it was mobile enough as not to contribute to the dipolar interactions. The cross-linking reaction could lead to the formation of hydrophilic and hydrophobic interactions. These two types of interactions could result in the coexistence of a network formed by hydrophilic and hydrophobic micropores. This knowledge could be useful for the interpretation of results of hydrophobic drugs permeation across hydrophilic membranes. For example, the increment of estradiol fluxes across chitosan membranes with an increase in cross-linking density.     

Nb NMR and Fe EPR study of local magnetic properties of magnetoelectric Pb(Fe1/2Nb1/2)O3

The ⁹³Nb NMR and Fe³⁺ EPR spectra of a magnetoelectric Pb(Fe_1/2Nb_1/2)O₃ (PFN) single crystal as well as of PFN ceramics and nanopowder have been measured. The relation between the chemical disorder and the local magnetic structure has been determined. The data show the occurrence of an antiferromagnetic (AFM) phase transition at 145 and 135 K in both single crystal and ceramic samples, respectively. The AFM phase transition is however essentially smeared out in the nanopowder. Two different Nb sites exist which have different local magnetic fields which are essentialy distributed in value. These two Nb sites correspond to different types of “chemical” order: Fe rich, Nb poor and Fe poor, Nb rich regions. The temperature dependences of the sublattice magnetizations and the values of the superhyperfine fields at the Nb ions are obtained. NMR data suggest that a spin-glass like state of PFN below T = 20 K arises from the Fe poor, Nb rich regions.     

Comprehensive structural studies of ultra-fine nanocrystalline calcium hydroxyapatite using MAS NMR and FT-IR spectroscopic methods

Three nanoapatites produced using a microwave solvothermal synthesis (MSS), including one with a mean crystal size of only 6 nm, were characterized by high-resolution solid-state NMR (¹H and ³¹P) and FT-IR spectroscopies. It was found that nanoapatite particles had an inner crystal core covered by a non-apatitic surface hydrated layer with an outermost liquid-like zone. Various physicochemical properties of those apatite compartments were determined. Important structural details, such as deficiency and disorder of structural OH⁻ ions were discovered and discussed. The materials were also compared with respect to the reaction time and thermal post-synthesis treatment in order to optimize their preparation conditions to the required performance. The study emphasizes the influence of the surface hydrated layer on the physicochemical properties and biological activity of nanoapatites.     

核磁共振测井微弱信号DPSD提取算法研究

针对核磁共振测井中采集到的核磁回波信号十分微弱的特点,提出一种微弱信号的提取处理算法(DPSD,相敏检波算法),详细分析了DPSD算法的特点以及数字DPSD算法的实现方法,并对DPSD算法的频谱进行详细分析,提出一种通过FPGA并行执行DPSD算法提高运算速度、增强数据运算时实型的设计。目前已经成功实现该数字DPSD并行回波数据处理算法,并在实际测井过程中得到了很好的应用效果。     

(Na4BH4) guests inside aluminosilicate, gallosilicate and aluminogermanate sodalite host frameworks studied by H, B, and Na MAS NMR spectroscopy

We report tetrahydroborate aluminosilicate, gallosilicate and aluminogermanate sodalites studied by ¹¹B, ¹H and ²³Na MAS NMR spectroscopy. The spectral parameters are consistent with the local environments of each investigated nucleus obtained from the crystal structures. The ¹¹B MAS NMR spectra exhibit a sharp narrow line at about −49.0 ppm, which is assigned to BH₄⁻ enclathrated into the sodalite framework matrix. The lineshape of the signal shows no quadrupolar interactions due to discreteness and high symmetry of the BH₄⁻ unit as well as possible fast dynamic site exchange of hydrogen atoms. The ²³Na MAS NMR signals also show a narrow Gaussian lineshape, which clearly indicates a single type of sodium coordination, and a centrosymmetrical charge distribution around the sodium atom. The ¹H MAS NMR spectra can clearly distinguish between hydrogen in BH₄⁻ anions (−0.6 ppm), H₃O₂⁻ anions (1.2 ppm) and H₂O molecules (5.0 ppm). The structural properties of BH₄⁻ intercalation into sodalite framework matrix help connect the microporous materials to hydride-containing A, X and Y type zeolites.