三自旋体系长寿命核自旋单重态的制备

长寿命核自旋单重态的寿命（T_S）可以长于常规的纵向弛豫时间（T₁）,基于这个独特的优势,长寿命核自旋单重态具有较好的应用价值.本文对已有脉冲序列进行了改进,给出了适用于任意三自旋弱耦合体系长寿命核自旋单重态制备的参数计算方法,发现在同一个三自旋体系内核自旋单重态具有多种不同比例系数组合的形式,并以丙烯酸为例制备出了两种长寿命核自旋单重态,实验上观察到不同的核自旋单重态的寿命存在差异,另外我们还探究了温度对丙烯酸核自旋单重态寿命的影响.     

三种脉冲序列制备核自旋单重态效率的比较

核自旋单重态的制备和检测在化合物分子的相关分析中具有重要的应用价值和广阔的应用前景.本文以三肽分子AGG（Ala-Gly-Gly）中两个质子构成的一组孤立的自旋耦合体系为研究对象,利用三组基于不同原理的脉冲序列,分别对该体系制备了核自旋单重态;并分别测定和比较了三种方法制备核自旋单重态的效率及其寿命.研究结果表明,对于同一分子的同一自旋耦合体系中的质子,使用不同的脉冲序列制备得到的核自旋单重态的寿命并不会有明显差异,但基于不同脉冲制备核自旋单重态的效率差异相对较大.     

非均匀磁场和杂质磁场对自旋1系统量子关联的影响

通过负值度和测量诱导的扰动, 研究了非均匀磁场和杂质磁场对自旋为1的Heisenberg系统量子关联的影响. 研究发现非均匀磁场的增加会降低纠缠, 但也可用来产生纠缠, 并且会提高临界非线性作用K_c的值, 测量诱导的扰动的临界磁场要高于负值度的临界磁场, 而且测量诱导的扰动不会随着非线性作用|K| 的减小而消失, 它能全面反映量子关联的存在. 研究还发现, 不同杂质磁场对测量诱导的扰动的影响彼此间无交叉. 杂质磁场下, 相互作用|J| 必须小于非线性作用|K| 才会有纠缠存在, 但是测量诱导的扰动却可以在相互作用|J| 大于非线性作用|K| 时依然存在, |J| 与|K| 相同时只是测量诱导的扰动的最小取值点. 此外, 系统粒子数目对量子关联也具有重要影响.     

不同耦合构型多自旋体系单重态制备效率研究

核自旋单重态是一种特殊的量子态,其存在寿命能远长于纵向弛豫时间(T₁),能够被用于研究分子之间的慢扩散和慢运动等动力学过程．单重态的制备是其能成功应用的关键．目前文献中报道了多种制备单重态的方法,这些方法主要适用于孤立的两自旋体系,当单重态所涉及的核自旋受到其它自旋的耦合作用时,单重态的制备效率往往会降低．本文以三自旋体系为例,研究了不同耦合构型下单重态的制备效率受非单重态自旋耦合的影响,模拟结果表明当单重态自旋由弱耦合逐渐变为强耦合时,单重态的制备效率会在单重态自旋与非单重态自旋的耦合呈现对称性时保持一定的稳定性,此特性能够为复杂体系中选择合适的自旋制备单重态提供参考．我们利用N-乙酰-L-天门冬氨酸(NAA)分子的三个质子组成的自旋体系实验验证了以上结论,通过调节NAA分子的酸碱度,所选的三自旋体系能从弱耦合向强耦合转变,实验结果表明：当三自旋中单重态自旋处于强耦合时,单重态的制备效率明显高于弱耦合时的效率．     

基于纯化学位移的同核二维正交相敏J分解核磁共振波谱

二维J分解(2D J_RES)核磁共振(NMR)波谱是一种简单且用户友好的谱图表达形式,其将J偶合常数和化学位移信息分离到两个正交的频率维度上.自40年前首次被提出以来,2D J_RES技术在脉冲序列和方法的改进,以及实际应用的进展方面一直备受关注.本文回顾了新型2D J_RES脉冲序列,以及用于精确测量同核J偶合常数的2D J编辑谱的最近进展,特别是基于纯化学位移演化机制的正交相敏2D J_RES谱方法及其应用,并阐述其在克服强偶合效应和磁场不均匀性等方面的能力.     

三自旋体系核自旋单重态的制备与单重态二维谱的实现

核自旋单重态是一种特殊的自旋状态,其寿命远长于相应自旋的横向和纵向弛豫时间,能够被用于研究分子的慢扩散、慢运动、特征信号选择等过程.目前单重态的研究主要集中于孤立的两自旋体系.而本文以N-乙酰基天冬氨酸（NAA）分子中由亚甲基和次甲基的三个氢原子核构成的三自旋体系为研究对象,将亚甲基中的两个氢核制备成单重态.利用优化控制和数值计算方法,分别设计了包含和不包含次甲基氢核耦合的单重态制备脉冲,结果发现,不考虑次甲基氢耦合设计的优化脉冲,其在实际三自旋体系中的单重态制备效率会显著下降.另外,我们以单重态为起点,实现了针对次甲基和亚甲基的信号选择COSY谱和NOESY谱,结果表明基于单重态的二维谱能够有效避免谱峰重叠现象,提高谱图分辨率,并有助于提高分子结构解析的准确性.     

非马尔科夫环境中各向异性海森堡自旋链的量子失协

利用量子失协方法研究在非马尔科夫环境中具有时变磁场的两比特各向异性海森堡XYZ模型量子失协的动力学演化。海森堡XYZ系统的初始态为最大纠缠态 $\left|\psi_{A B}\right\rangle=(1 / \sqrt{2})(|11\rangle+|00\rangle)$ , 利用非马尔科夫量子态扩散方法解析求解非马尔科夫主方程, 得出系统的约化密度矩阵; 然后代入量子失协公式得出系统量子失协的演化动力学。讨论自旋耦合强度、环境关联系数γ和余弦磁场强度B对量子失协动力学的影响。研究发现: 当环境关联系数γ较小时, 系统的量子失协明显呈现上升趋势, 因此可以表明非马尔科夫环境具有增加系统量子失协的作用。同时较大的自旋耦合系数J和J_Z以及余弦磁场强度B也具有增加系统量子失协的作用。     

三角格点基底上磁性分形团簇形貌演化规律

在扩散限制凝聚模型基础上引入粒子的自旋自由度,将磁耦合系数扩展为随自旋间距离幂次变化的非常数项J/r^a,采用Monte Carlo方法研究在二维三角格点基底上具有幂次相互作用的磁性团簇形貌及其分形维数D_f的演化规律.模拟结果表明,对于较大的幂指数α值,即α≥5时,团簇形貌随耦合参数J的变化较小,其分形维数D_f在1.50~1.70之间;随着α值的减小,团簇形貌随参数J有一明显的演化过程,在模拟范围内,分形维数D_f在1.20~1.90之间.     

三维Ising模型的蒙特卡罗模拟

采用蒙特卡罗(Monte Carlo)重点抽样法对三维Ising模型进行计算机模拟,测量无外磁场时三维Ising模型中自旋键链的能量、磁化强度、比热及磁化率的统计平均值与标准误差(不确定度).结果表明,三维Ising模型在无外磁场时存在自发磁化现象,铁磁→非铁磁相变临界点在J/(k_BT_C)=0.222 0,或居里温度T_C=4.500 0处.并研究存在外磁场时上述物理量随温度与外磁场的变化规律,给出物理解释.     

用于指导仲氢诱导核极化状态保存的己烯分子中五自旋的单重态制备和寿命研究

仲氢诱导核极化（PHIP）技术能极大地增强核磁共振（NMR）信号的灵敏度,已被应用于磁共振成像、原位化学反应监测等领域．除了不断提高不同分子极化后的灵敏度外,延长和保存高极化度状态对PHIP技术的应用也至关重要,其中将极化后的状态制备成核自旋单重态是目前被研究较多的一种方法．本文以能被PHIP技术极化的己烯分子为研究对象,通过设计优化控制脉冲,对分子中的一个五自旋体系进行操控,制备了多种核自旋单重态,结果表明：己烯分子的碳-碳双键上存在三种不同的核自旋单重态,它们的寿命均长于仲氢极化后产生的初始态的寿命,可以作为延缓极化度衰减的一种中间态;通过对比单重态的寿命与相应自旋的纵向弛豫时间发现,将极化后己烯的状态转化为纵向磁化可能也是一种保存极化度的有效方法．     

Coherent storage of photoexcited triplet states using 29Si nuclear spins in silicon

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the m(s)=±1 sublevels differs significantly from that of the m(s)=0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ~100% electron-spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor (29)Si nuclear spin. We measure the coherence time of the (29)Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.     

<Emphasis Type="Italic">cis</Emphasis> Versus <Emphasis Type="Italic">trans</Emphasis>-Azobenzene: Precise Determination of NMR Parameters and Analysis of Long-Lived States of <Superscript>15</Superscript>N Spin Pairs

We provide a detailed evaluation of nuclear magnetic resonance (NMR) parameters of the cis- and trans-isomers of azobenzene (AB). For determining the NMR parameters, such as proton–proton and proton–nitrogen J-couplings and chemical shifts, we compared NMR spectra of three different isotopomers of AB: the doubly ¹⁵N labeled azobenzene, ¹⁵N,¹⁵N′-AB, and two partially deuterated AB isotopomers with a single ¹⁵N atom. For the total lineshape analysis of NMR spectra, we used the recently developed ANATOLIA software package. The determined NMR parameters allowed us to optimize experiments for investigating singlet long-lived spin states (LLSs) of ¹⁵N spin pairs and to measure LLS lifetimes in cis-AB and trans-AB. Magnetization-to-singlet-to-magnetization conversion has been performed using the SLIC and APSOC techniques, providing a degree of conversion up to 17 and 24% of the initial magnetization, respectively. Our approach is useful for optimizing the performance of experiments with singlet LLSs; such LLSs can be exploited for preserving spin hyperpolarization, for probing slow molecular dynamics, slow chemical processes and also slow transport processes.     

强脉冲磁场冲击处理对铝基复合材料塑性的影响机制

铝基复合材料在加入颗粒相之后, 延伸率和塑性变形能力明显降低. 为改善其塑性变形能力, 通过对比强脉冲磁场冲击处理前后试样内部组织和残余应力的变化特征, 研究了磁致塑性效应对铝基复合材料塑性变形能力的影响机理. 结果表明: 当磁感应强度从2 T变化到4 T时, 铝基复合材料中位错密度显著增加, 4 T时的位错密度是未加磁场时的3.1倍; 3 T, 30个脉冲处理后的复合材料中残余应力值从未加磁场时的41 MPa减小为-1 MPa. 从原子尺度来看, 强磁场导致了磁致塑性效应, 从而引起了位错的运动, 并促进了位错的退钉扎和可移动位错数量的增加; 从材料内部整体结构变化来看, 磁场加速了材料内应力的释放速率, 降低了材料内部的残余应力, 从而改善了铝基复合材料的塑性变形能力.     

Observation of intermolecular double-quantum coherence signal dips in nuclear magnetic resonance

The correlated spectroscopy revamped by asymmetric Z-gradient echo detection (CRAZED) sequence is modified to investigate intermolecular double-quantum coherence nuclear magnetic resonance signal dips in highly polarized spin systems. It is found that the occurrence of intermolecular double-quantum coherence signal dips is related to sample geometry, field inhomogeneity and dipolar correlation distance. If the field inhomogeneity is refocused, the signal dip occurs at a fixed position whenever the dipolar correlation distance approaches the sample dimension. However, the position is shifted when the field inhomogeneity exists. Experiments and simulations are performed to validate our theoretic analysis. These signal features may offer a unique way to investigate porous structures and may find applications in biomedicine and material science.     

Depolarization of the photoluminescence and spin relaxation in n-doped GaAs

Optically oriented electron spin lifetime in n-doped gallium arsenide was measured via depolarization of the photoluminescence (PL) in a transverse magnetic field (Hanle effect). In order to measure the PL polarization, a time-resolved pump-probe experiment, where a pump pulse generates spin-polarized electrons and a probe pulse monitors their polarization, was employed. The PL polarization in dependences of the pump-probe delay, external magnetic field as well as of the sample temperature was studied. The PL polarization was found to decay exponentially with the pump-probe delay, from which the spin lifetime of the electrons was measured. The measured value was found to depend on the strength of the magnetic field and sample temperature.     

Preparing high purity initial states for nuclear magnetic resonance quantum computing

Here we demonstrate how parahydrogen can be used to prepare a two-spin system in an almost pure state which is suitable for implementing nuclear magnetic resonance quantum computation. A 12 ns laser pulse is used to initiate a chemical reaction involving pure parahydrogen (the nuclear spin singlet of H2). The product, formed on the micros time scale, contains a hydrogen-derived two-spin system with an effective spin-state purity of 0.916. To achieve a comparable result by direct cooling would require an unmanageable (in the liquid state) temperature of 6.4 mK or an impractical magnetic field of 0.45 MT at room temperature. The resulting spin state has an entanglement of formation of 0.822 and cannot be described by local hidden variable models.