量子阱中电子自旋注入及弛豫的飞秒光谱研究

采用飞秒脉冲的饱和吸收光谱方法研究了GaAs/AlGaAs多量子阱中电子自旋的注入和 弛豫特性，测得电子自旋极化弛豫时间为80±10ps.说明了电子自旋 轨道耦合相互作用引 起局域磁场的随机化，是导致电子的自旋极化弛豫的主要机理. 关键词： 自旋电子学 半导体量子阱 飞秒激光光谱 自旋 轨道耦合     

瞬态自旋光栅系统的建设及其在自旋输运研究中的应用

简要介绍了瞬态光栅系统原理及光路的建设,包括瞬态光栅的产生与探测.采用了外差探测法（heterodyne detection）,大大提高了信噪比.利用瞬态自旋光栅系统,研究了(110)方向生长的本征GaAs/AlGaAs单量子阱中自旋输运特性,测得室温下电子自旋的扩散常数D_s=551 cm/s. 关键词： 瞬态自旋光栅 自旋扩散 自旋输运 自旋弛豫     

本征GaAs中电子自旋极化的能量演化研究

采用时间分辨圆偏振光抽运-探测光谱,研究9.6 K温度下本征GaAs中电子自旋相干弛豫动力学,发现反映电子自旋相干的吸收量子拍的振幅随光子能量的增加呈非单调性变化.考虑自旋极化依赖的带填充效应和带隙重整化效应,发展了圆偏振光抽运-探测光谱的理论模型.该模型表明量子拍的振幅依赖于所探测能级的电子初始自旋极化度,自旋探测灵敏度以及带填充因子,三者的乘积导致了量子拍振幅的非单调变化,与实验结果一致.给出了能级分裂的二能级系统中电子自旋极化度定义.发现在高能级上可以获得100%的初始电子自旋极化度. 关键词： 圆偏振光抽运-探测光谱 吸收量子拍 电子自旋极化度 GaAs     

本征GaAs中电子自旋极化对电子复合动力学的影响研究

采用时间分辨圆偏振光和线偏振光抽运-探测光谱,研究了9.6 K温度下本征GaAs中自旋极化电子与非极化电子的复合动力学及其随光子能量演化.发现自旋极化对电子复合动力学具有显著影响.仅在导带底附近测量时,两种方法测试到的复合寿命一致,而在高过超能量电子态测量时,两种方法测试到的复合寿命不一致.指出时间分辨法拉第光谱中,用于反演求解电子自旋相干寿命的电子复合寿命应该使用圆偏振光抽运-探测获得的复合寿命,而不是线偏振光抽运-探测获得的寿命.理论计算与实验结果吻合较好. 关键词： 圆偏振光抽运-探测光谱 自旋量子拍 自旋极化 GaAs     

InAs单层和亚单层结构中的自旋动力学研究

利用偏振时间分辨光谱和时间分辨Kerr旋转谱，研究了GaAs中的InAs单层和亚单层的电子自旋动力学.实验发现，在非共振激发条件下，厚度为1/3单层的InAs亚单层中电子自旋弛豫寿命长达3.4ns，而1个单层厚的InAs层的电子自旋寿命只有0.48ns；而在共振激发条件下，亚单层结构中的电子自旋寿命大大减少，只有70ps，单层InAs中电子自旋寿命没有显著变化.分析表明，低温下InAs单层和亚单层结构中，Bir-Aronov-Pikus(BAP)自旋弛豫机理占主导地位.通过改变材料结构特性和激发条件来改变电子空穴的空间相关性，从而达到控制自旋弛豫的目的. 关键词： InAs亚单层 自旋弛豫 BAP机理     

电子自旋偏振度及其弛豫过程的飞秒激光吸收光谱研究

基于二能级体系的速率方程，获得了非完全初始自旋偏振极化条件下的自旋偏振向上和向下 载流子布居弛豫的解析解. 基于小信号近似，给出了左、右旋圆偏振探测光的饱和吸收变化 的表达式. 此表达式中含有电子布居的初始自旋偏振度参数，因而用此表达式拟合实验数据 能够直接获取电子布居的初始自旋偏振度，而电子布居的初始自旋偏振度在自旋偏振输运研 究中是一个非常重要的关键参数. 实验获得了GaAs/AlGaAs多量子阱结构中光注入电子布居 的初始自旋偏振度及其弛豫时间常数. 关键词： 圆偏振抽运_探测技术 电子自旋偏振度 自旋偏振弛豫 GaAs量子阱     

InAs量子点中自旋-轨道相互作用下电子自旋弛豫的参量特征

本文在处理InAs单电子量子点哈密顿模型时,将自旋-轨道(SO)相互作用作为微扰项,计算在Fock-Darwin本征函数下SO相互作用的矩阵元,利用其对能级和波函数的二阶修正,并且考虑新的能级对g因子和有效质量m^*的影响,计算得到在声子协助下电子的自旋弛豫率Γ的表达式.给出了InAs量子点中声子协助的电子自旋弛豫率Γ对于限制势频率ω₀、温度T、纵向高度z_{0关键词： 自旋弛豫率 自旋-轨道(SO)相互作用 InAs量子点 Fock-Darwin本征函数}     

本征GaAs量子阱中电子自旋扩散输运的时-空分辨吸收光谱研究

发展了一种时-空分辨圆偏振光抽运-探测光谱及其理论,并用于本征GaAs量子阱中电子自旋扩散输运的实验研究.获得室温下本征GaAs量子阱中的“自旋双极扩散系数”为D_as=37.5±15 cm²/s.此结果比用自旋光栅法测量到的掺杂GaAs量子阱中电子自旋扩散系数小.解释为是由于“空穴库仑拖曳”效应减慢了电子自旋波包的扩散输运. 关键词： 时-空分辨抽运-探测光谱 电子自旋扩散 GaAs量子阱     

GaAs中电子g因子的温度和能量依赖性的飞秒激光吸收量子拍研究

采用时间分辨椭圆偏振光抽运-探测光谱研究磁场作用下本征GaAs中电子自旋弛豫动力学，观察到吸收量子拍现象.这种吸收量子拍起源于电子自旋的拉莫尔进动，因而其拍频成为高精度测量电子g因子的一种新方法.利用这种新方法研究了本征GaAs中电子g因子的温度和能量依赖特性，发现g因子随电子的温度和能量增加而增加，但与k·p理论预测相差甚大.基于实验结果拟合，我们给出了一个g因子的温度和能量依赖的经验公式. 关键词： 椭圆偏振光抽运-探测光谱 自旋量子拍 g因子 GaAs     

自旋电子学简介及其研究进展

自旋电子学主要研究电子自旋在固体物理中的作用,是一门结合磁学与微电子学的交叉学科,其研究对象包括电子的自旋极化、自旋相关散射、自旋弛豫以及与此相关的性质及其应用等.基于电子自旋的自旋电子器件能够大大提高信息处理速度和存储密度,而且具有非易失性、低能耗等优点.简单介绍了自旋电子学的概念及其研究内容,综述了自旋电子学目前的研究及应用进展.     

Saturation recovery electron spin–lattice relaxation studies on benzene anion radical and its derivatives: application of Kivelson–Orbach mechanism of electron spin relaxation

The role of low-lying excited states on the spin–lattice relaxation times (T₁) of organic radicals has been investigated. To test the applicability of Kivelson's electric field fluctuation model (D. Kivelson, J. Chem. Phys. 45, 1324 (1966)), based on the Orbach mechanism of spin relaxation, the T₁s of the anion radicals of benzene, benzene-1-d, toluene, ethyl benzene, isopropyl benzene, t-butyl benzene, p-xylene, 1,2,4-trimethyl benzene and 1,3,5-trimethyl benzene in liquid solutions, with potassium cation as the counter ion, have been measured by the pulse saturation recovery technique. The energy gap between the ground and the first excited electronic states changed with the substitutions to different extent. The spin–lattice relaxation rates showed correlation with this energy gap. Anion radicals of benzene and benzene-1-d showed the shortest T₁ among the radicals studied here. A small but measurable energy splitting due to the deuterium substitution in benzene-1-d radical was obtained from the temperature dependence of T₁. Spin–lattice relaxation times of benzene anion measured here decreased monotonically in the range of ?60 to ?125 °C, in contrast to some reported claims of very unusual temperature dependence, based on the continuous wave microwave power saturation studies. Our results also showed that the ion pairing between benzene anion and potassium cation did not significantly influence the spin–lattice relaxation times.     

Single-Pulse Echo and Its Secondary Signals in Two-Level Spin Systems

The dependences of the amplitudes of single- and two-pulse spin echoes and their secondary signals in NMR (protons of glycerin in an inhomogeneous magnetic field) in the exciting-pulse repetition period T _r have been compared. The difference in origin of the primary and secondary signals of a single-pulse echo in a two-level spin system has been confirmed. It is shown that only a primary single-pulse echo observed when T _r > T ₁ (T ₁ is the spin lattice relaxation time) results from single-pulse excitation. The secondary single-pulse echo signals are observed for T _r < T ₁ and are due to the multiphase formation mechanism. The results obtained for magnetically ordered substances are analyzed. Based on these data, it was inferred earlier that primary and secondary single-pulse echo signals were formed by one and the same multiphase mechanism.     

A μSR Study of Er Spin Dynamics in (Y1−x Er x )Ni2B2C Superconductors

Zero field μSR has been used to probe rare earth spin dynamics in the magnetic superconductors, Y_1−x Er _x Ni₂B₂C. The muon spin relaxation function is stretched exponential, exp (−(λt)^β), in form, as usually found for spin glass systems above the glass temperature. However, the Y_1−x Er _x Ni₂B₂C compounds show no evidence of coexisting superconducting and static spin glass ground states even at concentrations below the critical value (x=0.6) for long range antiferromagnetic order. The temperature dependence of both the muon spin relaxation rate λ and the exponent β suggests that Er spin dynamics change significantly at the superconducting transition temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

半导体量子阱中电子自旋弛豫和动量弛豫

根据电子自旋轨道耦合对自旋极化弛豫影响的DP机理进一步导出了半导体中电子自旋弛豫与动量弛豫及载流子浓度的关系，并采用飞秒抽运探测技术在室温下测量AlGaAs/GaAs 多量子阱中载流子浓度在 1×10¹⁷—1×10¹⁸cm^-3范围内，电子自旋弛豫时间由58ps增加至82 ps的变化情况，与理论计算值符合，说明了随着载流子浓度的增加，载流子间的频繁散射加速了电子动量驰豫，减弱了电子自旋轨道耦合作用，从而延长了电子自旋寿命. 关键词： 电子自旋轨道耦合 电子自旋弛豫和动量弛豫 飞秒光谱技术     

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T₁∼T₂ in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T₁ and spin–spin relaxation T₂ are longer for ¹⁵N- than for ¹⁴N-nitroxides. The dominant contributions to T₁ are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T₁ on nitrogen nuclear spin state m_I was observed for both ¹⁴N and ¹⁵N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.     

Nonexponential 1H spin–lattice relaxation and methyl group rotation in molecular solids

We report a quantitative measure of the nonexponential ¹H spin–lattice relaxation resulting from methyl group (CH₃) rotation in six polycrystalline van der Waals solids. We briefly review the subject in general to put the report in context. We then summarize several significant issues to consider when reporting ¹H or ¹⁹F spin–lattice relaxation measurements when the relaxation is resulting from the rotation of a CH₃ or CF₃ group in a molecular solid.     

Surface-enhanced nuclear spin conversion in CH3F

The nuclear spin conversion of a molecule is the modification of the total nuclear spin I of its equivalent atoms. This phenomenon is observed by measuring the relaxation rate of a gas sample initially prepared with a population of spin isomers far from the equilibrium given by nuclear spin statistics. New experimental data obtained at low pressure show a surface-induced enhancement of the nuclear spin conversion in ¹³CH₃F. Contrary to binary collisions in the gas phase, hitting the surface induces direct conversion. Several mechanisms are proposed.     

An investigation of the structural aspects of the tomato fruit by means of quantitative nuclear magnetic resonance imaging

In this study, magnetic resonance imaging (MRI) was applied to study the structural aspects of the tomato fruit. The main study was performed on tomatoes (cv. Tradiro) using a 0.2-T electromagnet scanner. Spin-echo images were acquired to visualize the tomato macrostructure. The air bubble content in tissues was evaluated by exploiting susceptibility effects using multiple gradient echo images. The microstructure was further studied by measuring spin–spin (T₂) and spin–lattice (T₁) relaxation time distributions. Nuclear magnetic resonance relaxometry, macro vision imaging and chemical analysis were used as complementary and independent experimental methods in order to emphasize the MRI results. MRI images showed that the air bubble content varied between tissues. The presence of gas was attested by macro vision images. Quantitative imaging showed that T₂ and T₁ maps obtained by MRI reflected the structural differences between tomato tissues and made it possible to distinguish between them. The results indicated that cell size and chemical composition contribute to the relaxation mechanism.     

Control of Charge Transfer Phase Transition in Iron Mixed-Valence System (n-C n H2n+1)4N[FeIIFeIII(dto)3] (n=3–6; dto = C2O2S2)

The spin flop behaviour of low anisotropy Mn ions within the mixed halide (⁵⁴Mn)Mn(Br _x Cl_1−x )₂·4H₂O is shown to be interpolative with that of the two terminal compounds, in striking contrast with the dynamics of nuclear spin lattice relaxation.