未掺杂反式聚乙炔(CH)x中核自旋晶格弛豫-孤子-维扩散模型

Nechtschein等人报道并分析了反式聚乙炔中质子自旋晶格弛豫时间对拉摩频率ω和温度T的依赖关系。观察到了质子自旋晶格弛豫速率T₁^-1和ω^-1/2的正比关系。但是在高频段,T₁^-1∝ω^-1/2关系发生偏离,且温度越低,发生偏离的频率也越低。 本文用另一种方法对这些实验结果作了分析。首先,论证了孤子一维扩散模型的合理性。排除了质子弛豫速率∝ω^-1/2的另一种解释,即仅仅是核自旋向着静止的顺磁中心扩散。孤子能处在运动状态或静止状态。当温度降低时,发生两个效应,即越来越少的孤子处于运动状态,且运动孤子的扩散系数减小。只有扩散的孤子对所观察到的质子弛豫有贡献,而固定孤子的贡献可以忽略。其次,描述了运动孤子的一维随机行走模型,计算了它的相关函数和谱密度函数。质子自旋晶格弛豫速率是: 其中C是运动孤子的浓度,τ是运动孤子沿链跳跃时,渡越相邻位置的跳跃时间,ω是质子的拉摩频率。 这个公式揭示了质子弛豫速率的频率和温度依赖关系的主要特征。它和Nechtschein的测量结果拟合得很好。从拟合中可以得到各个温度下运动孤子的跳跃时间和相对浓度。     

Cr2O3中反铁磁自旋波的低频拉曼光谱研究(英文)

反铁磁自旋波在高速和低能耗信息处理方面具有很大潜力。然而，在反铁磁体系中激发和检测太赫兹自旋波是具有挑战性的。在本工作中，我们验证了低频拉曼光谱可作为探测反铁磁体系中自旋波的有力工具。我们通过拉曼光谱系统研究了典型的单轴反铁磁体Cr₂O₃中的反铁磁自旋波，我们的测量范围低至2.3 cm^-1(69 GHz)。我们分析了自旋波的塞曼劈裂和自旋翻转相变。我们进一步通过偏振拉曼的方式确定了自旋波能支的角动量符号。我们还得到了Cr₂O₃的各向异性能，g因子和自旋翻转场随温度和磁场变化的函数关系。自旋波重整化理论解释了所有实验观测结果。     

基于磁动力学方程的电流感应磁化翻转效应的宏观模型

提出了一个基于磁动力学方程的宏观唯象理论模型，对纳米级赝自旋阀结构的电流感应磁化翻转效应给出了明晰的物理解释：流入自由层的净自旋流和自由层内的自旋弛豫过程的共同作用，导致自由层总磁矩随时间的改变，甚至产生磁化方向的翻转.模型将“铁磁/非铁磁”界面的自旋相关散射，以及铁磁层中的自旋积累和弛豫过程，统一于宏观的磁动力学方程中.通过求解该方程的解析解，给出了赝自旋阀在电流激励下的磁化翻转条件和临界电流密度的表达式.对该效应的定性解释和数值模拟结果都和实验报道良好符合.根据模型分析了影响临界电流密度的诸因素，并指出提高器件性能的途径. 关键词： 电流感应磁化翻转 磁动力学方程 自旋电子学     

一维自旋轨道模型中自旋-轨道能隙产生机理分析

讨论考虑洪特耦合的两带赫伯德模型得到的一维自旋轨道模型中自旋-轨道能隙的产生.运用SU(4)赝费米子表象下的平均场理论,计算求得价键序参数、准粒子激发谱能隙和自旋、轨道密度-密度关联函数随系统耦合参数变化的结果.随洪特耦合相互作用由零开始增强,系统激发谱能隙逐渐打开,并且系统在参数取值为J₁/J₂=1/3处由具有阻错的无序状态相变到自旋铁磁有序和轨道反铁磁有序的状态.     

Er3+-Yb3+共掺碲酸盐玻璃上转换绿光激发过程的研究

根据Er³⁺-Yb³⁺共掺碲酸盐玻璃绿光上转换发光的能级结构和相关跃迁建立的速率方程模型,分析了Er³⁺的⁴S_3/2能级上升特性与⁴I_11/2能级和Yb³⁺的²F_5/2能级的关系.通过模型对实验观测的上升和衰减曲线的拟合,确定速率方程模型的相关参数,计算⁴S_3/2能级粒子数布居趋于稳定时能量传递粒子数与激发态吸收粒子数之比.进一步分析了Er³⁺-Yb³⁺共掺绿光上转换发光的动力学过程. 关键词： 速率方程模型 上转换 3+-Yb³⁺')" href="#">Er³⁺-Yb³⁺ 方波激发     

第一性原理研究Mg，Si和Mn共掺GaN

采用基于密度泛函理论(DFT)的第一性原理平面波赝势法(PWP)计算Mg,Si和Mn共掺GaN电子结构和光学性质,分析比较计算结果.计算表明：掺杂后体系均在能隙深处产生自旋极化杂质带,具有半金属性,能产生自旋注入.与Mn掺杂GaN比较,Mg共掺后能使居里温度(T_C)升高,并在1.0eV出现源于Mn⁴⁺离子基态⁴T₁(F)到⁴T_{2关键词： Mg Si和Mn共掺GaN 电子结构 TC)')" href="#">居里温度(TC) 光学性质}     

ZnS:Cr2+中局域晶格结构和自旋单态对零场分裂参量的贡献

在统一配体场耦合图像的基础上,构造了d⁴电子组态过渡金属离子在强场图像下包括所有自旋状态的210×210维完全能量矩阵.通过对角化完全能量矩阵,研究了Cr²⁺掺杂ZnS的局域晶格结构和Jahn-Teller能.理论计算结果与实验值符合非常好.同时,还研究了Cr²⁺掺杂ZnS后体系自旋单态对零场分裂参量的贡献.结果表明:自旋单态对二阶零场分裂参量D的贡献可以忽略,但是对于四阶零场分裂参量a和F的贡献却 关键词： 2+')" href="#">ZnS:Cr²⁺ 统一配体场耦合 自旋单态 Jahn-Teller能     

Al2O3吸附[Cu(H2O)6]2+基团的自旋哈密顿参量和四角畸变

采用微扰方法和对角化完全能量矩阵法计算了Al₂O₃粉末吸附的四角对称[Cu(H₂O)₆]²⁺基团的自旋哈密顿参量（g因子g∥,g_⊥和超精细结构常数A_∥和A_⊥）. 计算结果表明用这两种理论方法计算的自旋哈密顿参量很接近,并且都与实验结果比较一致. 表明这2种方法都可用于晶体中3d9离子基团的自旋哈密顿参量的研究,通过计算,我们还获得了[Cu(H₂O)₆]²⁺基团四角畸变的大小,并对结果进行了讨论.     

测定温度因子的共振X射线动力学衍射法

提出了一种测定双原子晶体中不同原子温度因子的新方法 .利用原子吸收限附近完整晶体共振X射线动力学衍射的特征，得到了关于温度因子的两个方程.方程中的参数可以由实验和理论计算得到，求解这两个方程便可分别求出两种原子的温度因子.将该方法应用于GaAs完整晶体，300K时，B_Ga=0.4573×10^-20m²，B_As=0.7339×10^-20m².该方法可以推广到多原子化合物晶体情形 关键词：     

激光等离子体辐射温度时间特性测量研究

本文提出一种软X射线条纹相机与吸收膜相耦合测量激光等离子体辐射温度的实验方法,并且应用这个方法测量了平面金靶在1.053μm激光(LF-11^#激光装置)作用下,靶面功率密度约为10¹⁴W/cm²的辐射温度。对温度处理中存在的误差,以及把此方法推广到能谱的时间分辨和温度的空间分辨测量中的可能性进行了讨论。 关键词：     

The relevance of theA 2-term in the Dicke maser hamiltonian

We study the multi-mode Dicke maser model, and analyze the conditions under which theA ²-term may be dropped from the hamiltonian. As far as the dynamics is concerned, theA ²-term may be dropped at the very beginning. As far as thermodynamics is concerned, there is mostly no need to keep theA ²-term. An exception is the case where one of the two states involved in the two-level atom approximation, is the atomic ground state. In general, however, dropping theA ²-term is an irrelevant modification. This result is obtained by partly diagonalizing the hamiltonian and applying the results of Hepp and Lieb.     

Quantum tunneling with dissipation and the Ising model over ℝ

We consider a quantum particle in a double-well potential, for simplicity in the two-level approximation, coupled to a phonon field. We show that static and dynamical ground state correlations of the particle and of the field are expressible through expectations in an Ising model over (rather than ). Its free measure is a spin flip process with flip rate , the difference in energy between the ground state and the first excited state. The Ising model has a ferromagnetic pair interaction whose form depends on the couplings to the phonon field and on the dispersion relation of the phonon field. In physical applications the interaction is long ranged and decays ast ^–2 for large distances. In this case we prove that for sufficiently strong coupling the particle becomes localized in one of the wells. The effective tunnel rate is zero. The transition to localization is associated with the generation of an infinite number of low momentum phonons. We apply the Ising technology to our problem and discuss the phase diagram in some detail.     

Optical properties of citrate-stabilized CdS nanoparticles

Citrate-stabilized CdS nanoparticles of size 4 nm are obtained by varying the sulfide:citrate ion concentration in a simple aqueous synthesis method. The optical absorption and photoluminescence properties of the nanoparticles are studied. The size of the crystallites is found to be less affected by sulfide:citrate ratio. At lower concentrations of S²⁻, trap state emission is favoured and at higher concentrations excitonic transition is predominant as shown by optical absorption and photoluminescence spectra. Effective surface capping and optimum concentration of S²⁻ leads to the quenching of surface-defect-related emission. Increase in citrate ion concentration is found to increase the intensity of photoluminescence band arising from trap state emission revealing the role of sulfide:citrate ratio on surface modification of CdS nanocrystals. The nanoparticles are hexagonal as shown by the X-ray diffraction and selected area electron diffraction pattern.     

Λααα cluster model for the Λ 13 C nucleus

The _Λ ¹³ C hypernucleus is treated as a (1/2)⁺ bound state of the Λααα system. The s-wave model is used on the basis of differential equations for the corresponding Yakubovsky components. No account is taken of 2+2 clustering in the system. Phenomenological potentials are used to simulate the αα and αΛ interactions. The system as a whole is bound owing to the additional potential of three-body interaction between the alpha-particle clusters. The differential equations for the Yakubovsky components are solved numerically by the cluster-reduction method. The binding energies are calculated for the ground and the first excited state of the _Λ ¹³ C hypernucleus. It is shown that the dominant type of clustering in the system is (Λαα)α.     

Kramers-Krönig relations in nonlinear optics

We review dispersion relations, which relate the real part of the optical susceptibility (refraction) to the imaginary part (absorption). We derive and discuss these relations as applied to nonlinear optical systems. It is shown that in the nonlinear case, for self-action effects the correct form for such dispersion relations is nondegenerate, i.e. it is necessary to use multiple frequency arguments. Nonlinear dispersion relations have been shown to be very useful as they usually only require integration over a limited frequency range (corresponding to frequencies at which the absorption changes), unlike the conventional linear Kramers-Krönig relation which requires integration over all absorbing frequencies. Furthermore, calculation of refractive index changes using dispersion relations is easier than a direct calculation of the susceptibility, as transition rates (which give absorption coefficients) are, in general, far easier to calculate than the expectation value of the optical polarization. Both resonant (generation of some excitation that is long lived compared with an optical period) and nonresonant instantaneous optical nonlinearities are discussed, and it is shown that the nonlinear dispersion relation has a common form and can be understood in terms of the linear Kramers-Krönig relation applied to a new system consisting of the material plus some perturbation. We present several examples of the form of this external perturbation, which can be viewed as the pump in a pump-probe experiment. We discuss the two-level saturated atom model and bandfilling in semiconductors among others for the resonant case. For the nonresonant case some recent work is included where the electronic nonlinear refractive coefficient,n ₂, is determined from the nonlinear absorption processes of two-photon absorption, Raman transitions and the a.c. Stark effect. We also review how the dispersion relations can be extended to give alternative forms for frequency summation which, for example, allows the real and imaginary parts of ⁽²⁾ to be related.     

Hartree theory for matter in a photon or phonon field

For a Dicke-type Hamiltonian describing M two-level systems coupled to N modes of a Boson field, the expectation value is minimized with respect to unrestricted products of 2^M-level states and field states. In the resulting Hartree ground state(s) ? ? Π, ? is ground state of a cubic Schrödinger equation in $\text{C}$^2M and Π is the product of N coherent one-mode states depending on ?. It is proved that ? factorizes as well, into M two-level states determined by a nonlinear equation in $\text{R}$^M; that for weak coupling the Hartree ground state is unique and independent of the parameters in H; and that for strong coupling there are consecutively 2,…,2^L (and possibly even more) Hartree ground states where 1≤L≤min{M, 2N} counts certain reflection symmetries. Details of this symmetry-breaking bifurcation (such as structural stability of relaxed two-level systems) and connections to the true ground state(s) of H are worked out.     

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm² excitation and recovers with a fast time constant 34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS ₁ and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S ₁) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.     

影响固体材料激光冷却若干因素的研究

采用一个简单的二能级系统来分析激光冷却的微观物理过程，从微观的离子数等方面讨论制冷功率，从而计算出温度的变化，同时讨论了影响制冷功率的因素，找到了提高制冷功率的途径，详细分析了掺杂离子浓度、抽运功率、有效吸收截面对冷却极限的影响.最后比较了计算结果与实验数据，二者基本一致，从而验证了采用该二能级系统理论分析反斯托克斯荧光制冷的合理性.     

Determination of the saturation parameter of electronic transition in a uranium-neon hollow-cathode discharge by optogalvanic spectroscopy

Summary The optogalvanic signal (OGS) induced in a uranium-neon hollow-cathode discharge was measured as a function of the laser power density for the 0→16900 cm⁻¹ (591.5 nm) uranium transition. Theoretical relations derived by solving a two-level system rate equations showed the OGS dependence on the laser photon flux, for a modulated c.w. light and for stimulated transitions starting from the ground state. A fitting of the theoretical relations to the experimental measurements allowed the determination of the σ₀ τ product, that is, the saturation parameter of the transition. The results showed good agreement between the σ₀ τ values obtained by the optogalvanic and the usual optical absorption processes.