Some applications of polarized inelastic neutron scattering in magnetism

A brief account of applications of polarized inelastic neutron scattering in condensed matter research is given. We show that full polarization analysis is the only tool allowing to discriminate unambiguously between different magnetic modes in various magnetic materials. We show by means of recent results in the Heisenberg ferromagnet EuS that the effects of dipolar interactions can be studied on a microscopic scale. Moreover, we have found for the first time indications for the divergence of the longitudinal fluctuations belowT _c. In the itinerant antiferromagnet chromium we demonstrate that the dynamics of the longitudinal and transverse excitations are very different, resolving a long standing puzzle concerning the slope of their dispersion. Finally, we show that a measurement of the polarization-dependent part of the cross section of non-centrosymmetric MnSi proves directly that the chirality of the magnetic fluctuations is left-handed.     

Crystal structure and magnetic ordering of RNi Si compounds

The element distributions and the magnetic ordering behaviour of compounds RNi₁₀Si₂ (R = Tb, Dy, Ho, Er, Tm) have been studied by neutron powder diffraction down to temperatures of 1.6 K. The compounds crystallize in an ordered variant of the ThMn₁₂ structure type in the tetragonal space group P4/nmm. An ordered 1:1 distribution of Ni and Si on sites 4d and 4e, respectively, corresponds to a modulation vector [0, 0, 1] with respect to the space group I4/mmm of the ThMn₁₂ structure. TbNi₁₀Si₂ orders antiferromagnetically below T _N = 4.5 K with a magnetic propagation vector of [0, 0, 1/2]. The magnetic Tb moments, 8.97(2) /Tb atom at 1.6 K, are aligned along the c-axis. The Ni sites in TbNi₁₀Si₂ do not carry any ordered magnetic moments. The compounds with R = Dy, Ho, Er, and Tm are paramagnetic down to 1.6 K and 3.0 K, respectively. Received 10 July 2002 / Received in final form 12 September 2002 Published online 29 October 2002     

第一讲中子散射与散裂中子源

中子散射是研究物质微观结构和动态的理想工具之一,广泛地应用于凝聚态物质研究和应用的众多学科领域.散裂中子源能是新一代的加速器基脉冲中子源,能为中子散射提供高通量的脉冲中子.文章简明地介绍了中子散射的特点和它作为物质结构和动态探针的优越性,以及散裂中子源的基本原理、发展状况和多学科的应用优势.我国计划建设的散裂中子源CSNS中,靶站将由多片钨靶、铍/铁反射体和铁/重混凝土生物屏蔽体组成.质子束功率100kW下,脉冲中子通量约为2.4×1016n/cm2/s.第一期将设计建造高通量粉末衍射仪、高分辨粉末衍射仪、小角散射仪、多功能反射仪和直接几何非弹性散射仪等五台典型的中子散射谱仪,以覆盖大部分的中子散射研究领域.     

Carbide precipitates in solution-quenched PH13-8 Mo stainless steel: A small-angle neutron scattering investigation

This paper deals with the small-angle neutron scattering (SANS) investigation on solution-quenched PH13-8 Mo stainless steel. From the nature of the variation of the functionality of the profiles for varying specimen thickness and also from the transmission electron microscopy (TEM), it has been established that the small-angle scattering signal predominantly originates from the block-like metallic carbide precipitates in the specimen. The contribution due to double Bragg reflection is not significant in the present case. The single scattering profile has been extracted from the experimental profiles corresponding to different values of specimen thickness. In order to avoid complexity and non-uniqueness of the multi-parameter minimization for randomly oriented polydisperse block-like precipitate model, the data have been analyzed assuming randomly oriented polydisperse cylindrical particle model with a locked aspect ratio.     

Propagation of mesons in asymmetric nuclear matter in a density-dependent coupling model

We study the propagation of the light mesons σ,ω,ρ, and a₀(980) in dense hadronic matter in an extended derivative scalar coupling model. Within the scheme proposed it is possible to unambiguously define effective density-dependent couplings at the Lagrangian level. We first apply the model to study asymmetric nuclear matter with fixed isospin asymmetry, and then we pay particular attention to hypermatter in β-equilibrium. The equation of state and the potential contribution to the symmetry coefficient arising from the mean-field approximation are investigated. Received: 16 October 2001 / Accepted: 10 January 2002     

Strain-induced modification of magnetic structure and new magnetic phases in rare-earth epitaxial films

Rare earths exhibit complex magnetic phase diagrams resulting from the competition between various contributions to the magnetic energy: exchange, anisotropy and magnetostriction. The epitaxy of a rare-earth film on a substrate induces (i) a clamping to the substrate and (ii) pseudomorphic strains. Both these effects are shown to lead to modifications of the magnetic properties in (0 0 1)Dy, (0 0 1)Tb and (1 1 0)Eu films. In Dy and Tb films, spectacular variations of the Curie temperature have been evidenced. Additionally, Tb films exhibit a new large wavelength magnetic modulation. In Eu films, one of the helical magnetic domains disappears at low temperature whereas the propagation vectors of the other helices are tilted. The link between structural and magnetic properties is underlined via magnetoelastic models. Moreover, molecular beam epitaxy permits the growth of Sm in a metastable dhcp phase. The magnetic structure of dhcp Sm has been elucidated for the first time. In this review, neutron scattering is shown to be a powerful technique to reveal the magnetic structures of rare-earth films.     

超子中子星性质的温度效应

从相对论平均场理论出发,考虑核子、超子和介子的相互作用,研究了温度对中子星组成粒子、状态方程和中子星质量等的影响.发现温度越高,超子在中子星内部出现时的重子数密度越低.当密度较高时,中子星的核心区主要由超子组成,即中子星转变成以奇异粒子为主要成分的超子星,并且这种转变受到温度的影响,温度越高,转变密度越低.由于超子的出现,中子星核心高密度区域的状态方程,对于不同温度,差别不大,所以有限温度中子星的最大质量都在1.8M_⊙附近.这与观测结果相符.     

影响DPF焦点装置中子稳定性的因素和改进措施

简要叙述了稠密等离子体聚焦装置作为中子发生器的工作原理，对影响稠密等离子体聚焦装置放电出中子产额及其稳定性的一些因素和改进措施进行了简单讨论．通过使用不同电极形状的场畸变开关很好地提高了焦点装置的运行，使装置输出中子的起伏得到明显的改善．通过对腔内电极的清洁，在不更换气体的条件下，聚焦装置放电出中子产额稳定.The principle of dense plasma focus(DPF) as neutron generator has been investigated in this paper. We have stated briefly some factors that mainly influnce the DPF s neutron yield and some methods improving the stability of neutron yields on the dense plasma focus. It is showed that the fluctuation of DPF output neutron becomes smaller by using different types of field-distortion elements. Over 100 consecutive shots without purging the test gas the neutron yields has no obvious change.     

单粒子势模型下价核子的密度分布

依据实验事实，利用单粒子势模型，计算了一些核态外层价核子的密度分布.计算给出了价核子在核外部分布的概率和贡献，以此作为晕核态的判断标准.通过研究均方根半径随结合能变化的规律，指出了晕核态存在的条件，尤其是质子晕核态存在的条件.这些对判断和寻找晕核态有现实的指导意义. 关键词： 单粒子势模型 价核子 密度分布 中子晕核态 质子晕核态