基于冷中子谱的紧凑型中子自旋倒相器设计

为节省极化中子散射谱仪传输光路的空间,实现特定冷中子谱的极化中子高效率自旋翻转,使用在空间上自然衰减的前端多层膜极化器静磁场作为中子自旋倒相器的导向磁场,在空间上形成了紧凑型冷中子自旋倒相器设计模型。介绍了实际模型物理参数的计算方法。对前端极化器静磁场在空间上的自然衰减进行了实验测试,根据测试结果及拟使用冷中子波段,针对设计的紧凑型中子自旋倒相器的相关参数进行了优化计算。模拟了极化中子在实际复合磁场中的自旋翻转图像,计算了自旋倒相器的翻转效率。对设计的紧凑型中子自旋倒相器进行了翻转效率物理实验测试,测试结果表明：设计的中子自旋倒相器翻转效率可在99.2%以上,达到了预期设计指标,可用于极化冷中子散射谱仪。     

Development of neutron resonance spin flipper for high resolution spin echo spectrometer

A new type of neutron resonance spin flipper (RSF) with high frequency oscillating magnetic field has been developed for Modulated IntEnsity by Zero Effort (MIEZE) spectrometer at cold neutron beam line MINE1 at JRR-3M reactor in JAEA. Dipole magnets enable us to provide the strong static fields for the RSFs. MIEZE signals have been demonstrated with the effective frequency of 600 kHz by using the new RSFs. The contrast of the signals was 0.58. The MIEZE spectrometer is under final process to practical use. The spectrometers can also be applied to the pulsed neutrons like J-PARC.     

Inelastic neutron scattering study of spin waves in MnO

The dispersion relation E(q) for spin waves in MnO has been measured at 4.2°K by neutron inelastic scattering. The isotropic exchange integrals as well as the anisotropy constants have been determined by fitting the Hamiltonian to the data. It is found that the exchange striction plays main role in the anomaly in the magnetic interactions in MnO and the biquadratic exchange interaction j₁(S₁·S₂)² is almost absent (j₁/J₁ ? 0.002).     

Inelastic neutron scattering measurement of spin waves and magnetic interactions in NiO

The dispersion relation E(q) for spin waves in NiO, a Type II f.c.c. antiferromagnet, has been measured at 78°K. Values for the isotropic exchange interactions have been determined by fitting the Hamiltonian to the data. The next nearest neighbour interaction is found to dominate and the small nearest neighbour interaction is ferromagnetic in sign, in agreement with simple overlap theory.     

Inelastic neutron scattering studies of the spin and lattice dynamics in iron arsenide compounds

Although neutrons do not couple directly to the superconducting order parameter, they have nevertheless played an important role in advancing our understanding of the pairing mechanism and the symmetry of the superconducting energy gap in the iron arsenide compounds. Measurements of the spin and lattice dynamics have been performed on non-superconducting ‘parent’ compounds based on the LaFeAsO (‘1111’) and BaFe₂As₂ (‘122’) crystal structures, and on electron and hole-doped superconducting compounds, using both polycrystalline and single crystal samples. Neutron measurements of the phonon density-of-state, subsequently supported by single crystal inelastic X-ray scattering, are in good agreement with ab initio calculations, provided the magnetism of the iron atoms is taken into account. However, when combined with estimates of the electron–phonon coupling, the predicted superconducting transition temperatures are less than 1 K, making a conventional phononic mechanism for superconductivity highly unlikely. Measurements of the spin dynamics within the spin density wave phase of the parent compounds show evidence of strongly dispersive spin waves with exchange interactions consistent with the observed magnetic order and a large anisotropy gap. Antiferromagnetic fluctuations persist in the normal phase of the superconducting compounds, but they are more diffuse. Below T_c, there is evidence in three ‘122’ compounds that these fluctuations condense into a resonant spin excitation at the antiferromagnetic wavevector with an energy that scales with T_c. Such resonances have been observed in the high-T_c copper oxides and a number of heavy fermion superconductors, where they are considered to be evidence of d-wave symmetry. In the iron arsenides, they also provide evidence of unconventional superconductivity, but a comparison with ARPES and other measurements, which indicate that the gaps are isotropic, suggests that the symmetry is more likely to be extended-s_± wave in character.     

Inelastic electron tunneling spectroscopy of a single nuclear spin

Detection of a single nuclear spin constitutes an outstanding problem in different fields of physics such as quantum computing or magnetic imaging. Here we show that the energy levels of a single nuclear spin can be measured by means of inelastic electron tunneling spectroscopy (IETS). We consider two different systems, a magnetic adatom probed with scanning tunneling microscopy and a single Bi dopant in a silicon nanotransistor. We find that the hyperfine coupling opens new transport channels which can be resolved at experimentally accessible temperatures. Our simulations evince that IETS yields information about the occupations of the nuclear spin states, paving the way towards transport-detected single nuclear spin resonance.     

Inelastic neutron scattering from superconducting rings

For the first time the differential cross section for the inelastic magnetic neutron scattering by superconducting rings is derived taking account of the interaction of the neutron magnetic moment with the magnetic field generated by the superconducting current. Calculations of the scattering cross section are carried out for cold neutrons and thin film rings from type-II superconductors with the magnetic fields not exceeding the first critical field.     

Inelastic neutron scattering study of antiferromagnetic CoO

In order to study the energy levels of the magnetic excitations in antiferromagnetic CoO, we have measured the cross-section for inelastic n-scattering of CoO powder at various scattering angles θ and energy transfer from 15 to 70 meV. For large θ purely nuclear scattering is observed, while for small θ the cross-section is predominantly due to magnetic scattering with some nuclear contributions. The cross-section shows several peaks, which have not yet been observed.By comparison of the results of pure CoO and of Co_0.8Ni_0.2O two peaks in the cross-section can be assigned to single ion-like transitions in the ground state multiplet of Co²⁺ in the antiferromagnetic phase. The parameters of a simple molecular field model are discussed.     

Inelastic neutron scattering from cerium under pressure

Inelastic neutron scattering from Ce metal at 300 K was studied both below and above the first order γ-α phase transition, using a triple axis spectrometer. It was found that (a) there is no indication of any residual magnetic scattering in the collapsed α phase, and (b) the energy width of the paramagnetic scattering in the γ-phase increases with pressure. We conclude that J_sf increases with pressure, as has been predicted byCoqblin.     

Inelastic neutron scattering to very high pressures

Progress in high-pressure and neutron scattering methods has recently allowed measurements of phonon dispersion curves of simple solids at high pressures to 10 GPa. In this technique single crystals of 10–25 mm³ volume are compressed by the Paris-Edinburgh cell and the phonon frequencies are measured on high-flux triple axis spectrometers. Detailed studies of the lattice dynamics of low-compressible systems are feasible, including measurements of mode Grüneisen parameters, elastic constants, and precursor effects of phase transitions. We describe the experimental set-up and illustrate its potential by results on semiconductors (Ge and GaSb) and metals (Fe and Zn) obtained at the LLB (Saclay) and ILL (Grenoble) reactor sources. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inelastic neutron scattering and lattice dynamics of minerals

We review current research on minerals using inelastic neutron scattering and lattice dynamics calculations. Inelastic neutron scattering studies in combination with first principles and atomistic calculations provide a detailed understanding of the phonon dispersion relations, density of states and their manifestations in various thermodynamic properties. The role of theoretical lattice dynamics calculations in the planning, interpretation and analysis of neutron experiments are discussed. These studies provide important insights in understanding various anomalous behaviour including pressure-induced amorphization, phonon and elastic instabilities, prediction of novel high pressure phase transitions, high pressure-temperature melting, etc.      

Coupling between neutron spin states in a proposed new spin directional coupler

A new neutron spin directional coupler in a magnetic multilayer waveguide is proposed. Quantum interference between neutron spin states in the latter structure will take place. The proposed structure has spin switching properties. It is shown that this kind of structures can be realized in magnetic spin-switches.     

黑索金的非弹性中子散射及第一性原理计算

黑索金(环三亚甲基三硝胺, RDX, C₃H₆O₆N₆)是一种非常重要的次级炸药, 因其高能量密度及对外界刺激的低感度而具有广泛的军事和工业应用. 为了能在生产、运输、存储以及使用中对其行为进行有效控制, 人们对它的化学性质、力学性质, 尤其是起爆进行了大量的研究. 炸药的起爆是一个非常复杂的过程, 其中最主要的问题之一就是能量是如何从连续介质尺度的刺激转移到原子尺度引起吸热分解的. 根据冲击波致爆的非平衡态Zel'dovich-von Neumann-Doering模型, 声子作为最初的热载体在整个过程中起着非常重要的作用. 实验上, 非弹性中子散射技术是研究晶体中原子和分子运动动力学的有力手段, 尤其是对于包含了大部分声子晶格模式的低频区域来说极具优势. 利用非弹性中子散射技术测得了RDX 在10–104 cm^-1 范围内的振动谱, 结合固态量子化学计算, 对所测的12个振动模式进行指认. 研究结果有助于人们对起爆详细机理的认识.     

Design of a pulsed spatial neutron magnetic spin resonator

It is well known that upon passage through a spatially alternating transverse magnetic field, produced by a meander-shaped conducting foil, in its rest frame each neutron creates its individual frequency which depends on its velocity and the period of the meander. A resonant spin flip process takes place, if this frequency equals the Larmor frequency determined by a homogeneous vertical field. Clearly, this effect can be used to monochromatise a polarised neutron beam. Here we propose a novel design of such a magnetic resonator consisting of a sequence of separate compact modules, which provide high homogeneity of the transversal field oscillations and allow rapid beam chopping since they meet the specifications of fast electronic switching. The wavelength resolution of this resonator device can be changed in an instant and likewise an arbitrary amplitude modulation of the transversal field can be established, which is required for an efficient suppression of subsidiary maxima of the wavelength-dependent spin flip probability.     

Development of a compact on-beam SEOP neutron spin filter

We have designed and demonstrated a prototype on-beam spin-exchange optical pumping (SEOP) ³He neutron spin filter (NSF). It is designed as the incident neutron polarizer for spallation neutron sources, where the installation space is limited due to thick radiation shielding. The size of the NSF is roughly 50 cm×50 cm×25 cm including the diode-laser optics with a frequency narrowing external cavity, and a cylindrical ³He cell as large as a diameter of 5 cm and a length of 10 cm can fit. A neutron beam test was performed at the NOP beamline of JRR-3 to see the performance of the NSF.     

Interfering with the neutron spin

Charge neutrality, a spin 1/2 and an associated magnetic moment of the neutron make it an ideal probe of quantal spinor evolutions. Polarized neutron interferometry in magnetic field Hamiltonians has thus scored several firsts such as direct verification of Pauli anticommutation, experimental separation of geometric and dynamical phases and observation of non-cyclic amplitudes and phases. This paper provides a flavour of the physics learnt from such experiments.     

Inelastic neutron scattering study of acoustic phonons of black phosphorus

Dispersion relations of acoustic phonons of the orthorhombic black phosphorus have been measured at room temperature and atmospheric pressure. Very anisotropic dispersion surfaces were observed along the three principal directions. Comparison is made between the present measurements and the recent calculations of dispersion curves by Kaneta et al.     

Inelastic neutron scattering study of commensurate-incommensurate phase transition in thiourea

This paper presents a study of the nature of the incommensurate lattice instability in deuterated thiourea by inelastic neutron scattering. It shows clearly a “soft mode” which condenses at the phase transition in a satellite reflexion. A study of related dispersion curves along the [O ξ O] direction shows an unusual spectral shape in a large temperature range above T_c.