Neutron scattering study of the field-induced soliton lattice in CuGeO3

CuGeO3 undergoes a transition from a spin-Peierls phase to an incommensurate phase at a critical field of H(c) approximately 12.5 T. In the high-field phase a lattice of solitons forms, with both structural and magnetic components, and these have been studied using neutron scattering techniques. Our results provide direct evidence for a long-ranged magnetic soliton structure which has both staggered and uniform magnetizations with amplitudes that are broadly in accord with theoretical estimates. The magnetic soliton width gamma(m) and the field dependence of the incommensurability deltak(SP) are found to agree well with theoretical predictions.     

Neutron inelastic scattering spectrum and valence force field for neopentane

The neutron inelastic scattering spectrum of neopentane has been measured between 10 and 2000 cm^?1. A partial reassignment of the normal modes of vibration is presented. A vibrational analysis based on the Snyder and Schachtschneider valence force field has been carried out using the frequencies of neopentane and neopentane-d₁₂. The calculated neutron spectrum, using our new force field, is found to be in good agreement with the observed neutron data.     

Neutron scattering in intermetallics

Inelastic and elastic neutron scattering as a probe of long range and disordered magnetism is discussed for example cases in rare earth intermetallic crystalline compounds and amorphous analogues. The determination of the sublattice site magnetizations is illustrated for RFe₂ and Y₆(Mn_xFe_1−x)₂₃ compounds. The latter are shown to exhibit strong exchange disorder effects for intermediate compositions, as manifested by the development of short range antiferromagnetic clusters. The effect of randomization of the crystal field interaction in destroying long range order is illustrated in amorphous rare earth compounds and in rare earth compound hydrides. Results of inelastic scattering measurements on Laves phase Fe, Co, and Al compounds are summarized which yield exchange and crystal field parameters.     

Neutron scattering study of the excitation spectrum of solid helium at ultra-low temperatures

There has been a resurgence of interest in the properties of solid helium due to the recent discovery of non-classical rotational inertia (NCRI) in solid ⁴He by Chan and coworkers below 200 mK which they have interpreted as a transition to a ‘supersolid’ phase. We have carried out a series of elastic and inelastic neutron scattering measurements on single crystals of hcp ⁴He at temperatures down to 60 mK. While we have found no direct evidence of any change in the excitation spectrum at low temperatures, we have found that the excitation spectrum of solid ⁴He shows several interesting features, including extra branches in addition to the phonon branches. We interpret these extra branches as single particle excitations due to propagating vacancy waves, which map on to the famous ‘roton minimum’ long known in the excitation spectrum of superfluid liquid ⁴He. The results show that in fact solid ⁴He shares several features in common with the superfluid.      

Neutron scattering in superionic conductors

The coherent dynamical structure factor S(q,ω) of one-dimensional superionic conductors is studied in a model of Brownian particles with harmonic interactions moving in a periodic potential. We present results for the q-dependence of the halfwidth of the quasielastic peak, which reflects the commensurability ratios of the periodic potential and the molten sublattice. Furthermore the influence of the periodic potential on the collective excitations of the mobile ions is discussed.     

Neutron scattering from a substitutional mass defect

The dynamic structure factor is calculated for a low concentration of light mass scatterers substituted in a cubic crystal matrix. A new numerical method for the exact calculation is demonstrated. We derive a local density of states for the low momentum transfer limit, and derive the shifts and widths of the oscillator peaks in the high momentum transfer limit. We discuss the limitations of an approximation which decouples the defect from the lattice.     

Neutron scattering from FeSi

The inelastic neutron scattering from the unusual paramagnet FeSi has been measured at temperatures of up to 700 K in order to investigate the possible existence of a local magnetic moment at high temperatures. The estimated intensities of the central peaks around $\text{h}\text{?}\text{ω = 0}$ and the nearly constant backgrounds ($\text{h}\text{?}\text{ω ? 16 meV}$) indicate that the paramagnetic state above 600 K in FeSi cannot be interpreted in terms of a simple localized spin model with $\text{S ～}\text{1}\text{2}$.     

Neutron scattering from PEEK

An alkali‐soluble polysaccharide, GM5‐1, from the mycelium of Ganoderma tsugae, was fractionated by the nonsolvent addition method. The weight‐average molecular mass (M _w) and intrinsic viscosity ([η]) of fractions were studied by laser light scattering and viscometry with dimethylsulfoxide (DMSO) containing 0.25M LiCl (0.25M LiCl/DMSO) as the solvent at 30°C. The dependences of [η] and the radius of gyration (?S ²?_z ^1/2) on M _w were found to be [η]=9.8×10^?2 M _w ^0.57 (cm³g^?1) and ?S ²?_z ^1/2=1.65×10^?2 M _w ^0.58 (nm) in the M _w range from 8.6×10⁵ to 2.6×10⁶. The analysis based on current theories for a wormlike chain showed that the chain conformational parameters of GM5‐1 were 4.2 nm for persistence length (q) and 833 nm^?1 for linear mass density (M_L) with an obvious excluded volume effect. The results suggested the alkali‐soluble polysaccharide behaves like a semiflexible chain in 0.25M LiCl/DMSO at 30°C.     

Neutron scattering in impure anharmonic solids

The Debye-Waller factor and the coherent inelastic differential scattering cross section for the one- and two-phonon band modes have been investigated for an anharmonic crystal containing randomly distributed substitutional impurities of low concentration. The method adopted in the evaluation is that of double time temperature dependent Green's functions. The quantities thus evaluated have been separated into diagonal and nondiagonal terms. The diagonal terms are further separated in three contributions, namely the contributions from defects, anharmonicity, and simultaneous involvement of impurity and anharmonicity. A new mode, the impurity-anharmonicity interference mode, is the salient featue of the theory, and arises due to the interaction between local and anharmonic fields.     

Electronic spectrum of a two-dimensional Fibonacci lattice

The electronic spectrum and wave functions of a new quasicrystal structure—a two-dimensional Fibonacci lattice—are investigated in the tight-binding approximation using the method of the level statistics. This is a self-similar structure consisting of three elementary structural units. The “central” and “nodal” decoration of this structure are examined. It is shown that the electronic energy spectrum of a two-dimensional Fibonacci lattice contains a singular part, but in contrast to a one-dimensional Fibonacci lattice the spectrum does not contain a hierarchical gap structure. The measure of allowed states (Lebesgue measure) of the spectrum is different from zero, and for “central” decoration it is close to 1. The character of the localization of the wave functions is investigated, and it is found that the wave functions are “critical.” Zh. éksp. Teor. Fiz. 116, 1834–1842 (November 1999)     

Neutron scattering from natural silicon

Differential cross sections for elastic and inelastic scattering of neutrons from natural silicon were measured in the energy region between 4.00 and 5.75 MeV, in steps of about 250 keV in the primary neutron energy. The results are presented in form of coefficients of Legendre polynomial expansions.     

Neutron scattering from elemental silver

Differential neutron elastic-scattering cross sections of elemental silver were measured from 1.5 → 10 MeV at scattering angles distributed between ≈17° and 160°. Cross sections for ten inelastically scattered neutron groups were determined, corresponding to observed excitations of 328 ± 13, 419 ± 50, 748 ± 25, 908 ± 26, 1150 ± 38, 1286 ± 25, 1507 ± 20, 1623 ± 30, 1835 ± 20, and 1944 ± 26 keV, all of which were composites of contributions from the two isotopes ¹⁰⁷Ag and ¹⁰⁹Ag. The experimental results were interpreted in terms of the spherical-optical and rotational and vibrational coupled-channels models.     

Neutron scattering of iron-based superconductors

Low-energy spin excitations have been studied on polycrystalline LaFeAsO_1?xF_x samples by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ″(ω) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, χ″(ω) almost vanishes at x = 0.158, where the superconducting transition temperature T_c is suppressed to 7 K. In addition, χ″(ω) in optimally doped LaFeAsO_0.918F_0.082 with T_c = 29 K exhibits a spin resonance mode. The peak energy, E_res, when scaled by k_BT_c is similar to the value of about 4.7 reported in other high-T_c iron-based superconductors. This result suggests that there is intimate relationship between the dynamical spin susceptibility and high-T_c superconductivity in iron-based superconductors, and is consistent with a nesting condition between Fermi surfaces at the Γ and M points.     

中子散射技术及其应用

热中子的波长和凝聚态物质的原子/分子间距具有相同的量级,而其能量又和原子/分子的热运动能量相近.因此,利用热中子的弹性和非弹性散射效应,可以从微观层次上获取物质的结构和动力学知识.目前,中子散射技术在物理、化学、化工、生物和材料科学等研究领域的应用已经获得了许多用其他方法无法得到的知识. 文章介绍了中子散射的基本原理和特点,并列举了中子散射技术在相关研究领域中的典型应用     

Neutron scattering from 26Mg

Elastic and inelastic cross sections have been measured for 24 MeV neutrons incident on ²⁶Mg using the time-of-flight technique. These cross sections and existing proton data were analyzed in terms of a Lane-consistent optical potential. Distorted-wave and coupled-channel calculations are presented. Deformation parameters for the first 2⁺ state (1.81 MeV) and the second 2⁺ state (2.94 MeV) derived from both the (n, n') and (p, p') data are used to obtain the ratio of neutron and proton transition matrix elements M_n(E2) and M_p(E2) for these states. Comparison of results for $\text{M}{}_{\mathrm{<mtext>n</mtext>}}\text{(}\text{E}\text{2)}\text{M}{}_{\mathrm{<mtext>p</mtext>}}\text{(}\text{E}\text{2)}$ with those obtained from pion work, from electromagnetic (EM) rates and theoretical evaluations are presented.