SU(6)破缺效应对核子形状因子的贡献

最近杰斐逊实验室测量了中子的两个弹性电磁形状因子之比GEn/GMn,它反映了中子内部的电磁分布并能帮助理解中子的内部结构.传统的相对论组分夸克模型在解释质子的两种电磁形状因子之比GEp/GMp是很成功的,但在解释中子的该比值时计算值比实验值要低.为了解释这种现象,我们在相对论组分夸克模型框架下计算考察了SU(6)破缺效应的贡献.发现考虑该破缺效应后的计算结果同实验符合有明显改善.     

Quasi-Elastic Electron-Deuteron Scattering and Calculation of Neutron Electromagnetic Form Factors at Q2 = 1.75 to 4.00 (GeV/c)2

Electric and Magnetic form factors of neutron are calculated via electron-deuteron scattering at 1.511 ～5.507 GeV energy using SLAC group data. Our results show that the neutron electric form factor is not equal to zero;rather it has a small value, indicating that in spite of the fact that total charge is almost neutral, there is a nonuniformcharge distribution within the neutron, and that magnetic form factor follows the dipole fit.     

A polarised neutron study of magnetic moment density in Cu2MnAl

A polarised neutron study of the ferromagnetic Heusler alloy Cu₂Mn_0.863Al_1.057 has been made. It has been concluded that the magnetic moment density is primarily situated on the Mn ions. On assigning the Mn-moment value, the observed magnetic form factor is found to be in good agreement with the Mn²⁺ free ion form factor calculated by Watson and Freeman. A slight asphericity has been observed in the moment density. It is estimated that there are about 3% excess 3d-electrons in the Eg states compared to spherical distribution. There is evidence of a very small positive polarisation of the Cu atoms. No appreciable conduction electron polarisation is found.     

Thermodynamic functions of degenerate magnetized electron gas

The Fermi energy, pressure, internal energy, entropy, and heat capacity of completely degenerate relativistic electron gas are calculated by numerical methods. It is shown that the maximum admissible magnetic field on the order of 10⁹ G in white dwarfs increases the pressure by a factor of 1.06 in the central region, where the electron concentration is ∼10³³ cm⁻³, while the equilibrium radius increases by approximately a factor of 1.03, which obviously cannot be observed experimentally. A magnetic field of ∼10⁸ G or lower has no effect on the pressure and other thermodynamic functions. It is also shown that the contribution of degenerate electron gas to the total pressure in neutron stars is negligible compared to that of neutron gas even in magnetic fields with a maximum induction ∼10¹⁷ G possible in neutron stars. The neutron beta-decay forbiddeness conditions in a superstrong magnetic field are formulated. It is assumed that small neutron stars have such magnetic fields and that pulsars with small periods are the most probable objects that can have super-strong magnetic fields.     

A coherent interpretation of the form factors of the nucleon in terms of a pion cloud and constituent quarks

The recent measurements of the electric form factor of the neutron suggest that its shape may be interpreted as a smooth broad distribution with a bump at 0.3 (GeV/c)² superimposed. As a consequence the corresponding charge distribution in the Breit frame shows a negative charge extending as far out as 2 fm. It is natural to identify this charge with the pion cloud. This realisation is then used to reanalyse all old and new data of the electric and magnetic from factors of the proton and the neutron by a phenomenological fit and by a fit based on the constituent quark model. It is shown that it is possible to fit all form factors coherently with both ansaetzen and that they all show the signal of the pion cloud.Received: 6 March 2003, Published online: 17 June 2003PACS: 14.20.Dh Protons and neutrons - 13.40.Gp Electromagnetic form factors - 21.10.Ft Charge distribution     

Specific features of neutron propagation in a medium with a helical magnetic structure

The features of neutron motion in a noncoplanar magnetic field have been considered in the example of a specific magnetization distribution in the form of a conical helix. The neutron reflectances for holmium crystals have been calculated. It is shown that, in the case of noncoplanar distribution of magnetic field in a crystal, the spin-flip neutron reflectance has an additional singularity.     

Magnetic form factor determination by inelastic scattering of neutrons as compared to elastic scattering measures

The difference between the magnetic form factor for elastic and inelastic scattering of neutrons is derived to interpret recent neutron scattering experiment.     

Phase relation between SDW and strain wave in chromium

The phase relation between SDW and strain wave in Cr has been studied by neutron diffraction and analysed on the basis of the rigid spin model. The intensity ratio between two satellites (1 ± δ 0 0) has been determined from observed results as 1.044, after making corrections mainly due to the magnetic form factor and the Lorentz factor. This result can be understood by a model that the largest elongation of the lattice spacing appears at atoms having the maximum spin magnetic moment.     

Form factors in the projected linear chiral sigma model

Several nucleon form factors are computed within the framework of the linear chiral soliton model. To this end variational means and projection techniques applied to generalized hedgehog quark-boson Fock states are used. In this procedure the Goldberger-Treiman relation and a virial theorem for the pion-nucleon form factor are well fulfilled demonstrating the consistency of the treatment. Both proton and neutron charge form factors are correctly reproduced, as well as the proton magnetic one. The shapes of the neutron magnetic and of the axial form factors are good but their absolute values at the origin are too large. The slopes of all the form factors at zero momentum transfer are in good agreement with the experimental data. The pion-nucleon form factor exhibits to great extent a monopole shape with a cut-off mass ofΛ=690 MeV. Electromagnetic form factors for the vertexγNΔ and the nucleon spin distribution are also evaluated and discussed.     

Comparison of spectroscopic information given by the peripheral model and DWBA

It is shown that the one parameter form of the peripheral model gives the same spectroscopic information as DWBA does for neutron transfer reactions on light nuclei at medium bombarding energies.     

Neutron magnetic form factor in strongly correlated materials

We introduce a formalism to compute the neutron magnetic form factor FM(q) within a first-principles density functional theory and dynamical mean field theory. The approach treats spin and orbital interactions on the same footing and reduces to earlier methods in the fully localized or the fully itinerant limit. We test the method on various actinides of current interest NpCoGa5, PuSb and PuCoGa5, and we show that PuCoGa5 is in mixed valent state, which naturally explains the measured magnetic form factor.     

Field-induced magnetic form factor of lutetium

The conduction electron field-induced magnetic form factor of Lu, a rare earth metal with a closed 4f shell, has been measured using the polarized neutron technique. The measurements demonstrate that the conduction electron density is more spatially extended than in the free atom. The experimental results are in good agreement with band calculations of the $\text{spin}$ magnetic form factor.     

Nature of the magnetic excitation spectrum in (Sm,Y)S: CEF effects or an exciton?

The dynamic magnetic susceptibility spectrum in a single-crystal sample of the intermediate-valence compound Sm_0.67Y_0.33S is studied by inelastic neutron scattering with neutron momentum transfer and sample temperatures varying over wide ranges. Two coupled collective modes have been found in the spectrum. Unlike the higher energy mode, whose intensity approximately follows the form factor of Sm²⁺, the lower energy mode exhibits a stronger angular dependence than could be expected from the form factor for the localized f electrons. The total intensity of the inelastic component of the magnetic response decreases with increasing temperature; this is accompanied by the appearance of a broad quasi-elastic signal of a magnetic nature at significantly lower temperatures than follows from the calculated intensities of the transitions within the excited multiplet of the Sm²⁺ ion. An analysis of the observed features allows the suggestion to be made that the fine structure of the magnetic excitation spectrum in (Sm,Y)S is associated with the formation of an exciton-like intermediate-valence state on Sm ions rather than with the crystal-electric-field effects.     

Analysis of form factors observed with polarized neutrons; comparison with band structure calculations

For the analysis of observed magnetic neutron form factors of transition metals and alloys different methods are being used. These methods are i) fitting of theoretical free-ion form factors to the experimental data, ii) Fourier analysis, as introduced by Moon, which does not require any a priori knowledge of the form factor but, instead, extracts its spin and orbital parts from the data, and iii) a modified version of ii) that properly takes into account the full symmetry of the crystal lattice. Spherical spin form factors appearing in i), ii) and iii) are compared with those based upon band structure calculations for Fe and Ni. It is found that only the results of iii) are in agreement with band theory. Since neither band theory nor method iii) leads to the existence of a uniform negative spin density this concept should be abandoned.     

Pauli paramagnetic effects on vortices in superconducting TmNi2B2C

The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.     

Induced neutron magnetic form factor of LaSn3

Polarized neutron scattering techniques have been used to study the spatial distribution of the magnetization induced in a single crystal of LaSn₃ by a magnetic field of 42.5 kG at 100 K. We find that the magnetic form factor decreases very rapidly with increasing scattering angle, and bears no resemblance to the spin or orbital free-atom magnetic form factor. The experimental results are in reasonable agreement with band theoretical calculations of the spin magnetic form factor of LaSn₃. We conclude that (a) the spin part is the dominant contribution to the bulk susceptibility of LaSn₃ and (b) there is a substantial amount of Sn-5p electronic character in the wavefunctions near the Fermi level.     

Thermodynamic functions of a nonrelativistic degenerate neutron gas in a magnetic field

The Fermi energy, partial concentrations of polarized neutrons, pressure, and volume energy density of a degenerate nonrelativistic neutron gas in a magnetic field are calculated using numerical methods taking into account the anomalous magnetic moment of a neutron. The results of calculations are a generalization of relations underlying the Oppenheimer-Volkov model of a neutron star to the case of an applied magnetic field. An ultrastrong (up to 10¹⁷ G) magnetic field changes the pressure and internal energy of the star and affects it static configuration and evolution. It is shown that a degenerate neutron gas in ultrastrong and weak magnetic fields is paramagnetic; the corresponding values of magnetic susceptibility differ by a factor on the order of unity. The possibility of experimentally verifying the results from analysis of pulsar-emitted radiation is discussed.     

Electronic structure,field-induced magnetization density and neutron magnetic form factor of palladium

The magnetic field induced magnetization density and neutron magnetic form factor of Pd metal is obtained from an abinitio APW energy band study of its electronic structure and properties. The magnetization consists of the spin density calculated for states on the Fermi surface and a much smaller orbital contribution. The solid state wavefunctions are found to yield a spatial localization of the spin density which is greater than that of the very contracted Hartree-Fock density of the free Pd²⁺ ion. The theretical magnetic form factor, which is dominated by the contribution of the fifth band, is found to be in excellent agreement with the measurements of Cable, Wollan, Felcher, Brun and Hornfeldt.     

First evidence for a Sm<Superscript>3+</Superscript>-type contribution to the magnetic form factor in the quasielastic spectral response of intermediate valence SmB<Subscript>6</Subscript>

The momentum-transfer dependence of the magnetic form factor associated with the quasielastic spectral component in the dynamic magnetic response of intermediate valence SmB₆ has been measured using inelastic neutron scattering on a double-isotope (¹⁵⁴Sm, ¹¹B) single crystal. The experimental dependence differs qualitatively from those obtained earlier for the inelastic signals, as well as from the field-induced magnetic form factor of SmB₆ obtained by polarized neutron diffraction. This observation is interpreted by specifically considering the Curie-type contributions to the dynamic susceptibility, which arise from the mixing of 4 f⁵ and 4 f⁶J-multiplets into the intermediate valence state wavefunction.