High temperature spin wave dynamics of the uniaxial antiferromagnets

In the framework of the spin operators diagram technique the dynamical properties of the easy-axis type antiferromagnets at high temperatures are considered. The diagonalization procedure generalizing the Bogoliubov u-v transformation on the spin operators is presented. The spin wave (SW) spectrum allowing for the temperature and the magnetic field dependence of the sublattice magnetizations at T～T_～ is obtained. The SW relaxation frequencies due to the processes of SW scattering on the thermal spin fluctuations are calculated. The latter are in agreement with the data obtained on MnF₂ near the spin-flop transition¹⁰.     

Spin waves transport across a ferrimagnetically ordered nanojunction of cobalt-gadolinium alloy between cobalt leads

A model calculation is presented for the spin wave scattering and coherent transport at the ferrimagnetically ordered cobalt-gadolinium alloy nanojunction between cobalt leads. The structural model for the amorphous alloy nanojunction [Co_1/2Gd_1/2]₃ is considered as an ordered alloy hcp structure of three (0001) atomic planes between the leads. To analyze the spin dynamics and spin wave scattering at the nanojunction boundary, the phase field matching method (PFMT) is implemented over the ground state of the system, in the Heisenberg Hamiltonian representation. The coherent reflection and transmission probabilities of spin waves from the cobalt leads incident onto the nanojunction boundary are calculated, and numerical results are presented for the coherent SW transport across the nanojunction over the entire range of their frequencies. The results are especially valid in the interval between nanometric SW wavelengths greater than the nanojunction width and macroscopic wavelengths. They demonstrate in particular, the possibility of the resonance assisted maxima for the SW transmission spectra owing to the interactions between the incident spin waves and the localized spin resonances on the nanojunction. This effect is general and may be observed at different characteristic frequencies and corresponding incident angles.     

Electron-spin-dependent 4He+ ion scattering on Bi surfaces

We studied low-energy (～ 1.55 keV) electron-spin-polarized ⁴He⁺ ion scattering on a Bi(111) ultrathin film epitaxially grown on a Si(111) substrate. We observed that the scattered ion intensity differed between the incident He⁺ ions with up and down spins even though Bi is a non-magnetic element. To analyze the origin of this spin-dependent ion scattering (the spin asymmetry), we investigated the detailed relationship between the spin asymmetry and the incident angle, the azimuthal angle, the scattering angle, and the incident energy. All the data indicate that the spin asymmetry originates from the scattering cross section owing to the non-central force in the He⁺–Bi atom binary collision. The non-central force is most likely attributed to the spin–orbit coupling that acts transiently on the He⁺ 1s electron spin in the binary collision.     

Intense interactions of molecules with a short-wavelength electromagnetic radiation field: I. The fundamentals of the nonadiabatic theory

The intense interactions between short-wavelength (SW) electromagnetic radiation with a wavelength λ ≥ 1 Å and intensity up to 10¹⁴ W/cm² and simple and polyatomic molecules are studied with the coherent excitations of high-lying Rydberg and autoionizing states taken into account. The Hamiltonian of a system “molecule + SW radiation” is obtained by using the methods of quantum electrodynamics. Conditions for the applicability of the dipole approximation to describe the interactions of molecules with radiation of the UV, VUV, XUV, and soft X-ray range are found. The fundamentals of the theory of resonance scattering of SW radiation from diatomic, triatomic, and symmetric-and asymmetric-top polyatomic molecules are outlined.     

SPECIFIC HEAT OF A HEISENBERG SYSTEM WITH FINITE SIZE

Based upon the spin wave theory, the influence of the size of a three-dimensional Heisenberg system on its thermodynamic properties was studied. It is found that the specific heat increases due to the finite size and free surface of the system. For a magnetic film with finite thickness, the interaction of spin waves was also discussed. There exist three additional scattering processes, namely, the scattering between spin waves with wave-vectors parallel to the surface of the film (two-dimensional spin wave), the scattering between two and three-dimensional waves, and the scattering between those waves with the same component in the direction along the thickness of the film. As a result, the T⁴ term, arising from the coupling of spin waves, in the expression of the specific heat of the system, splite into three parts proportional to T^5/2,T^7/2 and T⁴, respectively. Here T is the temperature.     

Investigation of three-particle spin correlations during critical scattering of polarized neutrons from a nickel single crystal

The critical small-angle scattering of polarized neutrons from spin fluctuations in a nickel single crystal with a special inclined geometry of the magnetic field has been studied. The method of inclined geometry makes it possible to investigate not only two-particle spin correlations but also three-particle spin correlations that determine the polarization-dependent contribution to scattering, which is asymmetric with respect to the momentum transfer q. This contribution depends on the momentum transfer q as 1/(q ² + ξ^?2)^5/2, where f is the neutron scattering correlation length; it linearly increases with an increase in the magnetic field H in the low-field range and then reaches saturation. The results obtained are in good agreement with the similarity theory.     

Neutron scattering length of lithium and boron and their isotopes

Coherent neutron scattering lengths and free cross sections were measured in order to determine the spin dependent scattering lengths of the isotopes of Li and B. The transmission measurements on Li at neutron energies of 0.51 meV, 1.26 eV and 5.19 eV delivered data for the⁶Li-abundances in the samples and for the free scattering cross section of⁷Li. By means of experiments on 15 different samples of lithium compounds we obtained the complex spin state scattering lengths for the bound atoms to be: (ie81-01) Measurements on 3 boron samples with different enrichments led to (ie81-02) A review on slow neutron scattering and resonance data shows an over all consistency of all values.     

A study of proton-12C inelastic scattering at 1 GeV

The inelastic scattering of protons on ¹²C is investigated on the basis of the first order term of the multiple scattering expansion of Kerman, McManus, and Thaler. The spin degrees of freedom are taken into account. The inelastic transitions to the three lowest excited states of the target are studied within the distorted wave impulse approximation, the distortion being evaluated using an eikonal approximation. The elementary nuleon-nucleon high-energy interaction employed contains a simplified but realistic spin dependence. Special emphasis is placed on the influence of the small-angle approximation, the effect of spin, and the sensitivity to different nuclear transition densities.     

Correlation,spin, and isospin effects in the scattering of kaons from nuclei

Elastic and inelastic multiple scattering of K⁺ and K^? on ¹²C and ⁴⁰Ca at 800 MeV/c incident momentum are investigated and the results are compared to experimental data. Effects of both c.m. and Pauli correlations on the differential cross sections and their relative importance are studied. The dispersive effects, i.e. virtual excitations and de-excitations of the target nucleus during the scattering process, are also analysed as well as the influence of the spin and isospin dependence of the kaon-nucleon interactions. Sensitivity of the angular distributions to the double spin and isospin-flip processes and to the double charge-exchange process is discussed.     

Messung der Spinabhängigkeit der Rayleighstreuung mit Hilfe der Interferenz von Mößbauer- und Rayleighstreuung

The interference of Mößbauer scattering and Rayleigh scattering was examined and it was found that this interference can be easily observed on certain conditions. The experiment was initially carried out with a single-line transition of the 14 keV energy level of Fe⁵⁷. From the experimental data the contribution of Rayleigh scattering could be determined relative to Mößbauer scattering and it could be shown that by measuring the interference of Mößbauer scattering and Rayleigh scattering one will have a sensitive detector for Rayleigh scattering. This was used to test the spin dependence of Rayleigh scattering. For this purpose, a similar experiment was carried out with circularly polarizedγ-quanta and polarized electrons, making use of the splitting of the Fe⁵⁷ hyperfine structure. By comparison of the measured values for left-handed and right-handed circularly polarized components a strong spin dependence of the Rayleigh scattering was found.     

A new approach for heavy ion fusion spin distribution

The method of optical model analysis of generalized elastic scattering angular distributions (GESA) has been applied to heavy ion scattering to derive fusion spin distributions. This method is used to reproduce the coupled channel fusion spin distributions. When applied to experimental data, particularly to the fissile systems like¹⁶O +²³²Th, the method gives large mean square spin values in agreement with “anomalous” values derived from experimental fission fragment anisotropies.     

On high-energy elastic scattering of protons by nuclei

The two coupled channel formalism for high energy elastic scattering [1] is extended to include spin and isospin effects. For a spin and isospin zero nucleus these manifest themselves by additional spin-orbit terms in the potentials. Explicit formulas for these potentials are obtained in terms of the fully spin and isospin dependent nucleon-nucleon scattering amplitude, the ground state nuclear form factor and the state dependent correlation functions. The coupling potential except for a small term arising from double spin and isospin flip process involving nuclear excitation depends only upon the pair correlations.Numerical calculations are performed for the elastic scattering of 1 GeV protons incident on ⁴He. Various phenomenological dynamical two-body correlations as well as correlations generated from the Reid soft-core and Tabakin potentials in an approximate Brueckner-Hartree-Fock calculation are considered. The angular distribution beyond its first diffraction minimum as well as the polarization in the same angular range are shown to be sensitive to these correlations. However, the present accuracy of the experimental data and the lack of knowledge of the nucleon-nucleon scattering amplitude prevent any definitive conclusion about their nature.     

Active polarized target: Measurement of proton spin polarizabilities

The static (electric and magnetic) polarizabilities of the proton are known from measurements of low-energy Compton scattering of real photons. However, no experimental data are so far available about its spin polarizabilities, which describe the response of the proton spin to a varying electromagnetic field. These fundamental structure constants can be obtained in the most model-independent way from measurements of double-polarized Compton scattering at energies below the pion threshold using a polarized photon beam and a polarized proton target. An active polarized target based on polystyrene doped with a scintillator allows recoil protons to be detected directly in the target material, background from the coherent Compton effect on heavier nuclei (¹²C etc.) to be eliminated and precise data on spin polarizabilities to be obtained.     

Interactions of neutrons with gallium and its isotopes

Coherent neutron scattering lengths and total cross sections were measured on elemental Ga, Ga-compounds and on isotopically enriched samples at neutron energies from 0.5 meV up to 143 keV using different techniques. From the experimental data the following quantities could be obtained:

the coherent scattering length of Ga and the spin state scattering lengths of the isotopes.

the potential scattering radii (R′)

the absorption cross sections of Ga,⁶⁹Ga,⁷¹Ga and⁷⁵As.

As derived quantities are given the coherent and incoherent cross sections and the resonance spin scattering lengths for theJ=1 andJ=2 states of⁷⁰Ga and⁷²Ga. The physical meaning of these data and ofR′ and the comparison with the resonance parameters are considered.     

Elastic Deuteron Scattering on the <Superscript>12</Superscript>C Nucleus within a Three-Body Model

The formalism developed earlier for elastic pd scattering on the basis of Glauber theory with allowance for a total spin dependence is modified by replacing pN amplitudes by amplitudes for N¹²C scattering and is applied to elastic deuteron scattering on the ¹²C nucleus. The amplitudes for elastic N¹²C scattering are obtained within the optical model. Respective numerical calculations performed at the kinetic deuteron-beam energy of 270 MeV lead to results that agree well with data on the differential cross section for d¹²C scattering into the forward hemisphere, but the calculated spin observable A _y ^d agrees with experimental data only qualitatively.     

半导体量子阱中电子自旋弛豫和动量弛豫

根据电子自旋轨道耦合对自旋极化弛豫影响的DP机理进一步导出了半导体中电子自旋弛豫与动量弛豫及载流子浓度的关系，并采用飞秒抽运探测技术在室温下测量AlGaAs/GaAs 多量子阱中载流子浓度在 1×10¹⁷—1×10¹⁸cm^-3范围内，电子自旋弛豫时间由58ps增加至82 ps的变化情况，与理论计算值符合，说明了随着载流子浓度的增加，载流子间的频繁散射加速了电子动量驰豫，减弱了电子自旋轨道耦合作用，从而延长了电子自旋寿命. 关键词： 电子自旋轨道耦合 电子自旋弛豫和动量弛豫 飞秒光谱技术     

Spin species conversion of rotational tunneling molecules studied by neutron transmission

The total proton spin of rotational tunneling molecules like CH₃, NH₃ or NH ₄ ⁺ groups has a strong influence on the total incoherent scattering cross section of long wavelength neutrons. At sufficiently low temperatures, a change in neutron scattering cross section after a rapid change in lattice temperature is the unequivocal signature of spin species conversion. This effect is used to study the time and temperature dependence of spin conversion. In a simple transmission experiment, we have screened 30 candidate substances of which 13 did not convert on a timescale of days. The other 17 converted: 10 contain paramagnetic ions and their spin temperature follows quasi instantaneously the lattice temperature. Seven substances convert slowly on a time scale of hours. Experimental results are presented and the advantages and limitations of the method are discussed.     

Element-selective spin polarization analysis of surfaces by spin-polarized ion scattering spectroscopy

We discuss the validity of surface spin polarization analysis with element selectivity using spin-polarized ion scattering spectroscopy (SP-ISS). We examined the control of the incident ⁴He⁺ spins and successfully conducted magnetic hysteresis measurement on an Fe(100) surface. The spin polarization of the Fe(100) surface exposed to O₂ atmosphere measured by spin-polarized ion neutralization spectroscopy was consistent with that reported by spin-polarized metastable de-excitation spectroscopy. The element selectivity of SP-ISS is discussed in terms of ion neutralization, re-ionization, and multiple scattering.     

Pion scattering on 3H and 3He nuclei in the Δ33(1232) region

Elastic scattering of π ^± mesons on ³H and ³He nuclei at energies around the Δ₃₃(1232) resonance is considered. The amplitudes for single and double scattering are calculated on the basis of the nonrelativistic diagram technique. The spin and isospin structure of pion-nucleon amplitudes is fully taken into account. The resulting theoretical predictions for the differential cross sections are compared with available experimental data.     

Using a grating analyser for SEMSANS investigations in the very small angle range

Spin-echo modulation small-angle neutron scattering (SEMSANS) is based on the detection of spatial beam modulation, which is induced by triangular spin echo precession regions and subsequent spin analyses. In order to detect such signal and exploit it for small angle scattering investigations neutron detection with sub-millimeter spatial resolution is required. Here an approach is reported where instead of a position sensitive detector an absorption grating is used to analyze the beam modulation stepwise. The spin-echo length scan in this case is performed by varying the sample-to-detector distance. The real space correlation functions of reference sample structures in the range 10² nm, i.e. giving rise to small-angle scattering in the very small-angle range, are recorded and analyzed successfully.