Phonon-Resonanzstreuung und Wärmeleitung in einer Silizium-Korngrenze

The thermal conductivity of a grain-boundary in silicon bicrystals has been measured in the temperature range from 12°K to 150°K using a newly developed method. The analysis of the experimental results shows that the thermal conductivity is governed by two scattering processes: the acoustical mismatch of both crystal halves and the resonant scattering of the phonons by impurity atoms of the grain boundary. A new method is thereby presented for the direct observation of phonon resonant scattering.     

On the neutron diffraction in crystals in the field of a sound wave

Diffraction of neutrons in crystals under influence of a sound wave is considered. The probability of scattering of neutrons at the elastic interaction with the crystal is calculated. On the contrary, scattering of neutrons by an acoustical phonon has inelastic character. The possibility to control the Debye-Waller factor is shown.     

中子非弹性散射对Bi12GeO20和Bi12SiO20旋声性的研究

通过用中子非弹性散射测量旋声晶体高对称方向的横声学支声子色散曲线的劈裂,从三个方面研究了同构异质晶体Bi₁₂GeO₂₀和Bi₁₂SiO₂₀的旋声特性。值得注意的是,沿〈111〉方向传播的左旋、右旋圆偏振声子具有不同的衰减。其中传播速度快的圆偏振声子衰减较大,该结论为这两种晶体所证实,这是在晶体旋声性研究中首次观察到的现象。实验还直接确定了衰减较大的这支声子的寿命约为2×10^-11S。 关键词：     

Phase problem in three-beam X-ray diffraction

It is shown that, apart from the purely elastic scattering of X-rays, their inelastic coherent scattering by phonons can play, in some cases, a significant part in the formation of reflection curves for multiple X-ray diffraction. This process may affect the interference pattern for weak reflection, and it must be taken into account when extracting the triplet phase, as was demonstrated by an analysis of the experimental rocking curves obtained for the coplanar three-beam diffraction by a KDP crystal.     

Thermal and subthermal neutron scattering

Several problems in thermal and subthermal neutron scattering are considered, such as long-wave phonon multiple scattering, interference, reflexion (sec. 1) and inelastic small angle scattering (sec. 2). A method for solving elastic scattering problems is given and applied to the problem of diffraction from a plane slit (sec. 3). These investigations were partly motivated by experimental results, a discussion of which will be found in the appendix. Finally, as of special interest for experiments with extreme subthermal energies, the contribution of phonons to refraction (sec. 4) and inelastic nuclear spinflip scattering, such as by a ferromagnet, are discussed (sec. 5).     

Combined nuclear-molecular resonance inelastic scattering of x rays

We report on theoretical and experimental studies of nuclear inelastic scattering in a molecular crystal, whose atoms experience both molecular and lattice vibrations. In this case scattering proceeds as combined nuclear-molecular resonance inelastic scattering. The lattice vibrations give rise to inelastic scattering around the molecular resonances with an energy dependence identical to that around the nuclear resonance. The incoherent nature of the scattering in the molecular resonances results in a proper balance of elastic and inelastic components, which has important implications for studies of heterogeneous systems.     

Influence of atomic order on TA1[110] phonon softening [4] and displacive phase transition in Fe72Pt28 Invar alloys

The acoustic phonon dispersions of two Invar crystals , one ordered with the () structure, the other disordered fcc, have been investigated between 3.4 K and 470 K by inelastic and elastic neutron scattering. For the ordered crystal, pronounced softening of the whole phonon branch is observed on cooling below the Curie temperature. Particularly strong phonon softening at the M-point zone boundary of the structure leads to a displacive, antiferrodistortive phase transition at low temperatures. For the disordered crystal, much weaker softening of the phonons is observed and restricted to the region near the Brillouin zone center, where increasing elastic scattering with decreasing temperature indicates the growth of local tetragonal strain. This strain is considered as a typical precursor of the transformation to bct martensite. Specific heat measurements, performed at low temperatures on both crystals confirm the neutron scattering results and reveal considerable enhancement of the low energy phonon density of states in the ordered crystal. Received 18 January 1999     

Inelastic scattering of He atoms from a Cu(110) surface

A He beam has been used to measure inelastic scattering from a Cu(110) surface. The scattering was a result of predominantly single phonon events and both energy gain and loss processes were observed. This was in contrast to Cu(001) where only energy loss was observed. For Cu(110), aligned in the [001] azimuth, partial dispersion relations have been measured which meant that phonons with ΔQ values up to the zone boundary value have not been detected under the present experimental conditions. The elastic incoherent component varied with azimuthal angle for Cu(110).     

Attenuation of phonon scattering intensities by point defects

The cross section of the coherent, inelastic neutron scattering is characteristically decreased in the presence of defect-induced lattice distortions. This effect is in analogy to the attenuation of Bragg intensities due to a static Debye-Waller factor. The integrated scattering intensities from transversal acoustic (TA-) phonons of the system NbN_0.014 have been measured and are shown to be attenuated with respect to the scattering intensities from the phonons of a pure Nb crystal. We discuss the obtained results through comparison with various model calculations.     

Lattice dynamics of a protein crystal

All-atom lattice-dynamical calculations are reported for a crystalline protein, ribonuclease A. The sound velocities, density of states, heat capacity (C(V)) and thermal diffuse scattering are all consistent with available experimental data. C(V) proportional, variant T(1.68) for T < 35 K, significantly deviating from a Debye solid. In Bragg peak vicinity, inelastic scattering of x rays by phonons is found to originate from acoustic mode scattering. The results suggest an approach to protein crystal physics combining all-atom lattice-dynamical calculations with experiments on next-generation neutron sources.     

Theory of multipole-dipole resonance Raman scattering by phonons

The theory of multipole-dipole resonance Ramon scattering by phonons is given. Under conditions of resonance of the incident or scattered photon with a multipole exciton, entirely new selection rules, and polarization properties govern the process of inelastic scattering. For example, in a cubic crystal, the ‘rule of mutual exclusion’ is broken. Some illustrations are given with particular attention to the experimental results for Cu₂O recently observed by Compaan and Cummins.     

A study of acoustical activity of BiP12GeO20

Acoustical activity of Bi₁₂GeO₂₀ has been studied by measuring the splitting of the degeneracy of transverse acoustical waves propagating along the directions corresponding to acoustical axes, [100] and [111] with neutron inelastic scattering. A comparison with the experimental results of α-SiO₂ and NaClO₃ has been done. Three striking features of acoustical activity can be obtained from the present study. One of them is that the attenuation is different for right circularly polarized mode of acoustical phonon propagating along acoustical axes from left one. This effect might have applications in the future.     

Elastic and inelastic diffraction of Mössbauer γ-rays from KCN

Mössbauer γ-ray diffraction was used to discriminate between the elastic and inelastic scattering intensities from the (1 1 1) to (5 5 5) Bragg reflections of a single crystal of KCN. The energy resolution of our experiment was 28 neV. We observe pronounced inelastic peaks at each Bragg point, while the elastic scattering dies out rapidly due to a large Debye-Waller factor. Thus in case of (4 4 4) and (5 5 5) the inelastic scattering is larger in magnitude than the elastic one.     

Lattice dynamics and inelastic neutron scattering experiments on andalusite,Al2SiO5

We report coherent inelastic neutron scattering measurements of the phonon dispersion relations and lattice dynamics shell model calculations of several microscopic and macroscopic properties of andalusite, Al₂SiO₅. Andalusite has an orthorhombic structure with 32 atoms/unit cell. The inelastic neutron scattering measurements (up to energy transfers of 45 meV) were carried out using the triple axis spectrometer at Dhruva reactor, India using a single crystal of andalusite and the phonon dispersion relations along the [100] direction have been measured. The shell model calculations have been used to compute the crystal structure, elastic constants, phonon frequencies, dispersion relations, density of states and the specific heat. The calculated results are in good agreement with available experimental data. The computed one-phonon neutron scattering structure factors based on the shell model have been very useful in the planning and analysis of the inelastic neutron scattering experiments.     

Elastic and inelastic scattering of 178 MeV protons from 58Ni and 60Ni

The elastic and inelastic scattering of 178 MeV protons from ⁵⁸Ni and ⁶⁰Ni has been studied. Angular distributions were measured for the differential cross sections for elastic scattering as well as inelastic scattering from excited states below about 5 MeV, all with natural parity. For the elastic and for the inelastic scattering from the first excited state (2⁺ in both nuclei, the angular distributions for the polarization were also measured. The measurements extend out to c.m. angles of about 60°, corresponding to a momentum transfer of about 600 MeV/c.The elastic and inelastic scattering data were compared to the results of coupled-channel calculations in the vibrational model using a deformed spin-orbit interaction of the full Thomas form. Good agreement was found in general showing that the main features of the experimental results are well described in this model.     

The inelastic scattering of He from Cu(001) at 20 K

The inelastic scattering of He atoms from a (001) surface of copper at 20 K has been studied. The energy analyzer consists of a LiF crystal situated close to the normal of the copper sample crystal. Incident energies of 23 and 64 meV have been used and evidence has been obtained of energy gain and energy loss events. The energy analyzer can be used in two modes: (1) to fix the analyzer crystal to receive atoms of a certain energy range while scanning the sample crystal or (2) to scan the analyzer crystal for a particular setting of the sample crystal. No evidence of phonons from a surface wave branch was found and most of the inelastic events occur very close to the forward direction.     

Multiphonon resonances in the Debye-Waller factor of atom surface scattering

He atom surface scattering by dispersionless phonons is treated employing coupled channel (CC) calculations. At low energies, they predict a behavior opposite to perturbative Born or "exponentiated" Born approximation: strong resonant phonon stimulated elastic and inhibited inelastic scattering. The corresponding resonances have not been observed in earlier CC results since these have considered only the temperature dependence of the Debye-Waller factor at higher energy or omitted the attractive well. The resonances can be interpreted in terms of bound states in the attractive well with several excited vibrational quanta. They may be observable for, e.g., He scattering by a cold Xe/Cu surface.     

Heat conduction in low-dimensional quantum magnets

Transport properties provide important information about the mobility, elastic and inelastic of scattering of excitations in solids. Heat transport is well understood for phonons and electrons, but little is known about heat transport by magnetic excitations. Very recently, large and unusual magnetic heat conductivities were discovered in low-dimensional quantum magnets. This article summarizes experimental results for the magnetic thermal conductivity κ_mag of several compounds which are good representations of different low-dimensional quantum spin models, i.e. arrangements of S=1/2 spins in the form of two-dimensional (2D) square lattices and one-dimensional (1D) structures such as chains and two-leg ladders. Remarkable properties of κ_mag have been discovered: It often dwarfs the usual phonon thermal conductivity and allows the identification and analysis of different scattering mechanisms of the relevant magnetic excitations.     

Elastic and the 2+ inelastic scattering of high energy protons on12C and the alpha-particle model

The alpha-particle model has been applied to calculate the elastic and the 2⁺ inelastic angular distribution of 1 GeV protons on¹²C within the framework of Glauber multiple scattering theory. The results are compared with the recent experimental data and with some previous calculations. The model gives a fairly good account of the inelastic data and its predictions for the elastic scattering are essentially the same as those of the shell model.     

Determination of the elastic constants by coherent neutron scattering in polycrystals

It is shown that elastic constants which usually can be determined in single crystals only may be measured in polycrystals by inelastic coherent neutron scattering. Measurements are reported for polycrystalline samples of aluminium, copper and stainless steel. The method is best suited for the determination of the elastic constants connected with transverse phonons.