Zeitschrift für Physik B Condensed Matter

An unusual peak has been found by Müller et al. for the attenuation of longitudinal sound as a function of temperature around 12 K for UPt3. Breathing mode coupling of the volume strain to the electronic energy states of a narrow band of high density at the fermi energy seems to be the origin of this peak.

We have extended the CTRW theory of Montroll and Weiss including the effect of extra variables, like the energy. This MCTRW scheme can be written in a simple matrix notation, that simplifies its solution. As an example of their usefulness we have studied a two-energy-group neutron diffusion problem. This has shown the peculiarities of the transient behaviour for the variance of the probability distribution, due to the coupling between the groups.

The structure of the distorted perovskite BaPbO3 was studied with high-resolution X-ray diffraction at 300 K and 26 K and with neutron diffraction at 300 K. Simultaneous refinement of the neutron and X-ray data sets (300 K) using the Rietveld method yields a monoclinic structure with the space groupI 2/m and lattice parametersa=6.0278 (1) Å,b=6.0664(1) Å,c=8.5109(1) Å, and γ=90.083 (2)o. The tilting of the oxygen octahedra is given asa − a − c 0 in Glazer's notation [11]. The monoclinic angle corresponds to the angle between the cubic directions [110] c and $$[1\bar 10]$$ c . This is in contrast to the observations in BaBiO3. The structure of BaBiO3 has the same space groupI 2/m, the similar dimensions of the unit cell and the same tilt system, but β=∢ ([110] c , [001] c ) as the monoclinic angle. As a consequence there is only one type and size of, PbO6 octahedra but two types of octahedra in BaBiO3. This fact may influence the occurrence of superconductivity in solid solutions (Ba(Pb1−x Bi x )O3 containing a large fraction of lead by enhancing valence fluctuations.

SnAu- and SnCu-alloys are vapour quenched at cryogenic temperatures. Resistivity and electron diffraction patterns were recorded in situ. The scattered intensities have been normalized to interference functions. The overall agreement with those of the corresponding liquid systems is quite well. The average interatomic distances obtained from the atomic distribution functions show a concentration dependence which corresponds closely to that found in liquid SnAu and SnCu respectively. This gives evidence to the statement that the nearest neighbour organisations of the films are very similar to those of the corresponding liquids. Furthermore it turned out that the local structures in the Sn-rich SnAu-system are substantial different from those in the Sn-rich SnCu-system, except in the vicinity of a minimum amount of noble metals (8–10 at.%), which is necessary to stabilize the amorphous state.

We present the results of inelastic neutron scattering experiments on the intermediate-valent system YbPd2Si2 to investigate the magnetic relaxation behaviour. We have performed measurements on polycrystalline samples with neutrons of incident energyE 0=3.1 meV at temperatures between 1.5 K and 250 K, and withE 0=12.7 meV andE 0=50.8 meV at temperatures between 5 K and 50 K using time-of-flight spectrometers. At temperaturesT>50 K we find a pure quasielastic magnetic response with a rather broad linewidth typical for intermediate-valent systems. AtT≈50 K an inelastic excitation line appears at about 21 meV; its intensity increases with decreasing temperature. In the same temperature range (T<50 K) the quasielastic linewidth decreases rapidly and atT=5 K the quasielastic response has been apparently transformed to a second inelastic feature at about 4.7 meV. The width of this low-energy excitation fits well to the temperature dependence of the quasielastic linewidth forT>5 K.

A new method for characterizing acoustic flux propagation in anisotropic media is introduced and developed. The technique, which we call ultrasonic flux imaging (UFI), utilizes a pair of water-immersion focused acoustic transducers as a point source and point detector. Raster scanning one transducer produces a transmission pattern which exhibits the anisotropies in acoustic flux known as “phonon focusing” modulated by interference between sheets of the acoustic wave surface. This “internal diffraction” is studied theoretically taking into account the anisotropy of the medium, the boundary conditions between the solid and the water, and the pressure fields produced by the transducers. In addition to bulk effects, the images reveal interesting critical-cone structures associated with the water/solid interface. The theoretical predictions agree well with experimental observations in silicon and a number of other materials, including single-crystal metals, insulators, and semiconductors. All measurements are made at room temperature, in contrast to the cryogenic requirements of previous phononimaging techniques. As a new method, UFI holds promise for examining anisotropies in the vibrational properties, and, possibly, electron-phonon coupling in metals and superconductors. The principles and techniques may also have application to non-destructive characterization of textured polycrystalline and composite materials.

A derivation of a transport equation for phonons, in terms of the microscopic description of an anharmonic crystal, is given. The starting point is a complete set of equations for the phonon Green function, the self-energy, and the vertex part, as given by a functional method using real times. The role of the external source field for introducing nonequilibrium into the system is discussed in detail. The essential equivalence of some recent theories is shown, which have been proposed in connection with phonon transport and second sound. The integral equation for the vertex part is related to a Boltzmann equation for a position and time dependent phonon density. For this quantity an operator, namely the Wigner operator, can be extracted from the energy density. It is shown, however, that the energy density can only approximately be expressed in terms of this phonon density. With the aid of the Wigner operator the variable phonon density is represented by a Kubo formula as the linear response of the phonon quasiparticle gas to the external source field. Cubic and quartic anharmonicities are taken into account and their influence on the collision operator and on the drift term of the transport equation is discussed.

Polarised neutrons and polarisation analysis were used in a neutron scattering study of UPt3 both above the superconducting transition temperatureT c as well as below. Thus the existence of magnetic fluctuations in UPt3 was established unambiguously. For varyingQ vectors the magnetic scattering intensity was integrated over energy transfers ±20 meV to obtain a measure of the corresponding magnetic correlation function. No indication for strong spatial correlations of magnetisation could be detected. Furthermore inelastic transitions up to energy transfers of 80 meV were observed.

We studied by high resolution inelastic neutron scattering the isotope effect of tunneling of coupled methyl groups in lithium acetate dihydrate (LIAC). Fully protonated, fully deuterated and mixed LIAC samples were investigated. The results are described by a simple model in which it is assumed that the single particle potential hindering rotation is strongly increased by deuteration, whereas the coupling potential is nearly unchanged. This allows to interpret the very strong isotope effect and also the spectra of the mixed compounds. The strong increase of the single particle potential by deuteration is tentatively explained by phonon mediated coupling.