[en] The distribution of electron momenta (electron density in momentum representation) of gases can be probed by Compton scattering of either photons (γ-rays or X-rays) or electrons. Recently it has been shown that Compton scattering of electrons is suited to the study of the electron momentum densities of solids on a microscopic scale. This technique, known as ECOSS, Electron Compton Scattering from solids can be done in the electron microscope by Electron Energy Loss Spectroscopy (EELS). The advantages and disadvantages of ECOSS as compared to photon Compton Scattering are discussed. It is shown that ECOSS is a method suitable to obtain information on the ground state wave function in condensed matter. After discussion of inherent approximations, especially the impulse approximation, the reciprocal form factor is introduced. A method is proposed in order to cope with the main difficulty, namely multiple scattering. Important applications of ECOSS are the study of anisotropy of momentum densities; correlation effects of conduction electrons in metals; and charge transfer in alloys

[en] Moessbauer diffraction experiments have been performed on a LiNbO₃ single crystal at room temperature for scattering angles between 14.5 and 40.5⁰. A large contribution of inelastically scattered intensity was detectable for all scattering angles. Radiation damage caused by the high-energy components of the radiation emitted by the ⁵⁷Co Moessbauer source was observed. An attempt was made to estimate the temperature factor B from the measured inelastically scattered radiation assuming that first order thermal diffuse scattering (TDS) is dominant. Both the TDS contribution to the integrated intensity within the Bragg peak and the ratio of the integrated first-order TDS included in the Bragg peak to that included in the background were calculated with various programs from the elastic constants and compared with the experimental values. (Auth.)

No abstract available

No abstract available

[en] Full text: Due to their low and 'glass-like' thermal conductivities intermetallic clathrates have a large potential for thermoelectric applications. The introduction of strong electronic correlations would further enhance the thermoelectric figure of merit Z = F²σ/κ; Kondo insulators are known to exhibit giant values of the thermopower. To date only europium clathrates were known which show however no sizeable interaction of 4f electrons with the conduction electrons. All previous attempts to incorporate other rare earth elements in clathrates based on the conventional synthesis techniques (crystallization from the melt followed by long annealing) have failed because of higher thermodynamic stabilities of other competing non-Eu rare earth ternary phases. A synthesis procedure that could use different formation kinetics of the participating phases was searched. Rapid quenching was tried in order to obtain the Yb substituted Eu_8-xYb_xGa₁₆Ge₃₀ phase (0 < x < 1.6) in a metastable state. Surprisingly well formed crystalline clathrate phases have yielded. The quenching procedure and the results of the characterization of the quenched phases by the x-ray powder diffraction, SEM/EDX and TEM are presented. The quenched clathrates show unusual electrical transport properties - unlike the conventionally synthesized Eu-clathrates which show metallic behavior (contrary to expectation from the Zintl concept), our materials are semiconductors (more detailed about the electrical properties see poster of S. Laumann et al 'Electric transport and magnetization investigation of the rapidly quenched clathrate Eu₈Ga₁₆Ge₃₀'). (author)

[en] Full text: Intermetallic clathrates have low thermal conductivities κ but high electrical conductivities σ ('phonon glass - electron crystal') which leads to a large ratio of σ/κ. The introduction of strong electronic correlations which is expected if adequate rare earth atoms are incorporated would in addition enhance the value of the thermopower S. To date the only rare earth containing clathrate is Eu₈Ga₁₆Ge₃₀. However, it shows no sizeable interaction of 4f electrons with the conduction electrons. In our search for ways to substitute Eu with other rare earth elements we have found an unconventional synthesis procedure s. The poster of A. Prokofiev et al., 'Rapid quenching as an unconventional synthesis technique for Eu₈Ga₁₆Ge₃₀-based clathrate phases'. The bonding situation in a clathrate Eu₈Ga₁₆Ge₃₀ can be understood in a first approximation with the Zintl concept. Ga and Ge build a covalently bonded tetrahedrally coordinated cage structure. The guest Eu atoms are ionically bonded to the framework. The electronegative Ga and Ge receive electrons from the electropositive Eu in accordance with the octet rule. Since all valence electrons are used in local covalent bonds, one expects clathrates to be semiconducters. Here we present the results of the magnetic and electric transport investigations. The resistivity and Hall-resistivity were measured over a broad temperature range from 350 to 2 K at magnetic fields from 0 to 9 Tesla. In contrast to conventional procedures a semiconducting compound has been produced by quenching of the melt. (author)

[en] Published in summary form only

[en] Films produced by quenching Cu-Ag vapour onto cooled substrates at liquid nitrogen temperature have been investigated using electron microscopy, electron diffraction and electrical resistivity measurements. In the composition range from 30 to 70 at% Cu the as quenched films are amorphous, and within the range of 35 to 63 at% Cu the amorphous phase is stable above room temperature with a maximum crystallization temperature Tsub(c)=381 K at 47.5 at% Cu. Crystallization results in the formation of a supersaturated fcc solid solution which decomposes in a second crystallization step. The effect of deposition rate, film thickness, temperature and surface of the substrate, and most importantly of the composition on the transition temperatures has been investigated. A comparative study of the formation of amorphous phases in a wide variety of Cu-based alloys is presented. (author)

[en] This study reports on the correlation between strain relaxation and nucleation kinetics during the Stranski-Krastanow growth of GaN on (0001)AlN by plasma-assisted molecular beam epitaxy. Using reflection high-energy electron diffraction and real-time desorption mass spectrometry, the strain-related Ga adatom detachment and desorption rates were determined, giving information about the average GaN island nucleation rate. Two different regimes were found: one at low-temperature growth (690< T_S<720 deg. C), where strain relaxation occurred slowly, yielding impeded island nucleation rates and small island sizes (diameter ∼8-12 nm and height ∼2.3-2.7 nm). In the other, i.e., high-temperature growth regime (T_S>720 deg. C), islands showed an abrupt relaxation mode, accompanied by a fast nucleation rate toward island sizes twice as large

[en] Since the availability of monochromators in transmission electron microscopes (TEMs), electron energy loss spectrometry (EELS) is widely used to determine band gaps and the dielectric properties of semiconductors on a nano-metre scale. Nevertheless, three physical effects hamper straightforward analysis: (a) relativistic energy losses, (b) the delocalization of the energy loss which is in the 10 nano-metreer range for valence losses, and (c) the presence of interface plasmons. When reducing the operation voltage of the TEM one can kill two birds with one stone: (a) the relativistic losses will disappear as soon as v_e< c₀/n(with v_e as the speed of the electron, c₀ as the vacuum speed of light and n as the refractive index of the investigated sample) and (b) the delocalization will decrease, because it also depends on the energy of the incident electron probe. The determination of the optical properties of quantum structures is discussed in the case of GaP/GaAs interface at 200 keV and 20 keV beam energy, respectively. Further, the influence of the delocalization of the energy loss signal is discussed theoretically and experimentally.