Diffusive motion of iron microcrystals studied by Mössbauer spectroscopy

A new model is presented which explains well the dramatic decrease of the Mössbauer line intensities with raising temperatures for freely dispersed iron microscrystals. In contrast to other theories which consider mainly vibration to be responsible we discuss here the decrease in terms of large amplitude diffusive rotational or translational jumps of the particles. Such diffusive jumps lead — in agreement with the observation — to a strong reduction of the Mössbauer-intensity without broadening the line width in a noticeable way. The typical potential well for a diffusing particle in an equilibrium position is derived quantitatively to be 13 meV. The model might be important also for a new understanding of the dynamics of catalytic clusters either in contact with each other or with larger solid surfaces.Part of this work was supported by the DFG Sonderforschungsbereich 306, Konstanz     

The effect of laser illumination on the propagation of ultrasonic waves in single crystalline lithium niobate

By means of ultrasonic experiments we have observed in Fe-doped LiNbO₃-crystals elastic inhomogeneities and an ultrasonic absorption which stronglydecrease under laser illumination. The elastic inhomogeneities before illumination lead to a non-exponential decay of the ultrasonic echo pattern. The corresponding variations of the acoustic velocity and absorption across the sound beam have been displayed directly by an ultrasonic topography. — In contrast to the photorefractive behaviour of LiNbO₃ the elastic inhomogeneities and the ultrasonic absorption are not enhanced but disappear as soon as the illumination is switched on: Under illumination the echo pattern becomes well exponential as expected for an ideal crystal. The physical nature of the elastic inhomogeneities and the acoustic absorption inside the sample which can be bleached out by optical radiation is not yet well understood. But there is evidence that Fe⁺⁺-impurities play an important role.Dedicated to Professor Harry Thomas on the occasion of his 60th birthdaySupported by the Science Fund of the Chinese Academy of Sciences     

Enhanced ionic conductivity at the interface between sapphire and solid lithium iodide

We have studied the enhanced ionic conductivity of thin films of LiI evaporated onto a planar sapphire surface carrying interdigital Au-electrodes. The interface conductivity parallel to the surface was measured in situ for increasing film thickness, up to 3,000 Å. The specific conductivity of LiI in the first 300 Å — adjacent to the sapphire — was found to exceed the bulk conductivity of LiI by nearly one order of magnitude. From our observations we conclude that the specific conductivity of LiI decreases exponentially with distance from the sapphire surface reaching the bulk LiI-value only at a distance of 3,000 Å. The conductivity of a 350 Å thick film varies with temperature (25°CT120°C) in accordance with an activation energy of 0.40±0.04 eV. This is in good agreement with the activation energy of bulk LiI in the extrinsic region.     

Anomalous ultrasonic attenuation and velocity change in LaAl2

The attenuation and velocity of longitudinal waves has been measured in superconducting and normal LaAl₂ in the frequency range from 15 to 225 MHz. Both in the superconducting and in the normal state an unusually large damping has been found superimposed on the relatively small attenuation due to the well known electron-phonon interaction. On cooling into the superconducting state the attenuation starts to drop only at temperatures well below T_c and — as in insulating glasses — it rises again below 0.8 K. In our view this indicates that two-level systems similar to the ones found in amorphous superconductors may be present in our crystalline LaAl₂ sample and are responsible for the observed acoustic anomalies.     

Molecular resolution of Langmuir-Blodgett monolayers on tungsten diselenide by scanning tunneling microscopy

Langmuir-Blodgett (LB) monolayers of 22-tricosenoic acid deposited on layered tungsten diselenide (WSe₂) have been investigated by Scanning Tunneling Microscopy (STM) in air. Reproducible molecular scale imaging of extended areas (16×16 nm) of the films was possible over more than two months after preparation. This indicates a high stability of the films on this type of polar substrates. The two-dimensional lattice of the organic film is incommensurate to the lattice of the WSe₂ substrates. The STM experiments also indicate that the tip partially penetrates the organic film without destroying the film structure.     

On the movement of iron microcrystals in high magnetic fields

The temperature dependence of the Mößbauer line intensities for iron particles coated with iron oxide is found to be the same for both the iron and the iron oxide subspectra. For the first time, this result gives direct experimental evidence that the dramatic decrease of the line intensities with raising temperaures /1,2/ is not due to changes in the phonon spectrum, but to motions of the particles instead. These motions are not influenced by external magnetic fields up to 10 T. The result is discussed in terms of different possibilities for the movement of the particles. Further more it is shown, that the geometric arrangement is different for the particles contributing to the Mößbauer effect at high, from those contributing at low temperatures.     

Ultrasonically tunable filter for ψ-rays

A new type of an ultra-narrow frequency transmission filter for ψ-rays is presented. The principle of its operation is based on the Mössbauer-sideband-spectroscopy [1–3]: A resonant absorber onto a thin piezo-electric polymer foil vibrates piston like at an ultrasonic frequency ω. At certain definite amplitudes the zero-order-sideband, i.e. the transition energy ω₀, is fully transmitted without any resonant absorption, whereas simultanously sidebands displaced by energies ±hhΩ from ω₀ are resonantly absorbed. Using several absorbers of this type in series having different frequencies a broadband filter for ψ-rays with a well defined transmission-profile can be designed. Symmetrically, on both sides of the central transmission window there exist resonant absorption bands. The energy width of these different bands is only limited by the possible ultrasonic frequencies and can be adjusted from 10^?o to 10^?5 eV. First experimental results using such an assembly of absorbers are presented. The limit of the efficiency of the filter, caused by non-resonant scattering is discussed.     

Der piezoelektrische Effekt in Polyvinyliden Fluorid und seine Anwendungen

Ohne ZusammenfassungMit 18 Abbildungen und 1 Tabelle     

Generation of acoustic surface waves on GaAs by photoexcitation

In the experiments reported here we observed the generation of non-thermal, large amplitude acoustic surface waves at GHz frequencies on the surface of a GaAs crystal under intense illumination with 514.5 nm radiation from an Ar⁺ laser at room temperature. The acoustic surface waves were detected by optical Brillouin spectroscopy.     

Debye-temperatures and Mössbauer-shift in high-Tc superconductor: Ba2EuCu3O7±δ

The high T_C superconductor Ba₂EuCu_307±δ with measured T_c=94±2 K was investigated by Mössbauer spectroscopy. It is shown that the material at low temperatures has either a very high Debye temperature θ_D > 800 K, or it is crystallographically not single phased with one component (87%) with θ_D = 378±30 K and the other (13%) with θ_D = 136±13 K.