Elektronen-Spin-Resonanz an (La1??xGdyCex)Al2

In the EPR of (La_1––x Gd _y Ce _x )Al₂ a bottleneck in the relaxation of the conduction electrons to the lattice is present at sufficiently high Gd-concentrations. The bottleneck can be broken by decreasing the Gd-concentration or by adding Cerium as a spin-flip-scatterer.g-factor and line broadening are measured for (La_1–y Gd _y )Al₂ and (La_1––x Gd _y Ce _x )Al₂ as a function of temperature and Gd- and Ce-concentrations. The Cerium induced relaxationsrate _eL (Ce) increases linearly with Ce-concentration up tox0.025. This corresponds to the fact, well known by measurements of the Kondo anomalies, that Cerium impurities show no significant interaction up to rather high concentrations.     

Influence of internal degrees of freedom on the structure of one-dimensional systems

Summary The influence of internal degrees of freedom on the behaviour of one-dimensional systems is discussed. For systems with half-filled bands the coupling to internalviz. lattice coordinates decides whether Peierls distortion is caused by intramonomer coordinates or by a lattice coordinate. Thereby the various intramonomer degrees of freedom act cooperatively. We show that there is a small regime of parameters where both kinds of distortion exist simultaneously. For increasing temperature we find that distortions can also move from the lattice coordinate to the intramonomer coordinate.     

Dynamics of vector spin glasses with random anisotropy

The dynamics of vector spin glasses with additional random anisotropy is investigated in the mean field approximation. We find a cross-over of the upper critical line from a behaviour of Heisenberg spins with a field-temperature dependence , for large fieldsH, to Ising like behaviourT _f –TT ^2/3 , for small fields and fixed anisotropy, in agreement with results of Kotliar and Sompolinsky. Here, andT _f are characteristic spin glass temperatures. In addition, one has a second line with reversed behaviour which presumably represents a cross-over line from weak to strong non-ergodicity. The local transverse susceptibility _T () varies for large fields and 0 along the upper critical line as ^vT , with a critical exponentV _T = 1/2 – 11D/60 J, whereD andJ are the anisotropy and exchange coupling constants, respectively. On the Ising-like part of the upper critical line one has isotropic spin glass parameters,q _L =q _T , and susceptibilities, and a critical exponent, which is similar to that of Ising spins along the de Almeida-Thouless line.Dedicated to B. Mühlschlegel on the occasion of his 60th birthdaySFB Aachen-Jülich-Köln     

Energy-dispersive x-ray laue measurements with anomalous dispersion effects

By using an energy dispersive solid state detector, harmonics were separated in a Lauetype diffraction experiment with a single crystal and white primary radiation (Bremsstrahlung) from a conventional x-ray tube. Intensity changes around the absorption edge of an atom species in the crystal could be verified according to theory.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Neutron diffraction study of the heavy fermion superconductors UM2Al3(M=Pd,Ni)

An elastic neutron scattering study was performed on the new superconducting heavy fermion systems UPd₂Al₃ and UNi₂Al₃. The neutron diffraction patterns reveal unambiguously long range antiferromagnetic order in UPd₂Al₃ with an ordered magnetic moment _U = (0.85±0.03) _B , which coexists with the superconducting state. This is by far the largest _U value observed for any heavy fermion superconductor. For UNi₂Al₃, no long-range magnetic order could be observed for temperaturesT1.5 K, yielding an upper limit of the ordered moment of 0.2 _B .     

Protein adsorption and interfacial rheology interfering in dilatational experiment

The static and dilatational response of β-lactoglobulin fibrils and native β-lactoglobulin (monomers) at water-air and water-oil interfaces (pH 2) was measured using the pendant drop method. The resulting adsorption behavior and viscoelasticity is dependent of concentration and adsorption time. The interfacial pressure of the β-lactoglobulin fibrils obtained in static measurements was 16–18?mN/m (against air) and 7?mN/m (against oil) for all concentrations. With higher concentrations, faster adsorption kinetics and slightly higher interfacial and surface pressure is achieved but did not lead to higher viscoelastic moduli. The transient saturation of the interface is similar for both the fibril solution and the monomers, however the fibril solution forms a strong viscoelastic network. To evaluate the superimposed adsorption behavior and rheological properties, the formed interfacial layer was subjected to dilatational experiments, which were performed by oscillating the surface area of the drop in sinusoidal and sawtooth (diagonal) deformation manner. The sinusoidal oscillations (time depended area deformation rate) result in a complex interfacial tension behavior against air and oil interfaces and show remarkable differences during compression and expansion as emphasized by Lissajous figures. For diagonal (constant area deformation rate) experiments, a slight bending of the interfacial tension response was observed at low frequencies emphasizing the influence of protein adsorption during rheological measurements.     

Solving coupled Dyson-Schwinger equations on a compact manifold

We present results for the gluon and ghost propagators in SU (N) Yang-Mills theory on a four-torus at zero and non-zero temperatures from a truncated set of Dyson-Schwinger equations. When compared to continuum solutions at zero temperature sizeable modifications due to the finite volume of the manifold, especially in the infrared, are found. Effects due to non-vanishing temperatures T, on the other hand, are minute for T < 250 MeV.     

Competing polarization mechanisms in the heavy-fermion paramagnet CeCu<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Nuclear magnetic resonance (NMR) and relaxation of ⁶³Cu and ⁶⁵Cu in a powder sample of the heavy-fermion paramagnet CeCu₆ is measured and analysed quantitatively. Five different Cu sites are accessible to a detailed analysis. We derive quadrupolar splitting frequencies, Ce to Cu transferred hyperfine field coupling constants, and transversal as well as longitudinal relaxation behaviour. Only small relaxation anomalies are observed at the orthorhombic to monoclinic structural phase transition of CeCu₆. We point to the different importance of transferred hyperfine interaction and local conduction electron density for static or dynamic part, respectively, of Cu hyperfine interaction. The different sign of the transferred hyperfine interaction from Ce³⁺ to different Cu neighbours reveals the different competing interaction mechanisms, giving rise to the heavy-fermion paramagnetic behavior of CeCu₆. Received 20 November 2001     

A model for the interaction of near-infrared laser pulses with metal powders in selective laser sintering

A thermal model of the interaction of pulsed near-infrared laser radiation from a Nd:YAG laser was made, taking the measured powder properties such as reflectance, optical penetration depth and thermal conductivity into account. It allows an estimation of the evolution of two different temperatures: the average temperature of the powder (taken over the grains in a volume given by the laser beam diameter and the optical penetration depth) and the temperature distinction within a single grain. It showed that in pulsed mode consolidation can be achieved at much lower average power as the surface of the powder particles are molten but their cores remain at nearly room temperature. This leads to a much lower average temperature and therefore a dramatic decrease in residual thermal stresses in the finished piece. The results of the model were experimentally tested and confirmed. Received: 26 July 2001 / Accepted: 23 November 2001 / Published online: 23 January 2002