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- Fachbereich Physik (261) (entfernen)
We report on the exchange bias effect as a function of the in-plane direction of the applied field in two-fold symmetric, epitaxial Ni80Fe20/Fe50Mn50 bilayers grown on Cu(110) single crystal substrates. An enhancement of the exchange bias field, Heb, up to a factor of two is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the ex-change bias field as a function of the in-plane angle of the applied field, the unidirectional, uniaxial and four-fold anisotropy contributions are determined with high precision. The symmetry direction of the unidirec-tional anisotropy switches with increasing NiFe thickness from [110] to [001].
The conversion efficiency of laser energy into kinetic ion energy in a laser-produced plasma has been investigated for two quite different targets: graphite and tantalum. The laser energy (intensity) varied from several mJ to 200 mJ (1O^9 to 7 x 10^10 W cm-2) which is appropriate to many applications of a laser produced ion source. The conversion efficiency as a function of the laser energy was directly determined by differential measurements of the charge, kinetic energy and angular emission distribution of the plasma ions in absolute units. Whilst for the Ta target a nearly constant efficiency of about 30% was observed, the graphite result shows an unexpectedly strong enhancement of the transfer efficiency of up to 80% in the laser intensity range around 1.5 x l0^10 W cm-2. It is assumed that the results are related to the difference in the surface roughness of the targets.
In der nichtrelativistischen Schrödinger-Theorie hat sich die nach, H. A. Kramers und L. Brillouin benannte WKB - Näherung eine weitreichende Akzeptanz verschafft. Obwohl diese Methode meist nur als WKB-Methode bezeichnet wird, war von H. Jeffreys zuerst eine solche Näherungsmethode vorgestellt worden. Es liegt nahe, in einer relativistischen Theorie eine entsprechende Näherungsmethode zu diskutieren. Ich habe verschiedene semiklassische Näherungen für die radiale Dirac-Gleichung untersucht. Analytische und numerische Diskussionen der verschiedenen Methoden haben gezeigt, daß die Güte der Näherung wesentlich von der Entkopplung der Radialgleichung abhängt. Speziell für das Coulomb-Potential habe ich eine näherungsfreie Entkopplung entwickelt, welche zu einer WKB-Quantenbedingung führt, die mit der Feinstrukturformel von Sommerfeld übereinstimmt. Für viele physikalisch relevante Potentiale, welche das Coulomb-Potential nur leicht abändern, wird somit eine weitaus größere Genauigkeit als bei herkömmlichen WKB-Näherungen erster Ordnung erreicht. Außerdem habe ich höhere Korrekturen zur WKB-Näherung untersucht.
Abstract: The classification of quasi - primary fields is outlined. It is proved that the only conserved quasi - primary currents are the energy - momentum tensor and the O(N)-Noether currents. Derivation of all quasi - primary fields and the resolution of degeneracy is sketched. Finally the limits d = 2 and d = 4 of the space dimension are discussed. Whereas the latter is trivial the former is only almost so. (To appear in the Proceedings of the XXII Conference on Differential Geometry Methods in Theoretical Physics, Ixtapa, Mexico, September 20-24, 1993)
The distribution of quasiprimary fields of fixed classes characterized by their O(N) representations Y and the number p of vector fields from which they are composed at N=infty in dependence on their normal dimension delta is shown to obey a Hardy-Ramanujan law at leading order in a 1/N-expansion. We develop a method of collective fusion of the fundamental fields which yields arbitrary qps and resolves any degeneracy.
Recently renewed interest in solitons has arisen in connection with exceptional statistics occuring in low-dimensional quantum field theory. The nonperturbative approach to quantum solitons [1, 2, 3, 4, 5], based on the notion of a disorder variable [6, 7], does not make use of the well-known semiclassical quantisation procedure around classical soliton solutions [8]. In a recent article [9] the author introduced multicomponent scalar field models, treated nonperturbatively on a Euclidean space-time lattice. The exponentially decaying disorder correlation functions are connected with soliton fields showing nonAbelian braid group statistics. It is the aim of this note to present the corresponding classical soliton solutions, which do not seem to have appeared in the literature.
Abstract: We calculate exact analytical expressions for O(alpha s) 3-jet and O (alpha^2 s ) 4-jet cross sections in polarized deep inelastic lepton nucleon scattering. Introducing an invariant jet definition scheme, we present differential distributions of 3- and 4-jet cross sections in the basic kinematical variables x and W^2 as well as total jet cross sections and show their dependence on the chosen spin-dependent (polarized) parton distributions. Noticebly differences in the predictions are found for the two extreme choices, i.e. a large negative sea-quark density or a large positive gluon density. Therefore, it may be possible to discriminate between different parametrizations of polarized parton densities, and hence between the different physical pictures of the proton spin underlying these parametrizations.
The particle flux produced by an obliquely incident Nd Q-switched pulse (20 ns) on a Ta target has been analysed with regard to its angular distribution resolved for both its neutral and ion components. The laser intensity has been varied in the range between about 10^10 - 10^11 W cm-2, which is appropriate for many low-irradiance applications. It is observed that, at all emission angles and for the whole range of laser intensities, the number of neutral species clearly dominates the composition of the particles. At 1.3 x 10^10 W cm-2 the total number of emitted particles is 4 x 10^14, scaling as E_L^¾ with the laser energy. While for relatively low laser energies the angular distribution shows the usual smooth cos-behaviour, an additional strong directive emission cone, superimposed upon the cos-distribution, develops if the laser energy is enhanced. Both the strength and the width strongly depend on the laser intensity. While at lower intensities a fit by a cos^n function with n ~ 10 seems appropriate, n increases to 26 at an intensity of 10^11 W cm-2 . It can be assumed that secondary energy transfer processes that are not yet fully understood are responsible for this anomalous emission.
