Regular Weyl-Systems and Smooth Structures on Heisenberg Groups

We study representation theory of the Weyl relations for infinitely many degrees of freedom. Differentiability of regular representations along rays in the parameter space E suggests to consider smooth structures on E. Switching from representations of CCR to group representations of the associated Heisenberg group over E we develop a framework for smooth representations of the Heisenberg group as an infinite dimensional Lie group. After careful inspection and translation of the necessary differential geometric input for Kirillov's orbit method we are able to construct a large class of smooth representations. These reproduce the Schr?dinger representation if E is finite dimensional. Received: 10 May 1996 / Accepted: 30 July 1996     

A Lieb-Thirring Bound for a Magnetic Pauli Hamiltonian

A Lieb-Thirring-Sobolev type inequality for Pauli Hamiltonians with magnetic fields is derived. The bound is in terms of an effective field, whose energy is comparable to that of the magnetic field itself. An application to the stability of matter in magnetic fields is given. Received: 19 September 1996 / Accepted: 3 January 1997     

Exploring new mass regions with the ISOLTRAP spectrometer

First direct mass measurements on rare earth isotopes around ¹⁴⁶Gd have been performed with the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN. More than 40 isotopes of the elements Pr, Nd, Pm, Sm, Eu, Dy and Ho have been measured with an accuracy of typically 1 × 10^-7. In the case of ¹⁴¹Sm isomeric and ground state (ΔE = 175 keV) were resolved. Since isobaric contaminations are present in the ISOLDE beam, these measurements on rare earth isotopes became only possible after the installation of a new cooler trap which acts an isobar separator. This revised version was published online in August 2006 with corrections to the Cover Date.     

μSR investigation of single crystalline dysprosium

Muon spin rotation (μSR) data on a single crystalline sample of Dy have been obtained as function of temperature and applied hydrostatic pressure. The discontinuity observed in our new data at the phase transition at 85 K strongly suggests that the muons occupy tetrahedral interstitial sites, with a contact field B_C=-2.5 T at the lowest temperatures. Measurements with applied pressures up to 0.8 GPa show that the magnitude of the local fields increases with pressure at 0.3 GPa, and that the muon signal in the antiferromagnetic phase has essentially disappeared at 0.8 GPa. This revised version was published online in July 2006 with corrections to the Cover Date.     

High‐pressure μSR studies of ferro‐ and antiferromagnetic metals

High‐pressure μSR experiments on ferromagnetic nickel and \alpha‐iron and antiferromagnetic chromium are reported. In Ni above 260 K B_Fermi was found to be proportional to the saturation magnetization, whereas at lower temperatures it is temperature independent apart from a small anomaly below 30 K which is presumably caused by a magnetoelastic interaction. There was no evidence for an occupation of metastable sites by the μ⁺ below the Curie temperature. By contrast, in \alpha‐Fe the temperature dependence of \curpartialB_μ/\curpartialp shows a structure which might be attributed to the occupation of excited muon states at elevated temperatures. High‐pressure zero‐field experiments on Cr performed in the temperature regime between 4.5 K and 8 K revealed a pressure dependence of B_μ as large as \curpartialB_μ/\curpartialp=-(89.15\pm 0.06)\times 10^-12 T/Pa. In terms of volume dependence a very large negative Grüneisen parameter \gamma =-27 was obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two-photon absorption and broadband optical limiting with bis-donor stilbenes

Large two-photon absorptivities are reported for symmetrical bis-donor stilbene derivatives with dialkylamino or diphenylamino groups. These molecules exhibit strong optical limiting of nanosecond pulses over a broad spectral range in the visible. Relative to bis(di-n-butylamino)stilbene, bis(diphenylamino)stilbene exhibits a 90-nm red shift of its optical limiting band but only a minimal shift of ~13 nm of its lowest one-photon electronic absorption band. Mixtures of these compounds offer an unprecedented combination of broad optical limiting bandwidth and high linear transparency.     

Studies of the out-of-plane emission of pions in symmetric heavy-ion collisions

Triple differential cross-sections of midrapidity pions from ²⁰⁹Bi+²⁰⁹Bi collisions were measured with the Kaon Spectrometer at SIS at incident energies of 400, 700 and 1000 AMeV. The azimuthal emission pattern of the pions has been investigated in dependence of beam energy and impact parameter. An enhanced emission of pions perpendicular to the reaction plane is observed. The strength of the anisotropy increases with beam energy and pion transverse momentum. In contrast to the nucleons the anisotropy varies only little with the impact parameter. No difference in the behaviour of positive and negative pions is observed.     

Tribological properties of chromium implanted silicon nitride ceramics correlated with microstructure

3 N₄ ceramic after chromium implantation were investigated for the dependence on implantation energy between 200 keV and 3 MeV at a fixed fluence of 10¹⁷ ions/cm². The wear of the modified material is reduced for a load of 2 N independent of ion energy accompanied by a slight increase of the friction coefficient. At higher loads only high-energy implantations result in improved wear behaviour. Structural investigations show the absence of any new phases formed by ion implantation. All energies result in an amorphous layer. For lower energies this amorphous layer reaches up to the surface whereas at higher energies it is covered by still-crystalline but damaged material. The observed wear behaviour can be explained with the amorphization of the near surface region and the stress generated by the volume swelling of the amorphous layer. Received: 25 November 1996/Accepted: 29 April 1997     

Polar angle dependence of the longitudinal polarization of quarks produced in e+e−-annihilation

We calculate one-loop radiative QCD corrections to the three polarized and unpolarized structure functions that determine the beam-quark polar angle dependence of the longitudinal polarization of light and heavy quarks produced in e⁺e^?-annihilations. We present analytical and numerical results for the longitudinal polarization and its polar angle dependence. We discuss in some detail the zero-mass limit of our results and the role of the anomalous spin-flip contributions to the polarization observables in the zero-mass limit. Our discussion includes transverse and longitudinal beam polarization effects.     

The structure of 32S I. Spectroscopy of highly-excited states

Assignments of I, π, T are made to 30 levels in ³²S between 7.35 and 11.76 MeV excitation energy, making the spectroscopy of the T= 0 states rather complete up to 10 MeV and that of the T = 1 states up to 12 MeV. A reassessment of existing data in the light of the new results clarifies the spectrum of I ^π = 1⁺, T = 1 states up to 15 MeV excitation energy. High-spin states (I = 52 - 7) below 10 MeV excitation energy have been investigated by n t γ angular-correlation measurements with the ²⁹Si(α, nγ) reaction at E _α 14.4 MeV. Five g-wave resonances of the ³¹P(p, γ) reaction, leading to the formation of I _π + 4⁺, 5⁺ states in ³²S, have been identified between 10 and 12 MeV excitation energy. The spectrum of T = 1 states between 10.7 and 12 MeV, has been investigated by measurements of γ-ray angular distributions on resonances of the ³¹P(p, γ) reaction and by measurements of resonance strengths. Several ³²S levels between 7.35 and 8.75 MeV excitation energy were studied as final states in resonance decays. Finally a search was performed for I ^π = 0⁺ resonances of the ²⁸Si(α, γ) reaction.