Groundstate properties of a generalized VBS-model

We study a generalized q-deformed VBS-model. This is a spin-1 quantum antiferromagnet with nearest neighbour interaction on a linear chain. The exact grounstate is determined in the form of a matrix-product of individual site-contributions. All relevant groundstate properties are calculated. The groundstate is unique, it has a finite gap to the excitations, and correlations decay exponentially. Thus the model has all the properties described by Haldane to be generic for certain quantum antiferromagnets with integral spin.Work performed within the research program of the Sonderforschungsbereich 341, Köln-Aachen-Jülich     

Planar microwave biased RF-SQUIDs in niobium technology

A planar version of microwave biased SQUIDs is described. In this type of SQUID, a superconducting half-wavelength microstrip resonator serves as tank circuit, into which the SQUID is integrated. For evaluation of this type of SQUID, samples were prepared from thin Nb films on sapphire substrates, using tunnel junctions as Josephson elements. When operated in hysteretic mode, signal voltages of up to 80 V were achieved, corresponding to a flux noise of 4×10^–6 ₀/Hz and an energy resolution of 2×10^–31 J/Hz.     

Nuclear moments and the change in the mean square charge radius of neutron deficient thallium isotopes

The hyperfine structure, isotope and isomeric shifts in the atomic transition 6p ² P _3/2–7s ² S _1/2, =535 nm have been measured for theI=7 andI=2 states of^{190, 192, 194, 196}Tl; theI=1/2 andI=9/2 states of¹⁹¹Tl and the I=7 isomer of¹⁸⁸Tl. The thallium isotopes were prepared as fast atomic beams at the GSI on-line mass separator following fusion reactions and — in some cases — subsequent-decay. The nuclear dipole moments, electric quadrupole moments and the change in the nuclear mean square charge radius are evaluated. Theuu-isotopes show an isomeric shift which changes sign between¹⁹²Tl and¹⁹⁴Tl.Dedicated to P. Armbruster on the occasion of his 60th birthday     

Wafer and epilayer improvement for integrated optics devices on InP: the ESPRIT project 2518

Ga _x In_1–x As _y P_1–y alloys lattice matched to InP substrates are currently used to fabricate optoelectronic and integrated optics devices. To achieve devices with high performances and high fabrication yield, the uniformity and reproducibility of the Ga _x In_1–x As _y P_1–y epitaxial layers (composition, thickness, doping, etc.) have become key parameters. These problems have been addressed in the frame of ESPRIT project 2518 and are presented in this paper. Several aspects have been considered starting from the optimization of InP substrates, the MOVPE growth of uniform GalnAsP layers, the material characterization to the validation of material uniformity on passive optical waveguides. Both scanning photoluminescence analysis and waveguide losses measurements performed on 2 inch wafers with a high lateral resolution have shown that high quality uniform GalnAsP layers can be obtained reproducibly on 2 InP substrates using a commercially available LP-MOCVD growth process. In particular, more than 60% of 36 mm long, 3m wide and 100m spaced rib waveguides exhibit losses below 0.8dBcm^–1.     

Upper limit for thermal direct photon production in heavy-ion collisions at 60 and 200A·GeV

The production of direct photons has been investigated in reactions ofp and¹⁶O projectiles at 60 and 200A·GeV with C and Au nuclei. Photon and ⁰ spectra have been measured in the pseudorapidity range 1.52.1 for the transverse momentum region 0.4 GeV/cp _T 2.8 GeV/c employing the lead-glass spectrometer SAPHIR. An upper limit of 15% at the 90% confidence level for the direct photon signal relative to the neutral pion production is obtained from the comparison of measured photon spectra with Monte Carlo simulations of the hadronic background based on the reconstructed yield of ⁰ and mesons. Consequences for a possible phase transition to a quark-gluon plasma are discussed.