Tellurium‐doped GaAs was grown under reduced gravity during the Second German Spacelab‐Mission D2. The growth experiment MD‐ELI‐TRABE is described and first results are presented.
In order to elucidate the nature of spin relaxation in nematic liquid crystals in the vicinity of the nematic-isotropic phase transition temperature, we present extensive measurements and a quantitative analysis of the proton spin, T,, relaxation dispersion for some homologues of the PAA series (PAA, PAB, HAB). The studies were made for the nematic state over a broad range of Larmor frequencies. By a detailed inspection of the frequency, temperature and chain length dependences we conclude that in all cases at least three kinds of molecular reorientations with rather different relaxation dispersion contribute to the total TI, i.e., order fluctuations of the director (OFN), self-diffusion (SD) and an additional mechanism, which we attribute to order parameter fluctuations (OFS). The combined OFN-SD-OFS model extends existing interpretations and allows a quantitative description of the observed phase transition effects. The effects of both OFN and OFS on TI clearly increase with the molecular length and we deduce from this that not only the ring protons, but also some chain protons undergo these types of fluctuation. Due to the rather high temperature of the mesophases, the diffusional contribution is hard to evaluate exactly and hence there are no significant variations between the homologues within the limits of experimental error. However, a small systematic discrepancy with respect to independent diffusion measurements, available for PAA, indicates that this relaxation model is still incomplete and that a fourth mechanism should be included in the analysis. Possibly, this could be some kind of rotation as previously reported for lowtemperature nematics.
A Tc-doped GaAs (100) single crystal of 20 mm diameter and 20 mm length was successfully grown by the floating-zone method during the 2nd German Spacelab mission D2. Dopant striations due to Marangoni convection as well as a reduction of the etch pit density compared to the starting material have been found. The last part of the crystal, grown with the quite high translation rate of 120 mm/h, showed no sign of unstable growth and exhibited properties similar to the parts grown with 3 and 6 mmih.WBhrend der zweiten deutschen Spacelabmission D2 konnte ein tellurdotierter (100) GaAs-Einkristall mit 20 mm Llnge und 20 mm Durchmesser nach der Floating-Zone-Methode gezuchtet werden. Der Kr/stall zeigt Dotierstoffstreifen durch zeitabhangige Marangonikonvektion sowie cine Reduktion der Atzgrubendichte gegeniiber dem Ausgangsmaterial. Der mit der hohen Ziehgeschwindigkeit von 120 mm/h geziiehtete letzte Abschnitt des Kristalls lie8 keinerlei Wachstumsinstabilitlten erkennen und zeigte Materialeigenschaften Bhnlich denen der mit 3 bzw. 6 mm/h geziichteten Bereiche.
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