Search for Dislocation Free Helium 4 Crystals.
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We found dislocation densities that are in the range of 10[Formula: see text]-10[Formula: see text] per cm[Formula: see text], that is several orders of magnitude larger than Ruutu et al.Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations.As for the dislocation nucleation mechanism, it remains to be understood.
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Affiliation: Laboratoire de Physique Statistique de l'École Normale Supérieure associé au CNRS et aux Universités P.M. Curie et D. Diderot, 24 rue Lhomond, 75231 Paris Cedex 05, France.
ABSTRACT
The giant plasticity of [Formula: see text]He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996-1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm[Formula: see text], sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10[Formula: see text]-10[Formula: see text] per cm[Formula: see text], that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood. No MeSH data available. Related in: MedlinePlus |
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Mentions: For every oriented crystal, a numerical model of the resonant cavity was produced with COMSOL software and used to predict the dependence of the resonance frequency on the value of . An example for the crystal P3 is shown on Fig. 3, for which the sixfold axis of symmetry is almost along to the axis. For this crystal, when is varied from zero to its intrinsic value , the resonance frequency varies almost linearly from to kHz. This calibration curve is subsequently used to convert the measured resonance frequencies into values. The accuracy of the numerical model was tested with several crystals in a fully pinned regime, where the value of is equal to the intrinsic value measured by Greywall [16] at K and MHz. Among different crystal orientations, the disagreement between the predicted value and the measured value of the resonance frequency never exceeded %. For example, for the measurement presented in Fig. 2, the measured frequency is kHz, while the predicted value is kHz. These % are likely due to a slight error in the cell size measurement and, in any case, compatible with Greywall’s error bars. This agreement confirms the validity of our numerical calculation.Fig. 3 |
View Article: PubMed Central - PubMed
Affiliation: Laboratoire de Physique Statistique de l'École Normale Supérieure associé au CNRS et aux Universités P.M. Curie et D. Diderot, 24 rue Lhomond, 75231 Paris Cedex 05, France.
The giant plasticity of [Formula: see text]He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996-1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm[Formula: see text], sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10[Formula: see text]-10[Formula: see text] per cm[Formula: see text], that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood.
No MeSH data available.