Magnetooptic techniques coupled with high-speed cinephotography have been used to study flux jumps in a Nb-25 % Zr superconducting disk triggered by the application of an external magnetic field. It is shown that the exponential time constant z u which characterizes the penetration of the magnetic flux and the.flux jumpfield HIj are independent of the sweep rate of the applied magnetic field over three orders of magnitude. It is also demonstrated that magnetic coupling between the disk-shaped superconductor and a high-purity normal metal beneath it can significantly damp the motion of magnetic flux into the superconductor, thus enhancing its stability against flux jumps.
The thermal boundary resistivity between cerium magnesium nitrate and liquid He' has been measured over the temperature range 0.07-0.7 K. The resistivity can be well described as (55 + 10)/T' cm~K W ' which compares with 150/T cm KW ' as deduced from the Khalat~ov acoustic-mismatch theory of therimd boundary resistance. The theoretical values includes the surface-wave contribution. This agreement with acoustic-mismatch theory is in turn compared with that obtained for other materials, and is seen to be relatively good. Finally, the thermal-boundary-resistivity result has been used to reinterpret other experiments that measure the temperature decay following a change of magnetic field. It is shown that these experiments can be described by a relaxation process consistent, as far as size dependence, temperature dependence, and imagnitude are concerned, with a phonon bottleneck.
The specific heat of cold-rolled Nb-25% Zr has been measured from 2 to 11.6 K in zero magnetic field. The transition temperature has been found to be 10.9±0.05 K and the specific heat below the transition temperature is reasonably well represented by C=1954 exp(−1.874Tc/T) +0.16T3 mJ/mole K.
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