Softening of Al Single Crystals due to Superconducting Transition

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Abstract
Using a 3 He cryostat, the flow stress increment Δτ sn associated with superconducting-normal transition has been investigated in the temperature range between 0.5 K and T c (1.183 K) for Al single crystals of three different purities, i.e., 99.999%, 99.99% and 99.7%. It was found that Δτ sn increased as the purity decreased and that in all cases values of Δτ sn in stage II were smaller than those in stage I in contrast to the results on Pb single crystals. The temperature dependence of Δτ sn for each deformation stage roughly agreed with that of the electron drag coefficient obtained from the BCS theory, i.e., Δ B e = B e n (1-Γ), Γ=2(1+exp (Δ T )/ k T )) -1 . The temperature dependence of flow stress below T c was positive in the superconducting state, while it is slightly negative in the normal state.