Softening of Al Single Crystals due to Superconducting Transition

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.