The impact of nuclear magnetism on superconductivity

The influence of nuclear magnetism on the superconducting critical field B- c has been measured in the nuclear magnets Al, AuIn2, and Sn, at ultralow t emperatures, 17 mu K much less than T much less than I It. The materials ha ve been chosen in respect to their distinctive Korringa constants kappa(Al, AuIn2, Sn) = (1.8.0.1,0.05) Ks. Both in the weakly and in the strongly cou pled nuclear magnets, Al and Sn, a reduction of B-c is observed being propo rtional to the nuclear magnetization at T less than or equal to 4 mK. Howev er using the high resolution of the experiment performed on Sn, the decreas e of B-c at cooling down to 70 mu K is found to be nonmonotonic. The featur es of the recently measured B-c( T) curve of Sn which are not understood ye t as well as the formerly observed results of the interplay of nuclear ferr omagnetism and superconductivity in AuIn2 have motivated ongoing experiment s which try to clear up the pair breaking contribution of nuclear magnetism . (C) 2000 Elsevier Science B.V. All rights reserved.