Nuclear spin relaxation and quasiparticle excitations in superconducting aluminum-3d alloys

Abstract

Aluminum-27 nuclear spin-lattice relaxation times T t have been measured in the normal and superconducting states of dilute alloys of Mn, Cr, and V in aluminum. These measurements prove the spectrum of thermally excited quasiparticle states in the superconductor and its modification by nonmagnetic resonant impurities. Theoretical studies of such systems indicate that in the Hartree-Fock (HF) approximation the BCS character of the host is basically retained in the alloy, with a reduced pairing interaction and a small additional broadening of quasiparticle states at the gap edge. Spin-fluctuation improvements on the HF approximation renormalize the parameters involved, but do not change the basic picture. Our TI data indicate gap edge broadening an order of magnitude greater than predicted. The discrepancy is nearly as large for V and Cr impurities as for Mn, which makes it unlikely that localized spin fluctuations play an important role in the broadening. Recent speculation on the effect of dislocation pinning by resonant impurities may be relevant to these results.

Critical temperature modification of low dimensional superconductors by spin doping

“…In case of resonant states, T c suppression is due to reduction of probability of a Cooper pair formation. The dependence of corresponding T c suppression rate on impurity concentration has weaker dependence compared to the magnetic pair-breaking mechanism [5][6][7][8][9][10][11][12][13][14][15] . One exception is non-magnetic, but VBS/spin fluctuating Mn in Al, 5,[10][11][12][13][16][17][18][19][20][21] which gives an initial T c suppression rate even stronger than magnetic Gd in La 22 .…”

“…The dependence of corresponding T c suppression rate on impurity concentration has weaker dependence compared to the magnetic pair-breaking mechanism [5][6][7][8][9][10][11][12][13][14][15] . One exception is non-magnetic, but VBS/spin fluctuating Mn in Al, 5,[10][11][12][13][16][17][18][19][20][21] which gives an initial T c suppression rate even stronger than magnetic Gd in La 22 . It is rather tempting to find out whether Fe and Mn impurities can provide distinct T c suppression in thin Al films compared to the bulk material in the limit of dilute impurity concentration.…”