Low-Field Cross Spin Relaxation Of Li-8 In Superconducting Nbse2

A low energy beam of spin polarized Li-8 has been used to investigate nuclear spin relaxation in the multiband superconductor NbSe2. In low magnetic fields there is significant cross relaxation between the Li-8 and the host Nb-93 spins, which is driven by low frequency fluctuations in the nuclear magnetic dipolar interaction. The rate of cross relaxation is strongly field dependent and thus the 1/T-1 spin relaxation rate of the Li-8 is a sensitive monitor of the static local magnetic field B just below the surface. This in turn is used to determine the absolute value of the magnetic penetration depth lambda in the Meissner state. The temperature variations in 1/T-1 and lambda are consistent with a wide distribution of superconducting gaps expected for a multiband superconductor.