  We report beta-NMR investigations of polarized 8Li implanted in thin Pb and
Ag/Nb films. At the critical superconducting temperature, we observe a singular
peak in the spin relaxation rate in small longitudinal magnetic fields, which
is attributed to fluctuations in the superconducting order parameter. However,
the peak is more than an order of magnitude larger than the prediction based on
the enhancement of the dynamic electron spin susceptibility by superconducting
fluctuations and reflects the presence of unexpected slow fluctuations.
Furthermore the fluctuations are rapidly suppressed in a small magnetic field,
which may explain why they have not been observed previously with conventional
NMR or NQR.
