  Evidence for quantized magnetic vortex entry at a defect into a Type-II
superconductor operating in the metastable state above Bc1 while under the
influence of RF electromagnetic fields has been observed in a polycrystalline
coating of Nb3Sn. The measurement was made using a 1.3 GHz microwave cavity for
use in particle accelerator R&D and equipped with a temperature mapping system
for precision measurements of local heating. At peak RF fields near the quench
field of the cavity, discrete jumps in local heating at the quench origin were
observed. A number of these jumps were found to be identical in height within
measurement error. This suggests entry of a limited number of vortices at a
defect. The behavior observed appears to be consistent with recent theoretical
predictions regarding vortex entry at grain boundaries in the material.
