Reduction of surface resistance in superconducting thin films by microwave irradiation

We have experimentally observed dramatic reductions in the microwave surface resistance R
s
of superconducting aluminum thin films due to microwave pumping. For temperatures T > 0.97 T
c
, nearly complete suppression of the temperature-dependent component of R
s
occurred when the films were subjected to a microwave magnetic field < 1 Oe at 11.4 GHz. Contrary to theoretical predictions, several abrupt transitions to superconducting states with lower R
s
were observed before the transition to the normal state. The reduction of R
s
may provide stability against thermal breakdown in rf cavities currently limited by defect-nucleated quench.