Reduction of surface resistance in superconducting thin films by microwave irradiation

We have experimentally observed dramatic reductions in the microwave surface resistance Rsof superconducting aluminum thin films due to microwave pumping. For temperatures T > 0.97 Tc, nearly complete suppression of the temperature-dependent component of Rsoccurred 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 Rswere observed before the transition to the normal state. The reduction of Rsmay provide stability against thermal breakdown in rf cavities currently limited by defect-nucleated quench.