MAGNETIC-FIELD-INDUCED RECOVERY OF SUPERCONDUCTIVITY IN EPITAXIAL YBA2CU3O7-X FILMS AT MICROWAVE-FREQUENCIES

An anomalous magnetic field-effect has been observed in the microwave surface impedance of epitaxial YBa2CU3O7-x films on LaAlO3 substrates. The application of de and microwave magnetic fields up to 20 mT cause d a significant decrease of microwave surface reactance X-s (less than or equal to 3%) and resistance R-s (less than or equal to 20%) at 8.5 GHz, 77 K and at 87 GHz, 4.2 K. The field effects on X, and R, were f ound to correlate within the framework of the two-fluid model. The obs erved effect is attributed to the presence of magnetic impurities in t he superconducting films, giving rise to pair breaking. Alignment of t he impurity spins suppresses the spin-flip scattering rate and, as a r esult, leads to a recovery of microwave superconductivity.