Millimeter-wave absorption in the new high-$T_c$ superconducting materials La-Ba-Cu-O and Y-Ba-Cu-O has been studied at a frequency of 80 GHz over the temperature range 4-300 K and in the presence of magnetic fields 0-5.5 T. It is found that there are strong correlations between the changes in microwave absorption and the superconducting transitions at 30 K for La-Ba-Cu-O and at 90 K for Y-Ba-Cu-O samples. The Y-Ba-Cu-O samples display additional microwave features near 250 K which may be associated with a higher-temperature superconductivity.

• Received 14 September 1987

DOI:https://doi.org/10.1103/PhysRevB.37.615