FAR IR STUDY OF THE LONDON PENETRATION DEPTH FROM 5 TO 70 K, IN A VERY THIN-FILM OF YBA2CU3O7-DELTA - COMPARISON WITH NBN, A ONE GAP SUPERCONDUCTOR

The far-IR transmission study of the classical superconductor NbN is e asier than for YBa2Cu9O7-delta, and shows a maximum at a frequency cor responding to the gap. For YBa2Cu3O7-delta the transmission powers con sidered are weaker and new experimental methods are needed. Here the P erot-Fabry resonances are used inside the MgO substrate plate. The Lon don penetration length is shown to be proportional to the square theta (2) of the temperature; the study also suggests a gap distribution.