YBA2CU3O7 AND YBA2(CU1-0.04FE0.04)3O-7 SINGLE-CRYSTAL THIN-FILMS - PREPARATION AND DETERMINATION OF THE GAP DISTRIBUTION

We report on far-infrared measurements of twinned YBa2(Cu1-0.04Fe0.04) 3O7 and YBa2Cu3O7 single crystal thin films oriented with the c-axis p erpendicular to the surface by using a reflection Fabry-Perot interfer ometer as a multireflection device. From these we could determine the dielectric function, the refractive index, the field penetration depth and the surface impedance of the material. The order of magnitude hig her sensitivity of this method compared to direct reflectance measurem ents allowed a more accurate gap determination. We find in the undoped 123 material an almost continuous gap distribution in the 70 - 215 cm -1 range together with a separate gap at about 330 cm-1. A quasi zero gap absorption is observed down to 20 cm-1 even at low temperatures. T he Fe doped sample shows a rearrangement of some parts of the gap dist ribution towards lower frequencies. A strong and sharp gap system is f ound just in the same range where earlier measurements on ceramics sho wed a gap.