YBa 2Cu 3O 7 and YBa 2(Cu 1−0.04Fe 0.040) 3O 7 single crystal thin films — Preparation and determination of the gap distribution

Abstract

We report on far-infrared measurements of twinned YBa 2(Cu 1-0.04Fe 0.04) 3O 7 and YBa 2Cu 3O 7 single crystal thin films oriented with the c-axis perpendicular to the surface by using a reflection Fabry-Perot interferometer 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 higher sensitivity of this method compared to direct reflectance measurements 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. The Fe doped sample shows a rearrangement of some parts of the gap distribution towards lower frequencies. A strong and sharp gap system is found just in the same range where earlier measurements on ceramics showed a gap.