INFRARED STUDY OF THE SUPERCONDUCTING PHASE-TRANSITION IN YBA2CU3O7-X

We observe a critical behaviour near T(c) in the infrared reflectivity of c-axis oriented Y1Ba2Cu3O7-x films prepared with pulsed in situ laser deposition. From an analysis of the temperature dependence of the spectra, which were measured with intervals of 5 K in a large temperature range around T(c) we obtain (1) the temperature dependence of the superfluid fraction, and (2) the temperature dependence of the absorption edge at 400 cm-1. We show that the superfluid fraction follows closely a 1 - (T/T(c))4 law. We show furthermore that the height of the absorption edge closely follows the behaviour of the superfluid fraction, whereas the energetic position of the edge is practically constant up to T(c). This behaviour can be understood in a semi-empirical way using a modified Gorter-Casimir two-fluid model using the concepts of a temperature independent gap and a temperature dependent density of the superconducting fraction. Above T(c) we observe a dip in the self-energy of the dielectric function at the position of the absorption edge existing in the superconducting state.