DIRECT SUBMILLIMETER ABSORPTIVITY MEASUREMENTS ON EPITAXIAL BA1-XKXBIO3 FILMS AT 2K

We have used a bolometric technique to obtain accurate low temperature loss data for epitaxial thin films of Ba0.6K0.4BiO3 from 30 cm-1 to 7 00 cm-1. These films were grown on MgO and SrTiO3 substrates by MBE, o ff-axis sputtering and laser deposition techniques. All films show a s trong absorption onset near the BCS tunneling gap of 3.5k(B)T(C). We h ave analyzed these data using a Kramers-Kronig transformation and have corrected for finite film thickness effects. Our results indicate tha t the absorption onset is consistent with a superconducting energy gap . Comparison is made with predictions based on strong coupling Eliashb erg theory using alpha2F(omega) spectra obtained from the literature. While we are able to fit the overall measured absorptivity, we are una ble to fit the structure observed in our data.