Submillimeter and microwave residual losses in epitaxial films of Y-Ba-Cu-O and Tl-Ca-Ba-Cu-O

We have used a novel bolometric technique and a resonant technique to obtain accurate submillimeter and microwave residual loss data for epitaxial thin films of YBa₂Cu₃O₇, Tl₂Ca₂Ba₂Cu₃O₁₀, and Tl₂CaBa₂Cu₂O₈. For all films we obtain good agreement between the submillimeter and microwave data, with the residual losses in both the Y-Ba-Cu-O and Tl-Ca-Ba-Cu-O films scaling approximately as frequency squared below ∼1 THz. We are able to fit the losses in the Y-Ba-Cu-O films to a weakly coupled grain model for the a-b plane conductivity, in good agreement with results from a Kramers-Kronig analysis of the loss data. We observe strong phonon structure in the Tl-Ca-Ba-Cu-O films for frequencies between 2 and 21 THz, and are unable to fit these losses to the simple weakly coupled grain model. This is in strong contrast to the case for other high-T_c superconductors such as YBa₂Cu₃O₇, where phonon structure observed in ceramic samples is absent in epitaxial oriented films and crystals because of the electronic screening due to the high conductivity of the a-b planes.