Penetration depth inYBa2Cu3O7thin films from far-infrared transmission

We compare the analytical expressions for the transmission and the surface resistance of a thin superconducting film and we point out the differences and similarities between the two quantities. We show that in the case of the transmission, this single quantity allows a straightforward determination of the electromagnetic penetration depth λ(T≪${\mathit{T}}_{\mathit{c}}$) at low temperatures. We illustrate this point by transmission measurements at different fixed far-infrared frequencies (4–60 ${\mathrm{cm}}^{\mathrm{-}1}$) in two purposely very different ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films. We find 2900 and 2000 Å, respectivley, for the absolute value of their electromagnetic penetration depth at T≪${\mathit{T}}_{\mathit{c}}$, which suggests an extrinsic origin of the field penetration in these samples. Such an assignment may further clarify the temperature dependence of the penetration depth which has been observed in thin films.