Surface-Resistance Of Yba2cu3o7 Films Deposited On Lagao3 Substrates

The surface impedance of several c-axis oriented YBa 2 Cu 3 O 7 thin-film samples was measured in a copper host cavity at 87 GHz between 4.2 K and 300 K. High quality films between 0.17 μm and 0.8 μm in thickness have been grown epitaxially on SrTiO 3 by pulsed excimer laser ablation. The partial penetration of the electromagnetic fields into the substrate leads to an enlarged surface resistance R s and reactance X S . By means of numerical simulations for the effective surface impedance of the film-substrate sandwich, the true values for R S at T C ∼ 88 K to less of YBa 2 Cu 3 O 7−x are extracted from the data. The best samples show a sharp drop of R S at T C ∼ 88 K to less than 10 mΩ at 77 K . These values exceed the best results ib polycrystalline samples and come close to the expectation from classical superconductors. Experimental results on X S for different thin-film samples are in good agreement to the numerical results if a temperature dependence of the penetration depth λ (T) according to the BCS-theory in the weak coupling limit is assumed. The extrapolated λ(0)-value is (160±20) nm.