Millimeter wave surface resistance of epitaxially grown YBa2Cu3O7−x thin films

Abstract
We have measured the surface resistance of two c‐axis oriented YBa2Cu3O7−x thin‐film samples in a copper host cavity at 86.7 GHz between 4.2 and 300 K. High quality films of 0.6 and 0.4 μm thickness have been grown epitaxially on SrTiO3 by pulsed excimer laser ablation. Their millimeter wave absorption drops sharply at a transition temperature of 86 and 88 K to a corresponding surface resistance at 77 K of 18 mΩ and less than 8 mΩ, respectively. These values exceed the best results on polycrystalline samples and come close to the expectation from classical superconductors. Therefore, applications of high Tc superconductors up to THz frequencies can be envisaged now.