Surface resistance and residual losses of Ag-doped YBa2Cu3O7−δ thin films on sapphire

High-quality Ag-doped YBa2Cu3O7−δ thin films have been grown by laser ablation on R-plane 〈11̄02〉 sapphire without any buffer layer. Thin films have been found to be highly c-axis oriented with Tc=90 K, transition width ΔT≤1 K, and transport Jc=1.2×106 A cm−2 at 77 K in self-field conditions. The microwave surface resistance of these films measured on patterned microstrip resonators has been found to be 530 μΩ at 10 GHz at 77 K which is the lowest reported on unbuffered sapphire. Improved in-plane epitaxy and reduced reaction rate between the substrate and the film caused due to Ag in the film are believed to be responsible for this greatly improved microwave surface resistance.