Large-area YBCO films on sapphire for microwave applications

Crack-free thick YBa(2)Cu(3)O(7-x) films are prepared on CeO(2), buffered r-cut sapphire (2 inch in diameter) with thickness up to 700nm, smooth surfaces ('peak-to-valley' roughness < 10 nn), high critical currents (J(c) > 2 MA/cm(2) at 77 K and 0T) and low microwave surface resistance (R(s)(77K)approximate to 1.4 m Omega and R(s)(4.2K) approximate to 110 mu Omega at 19 GHz), comparable to the best values reported for: YBCO films on structurally better matched substrates. The improvement of the critical thickness of YBa(2)Cu(3)O(7-x) on structurally poorly matched sapphire from typically d(c)approximate to 250-300nm to d(c)>700nm is achieved by introducing defects which do not hamper the superconducting properties of the films. The modified film grow-th was reflected by slightly broadened rocking curves and an altered temperature dependence of R(s). The thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B(HF)) of at least 54 and 37mT at 4.2 and 50K (limited by the available power), and 17mT at 77K (often limited by quenches). The excellent high-power performance was achieved without any degradation of the samples despite frequent thermal cycling.