Large-Area YBa₂Cu₃O_{7 − x} Films on Sapphire with Excellent Microwave Power Handling Capability

Abstract

Crack-free thick YBa₂Cu₃O_{7 − x} films are prepared on CeO₂ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (“peak-to-valley” roughness <10 nm), high critical currents (J _C > 2 MA/cm² at 77 K and 0 T), and low microwave surface resistances (R _s(77K) ≍.4mω and R _s(4.2K) ≍.110μω at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B _HF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa₂Cu₃O_{7 − x} films, was achieved without any degradation of the samples despite frequent thermal cycling.