Relationship between electrical properties and crystallinity of YBa/sub 2/Cu/sub 3/O/sub 7-x/ ultrathin films

The X-ray data, DC and microwave properties as functions of the YBaCuO epitaxial films thickness in the range from 5 to 300 nm are investigated. The films were deposited by laser ablation on LaAlO/sub 3/ substrates. Films over 9 nm thickness have T/sub c0/=89 K, J/sub c/(77 K)>3 MA/cm/sup 2/ and the thickness dependence of their effective microwave surface resistance R/sub eff/ at 77 K, 10 GHz is in good agreement with the local electrodynamic theory for the thickness-independent intrinsic surface resistance R/sub s/=0.25 m/spl Omega/ and penetration depth /spl lambda/=250 nm. Decreasing a film thickness to less than a few tens nanometers provides a decrease of orthorhombicity parameter of YBCO cell, and to less than 9 nm - a reduction of transition temperature. The ultrathin films showed no degradation during thermocycling without a protection layer.