Structural and electrical properties of PrBa2Cu3-xGaxO7 (x=0.2) thin films and [PrBa2Cu3-xGaxO7)(M)/(YBa2Cu3O7)(N)](P) superlattices grown by pulsed laser deposition

The low temperature resistivity (T < 200 K) of PrBa2Cu3-xGaxO7 (x = 0.2) thin films grown by pulsed laser deposition follows the Mott variable range hopping law. The positive lattice mismatch to SrTiO3 (Delta a/a = +0.15%) leads to alternately strained pseudomorphic superlattices with YBa2Cu3O7, when the elementary layer is ultra-thin (less than or equal to 5 nm). Chemical diffusion of gallium in YBa2Cu3O7/PrBa2Cu3-xGaxO7 heterostructures is neither detected from the refinement calculations of the XRD spectra nor from SIMS profile analysis. The T-c lowering of the superlattices is explained in terms of charge transfer effect. The shortest period superlattices without Josephson coupling between the YBa2Cu3O7 layers (M > 3, N less than or equal to 4) show Kosterlitz-Thouless bidimensional fluctuations due to the strong anisotropy of the structure.