Superconducting properties of ultra-thin YBa2Cu3O7 layers in (PrxY1-x)Ba2Cu3O7/YBa2Cu3O7/(PrxY1-x)Ba2Cu3O7 trilayer structures

We have studied the superconducting properties of ultra-thin YBa2Cu3O7 layers using trilayers of YBa2Cu3O7 sandwiched between semiconducting (PrxY1-x)Ba2Cu3O7 layers. We find that superconducting parameters such as the transition temperature T-c, the upper critical field H-c2(0), and the Pippard coherence length xi(0) deviate from bulk values when the YBa2Cu3O7 layer is thinner than four unit cells. The dependence of T-c on the thickness of YBa2Cu3O7 with different Pr composition x of the (PrxY1-x)Ba2Cu3O7 adjacent layers shows a similar behavior, even though T-c depends strongly on x. We observed a linear temperature dependence of H-c2(T) from 1 to 100 unit-cell thick YBCO layers. The plot of the Pippard coherence length xi(0) versus 1/T-c(R/R(n) = 0.1) of 1-8 unit-cell YBCO shows a linear relationship suggesting no appreciable change in Fermi velocity.