Artificially prepared YBa₂Cu₃O₇ PrBa₂Cu₃O₇ superlattices: Growth and superconducting properties

We report on the growth and superconducting properties of artificially prepared YBa₂Cu₃O₇ PrBa₂Cu₃O₇ ( YBCO PrBCO) superlattices. We have grown these superlattices by single target dc planar magnetron sputtering using the high sputtering pressure technique to obtain the in situ growth with the correct stoichiometry. We find in these modulated structures that the superconducting critical temperature, T_c, depends markedly on the individual YBCO thickness in the multilayer and on the insulating PrBCO separation thickness. For series of samples with constant 12Å or 24Å layers of YBCO (one or two unit cells), T_c initially decreases with increasing thickness of PrBCO, d-PrBCO, and then saturates for d-PrBCO larger than 60-70Å. We discuss this behavior in terms of a progressive decoupling of the ultrathin YBCO layers in the multilayer. Resistively measured parallel critical fields show, for structures with thick PrBCO layers, a transition from a flux flow regime for thick YBCO layers to a behavior with only very weak magnetic field induced broadening of the resistive transitions for thin YBCO layers. For multilayers with individual YBCO layers of 48Å or 96Å thickness we find a linear behavior of the resistively determined critical field with a slope of ≅7.5T/K. When the individual YBCO layer thickness is reduced to 24Å practically no effect of a 9T magnetic field could be observed. These different regimes are discussed in terms of the particular vortex structures in the multilayers.