Interlayer coupling in ultrathin high TC artificial structures

Artificially layered superlattices were grown, by stacking in sequence ultrathin epitaxial layers of (Ba(0.9),Nd(0.1))CuO(2+x) and CaCuO(2). Starting from [(Ba(0.9),Nd(0.1))CuO(2+x)](m)/[CaCuO(2)](2) superlattices with m = 2, the thickness of the Ba-Cu-O block, separating adjacent Ca-Cu-O superconducting blocks, was gradually increased up to m = 10. We found that, increasing m, superlattices show a moderate decrease of the critical temperature and a sizeable increase of the anisotropy. In the superlattices with m greater than or equal to4 the superconducting layers are completely decoupled, showing, relative to the m = 2 superlattice, a decrease of the transition temperature less than 10% and an increase of the anisotropy by a factor similar to2-3. Superconductivity at 55 K was found in ultrathin films only consisting of one (CaCuO(2))(2) layer sandwiched between five (Ba(0.9),Nd(0.1))CuO(2+x) layers, demonstrating that the interlayer coupling in m greater than or equal to4 superlattices is negligible.