Superconductivity and interlayer coupling in ultrathin artificially layered cuprates

摘要

A structural requirement for high-T_c (HTS) cuprates is the simultaneous presence in a layered structure of two different functional blocks, individually non-superconducting: the charge reservoir (CR) block and the infinite layer (IL) block. Artificially layered 2 x m superlattices were grown, by pulsed laser deposition, stacking epitaxially m layers of (Ba_(0.9)Nd_(0.1))CuO_(2+δ) and two layers of CaCuO_2. Starting from superlattices with m = 2, the thickness of CR block, namely the Ba-Cu-O block, which separates adjacent Ca-Cu-O IL blocks, was gradually increased up to m = 10. In the superlattices with m > 4 the superconducting layers are completely decoupled, showing, relative to the m = 2 su-perlattice, a negligible decrease of the transition temperature and a strong increase of the anisotropy. Superconductivity at 55 K was found in both the ultrathin films, only consisting of two CR blocks sandwiching a (CaCuO_2)_2 layer, and the superlattices with thick CR blocks, demonstrating that the interlayer coupling in such heterostructures is unimportant. Moreover it has been shown that a single CaCuO_2 layer is still superconducting.