Superconductivity and interlayer coupling in ultrathin artificially layered cuprates S. Lavanga * , G. Balestrino, P.G. Medaglia, P. Orgiani, A. Tebano INFM, Dipartimento Scienze e Tecnologie Fisiche ed Energetiche, Universit  a di Roma Tor Vergata, Via di Tor Vergata 110, 00133 Roma, Italy Abstract 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  m superlattices were grown, by pulsed laser deposition, stacking epitaxially m layers of ðBa 0:9 Nd 0:1 ÞCuO 2þd and two layers of CaCuO 2 . Starting from superlattices with m ¼ 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 ¼ 10. In the superlattices with m > 4 the superconducting layers are completely decoupled, showing, relative to the m ¼ 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. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Ultrathin films; Interlayer coupling; Superlattices; Multilayers All existing high-T c (HTS) cuprates exhibit a layered structure, with a stacking sequence of two distinct structural sub-units having different func- tions: the BO–A m O d –BO block and the ðCuO 2 Þ n – D n1 block, where A, B, D represent appropriate cations [1]. The BO–A m O d –BO sub-unit, usually named the charge reservoir (CR) block, easily incorporates extra-oxygen ions, uncompensating the electrical charge of the whole structure. It is commonly accepted that some of the excess charge carriers (generally holes) can be transferred to the ðCuO 2 Þ n –D n1 layer, named the infinite layer (IL) block, giving rise to superconductivity in the otherwise insulating CuO 2 planes [1]. Due to the extremely short superconducting coherence length along the c-axis, adjacent IL conducting blocks are only weakly coupled, so that HTS materials show a quasi two-dimensional character in a large number of their physical prop- erties. The zero-temperature coherence length of YBa 2 Cu 3 O 7d (YBCO),forinstance,is2–3  Aalong the c-axis (15  A in the a–b plane), much shorter than the c-axis lattice parameter (11.7  A). Al- though all HTS materials are strongly anisotropic, Physica C 372–376 (2002) 590–595 www.elsevier.com/locate/physc * Corresponding author. Fax: +39-6-7259-7145. E-mail address: simone.lavanga@uniroma2.it (S. Lavanga). 0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4534(02)00802-X