High TC superconductivity in engineered cuprate heterostructures

Given the layered structure of cuprate high-transition-temperature superconductors (HTS), essentially constituted by infinite layer (IL) blocks, containing the CuO 2 planes, separated by charge reservoir (CR) blocks, the use of heterostructures and superlattices, made by stacking HTS with different magnetic and conductivity properties (metallic, superconducting, insulating), such as YBa 2 Cu 3 O 7−x, PrBa 2 Cu 3 O 7−x, and Bi 2 Sr 2 Cu n Ca n−1 O x, have been extensively exploited to investigate many fundamental issues in HTS physics. Successively, superconducting heterostructures made by a metallic (but non-superconducting) cuprate and an insulating cuprate have been successfully synthesized, such as CaCuO 2 /BaCuO 2+x and La 2 CuO 4 /La 2−x Sr x CuO 4, with the idea to mimic the intrinsic IL/CR structure of HTS, revealing the circumstance that interface effects are important for the occurrence of high T c superconductivity. The discovery of 2D electron gas at the interface between two insulating perovskites induced us to develop a new kind of heterostructures based on an insulating cuprate with IL structure, CaCuO 2 (CCO), and an insulating perovskite, SrTiO 3 (STO), with the idea that the 2D electron gas eventually formed at the interface could dope the CuO 2 planes in CaCuO 2 block, giving rise to high T c superconductivity. Indeed, CCO/STO superlattices are superconducting at T c = 50 K. The doping of CCO occurs thanks to extra oxygen ions, which are incorporated at the CCO/STO interface during the growth in strongly oxidizing conditions. The CR role is thus here played by the interface layer, and the superconductivity is confined at the interface within few CCO unit cells. Structural features of these heterostructures can be engineered in a wide range and, consequently, their superconducting properties are studied. The highlights of these investigations are reviewed here. © Springer International Publishing Switzerland 2015.