High-temperature superconductivity without doping-synthesis of conceptually new superconductors

Abstract

High-temperature superconductivity is commonly associated with cuprate superconductors as their superconducting transition temperatures are still highest among all of the superconducting materials. It is these cuprate superconductors that drive scientists to reveal the very mechanism of its superconductivity and to derive models capable of predicting new superconducting material systems. Cuprate superconductors are built up of copper-oxygen planes, sandwiched by rare-earth or alkaline-earth oxide layers. The copper-oxygen layers form infinitely long planes, and it is those planes where superconductivity takes place. In particular, the copper ions in those planes may have three distinct coordinations, i.e., octahedral, pyramidal, and square-planar. Quite generally, electronic states are a function of crystal structure, crystal symmetry, and the elements building up the crystal structures. Nevertheless, it has been widely assumed that the copper-oxygen planes are inherently insulating and superconductivity is induced by doping carriers into those copper-oxygen planes. Indeed, such an approach is suitable for the copperoxygen layers with octahedral and pyramidal coordinated copper, where superconductivity is induced by hole doping. For cuprates with square-planar coordinated copper, however, we demonstrate here that elimination of defects by annealing is the key to inducing superconductivity and doping is not a prerequisite for the emergence of superconductivity.