Electronic Energy Levels in High-Temperature Superconductors

Parent materials of high-temperature superconductors (HTSC) need to be doped to become superconducting. The optimum doping for maximum critical transition temperature T c has been analyzed for more than 20 materials. Assuming a uniform doping distribution the distance x between doped unit cells—projected into the CuO 2 plane for cuprates—shows a strong linear correlation to the inverse of T c in the form (2 x ) 2 = m 1 1/ T c with a slope of m 1 =(2.786±0.029)×10 −15 m 2 K. The mercury cuprate homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+ δ with n =1,2,3 has been used to demonstrate the procedure deriving the doping distance x from the optimum doping value δ .