General Rule and Materials Design of Negative Effective U System for   High-T_c Superconductivity
  Based on the microscopic mechanisms of (1) charge-excitation-induced negative effective U in s^1 or d^9 electronic configurations, and (2) exchange-correlation-induced negative effective U in d^4 or d^6 electronic configurations, we propose a general rule and materials design of negative effective U system in itinerant (ionic and metallic) system for the realization of high-T_c superconductors. We design a T_c-enhancing layer (or clusters) of charge-excitation-induced negative effective $U$ connecting the superconducting layers for the realistic systems. 
Hiroshi Katayama-Yoshida, Koichi Kusakabe, Hidetoshi Kizaki, and   Akitaka Nakanishi