Computational materials design of negative effective U system in hole-doped chalcopyrite CuFeS2

A general rule of negative effective U (U-eff) system caused by (i) exchange correlation and (ii) charge excitation mechanisms is proposed. Based on the general rule, we perform ab initio electronic structure calculations by generalized gradient approximation (GGA)+U method for hole-doped chalcopyrite CuFeS2 [Cu+(d(10))Fe3+(d(5))S2-(s(2)p(6))(2)]. It is found from our calculations that the hole-doped CuFeS2 has the negative U-eff = -0.44 eV, where U-eff = E(N + 1) + E(N - 1) - 2E(N) < 0 and E(N) is the total energy of the hole-doped CuFeS2. The negative U-eff is caused by the charge-excitation in the hole-doped Cu2+(d(9)) and S-(s(2)p(5)), and also caused by the exchange-correlation in the hole-doped Fe4+(d(4)). The strong attractive electron-electron interaction (U-eff = -0.44 eV similar to - 5000 K) originates from the purely electronic mechanism. The closed shell of the d(10) electronic configuration is more stable than the d(9) electronic configuration, since the first excited state with the d(9)s(1) electronic configuration and the ground state with the d(10) electronic configuration are very close, then these two states repel very strongly through the second order perturbation. Therefore, the spin-polarized total energy curve for the hole-doped CuFeS2 shows the strong upward convexity with N - 1, N and N + 1 electronic configurations leading to the negative U-eff. The hole-doped paramagnetic and metallic CuFeS2 with the negative U-eff may cause a possible high-T-c superconductor (T-c similar to 1000 K, if 2 Delta/k(B)T(c) approximate to 10 by assuming a strong coupling regime) because of the strong attractive electron-electron interactions (superconducting gap Delta approximate to vertical bar U-eff vertical bar similar to 5000 K). Finally, we propose a new computational materials design methodology to design ultra high-T-c superconductors by using three steps starting from the atomic number only.