Computational materials design of attractive Fermion system with large negative effective U in the hole-doped Delafossite of CuAlO₂, AgAlO₂ and AuAlO₂

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In order to realize super-high-critical temperature (Tc) superconductors (Tc>1,000 K) based on general design rules for negative effective U (Ueff) systems by controlling purely-electronic and attractive Fermion mechanisms, we perform computational materials design for the negative Ueff system in hole-doped two-dimensional (2D) Delafossite CuAlO₂, AgAlO₂ and AuAlO₂ from abinitio calculations. It is found that the large negative Ueff in the hole-doped attractive Fermion systems for CuAlO₂ (Ueff = -4.53 eV), AgAlO₂ (Ueff = -4.88 eV), AuAlO₂ (Ueff = -4.14 eV). These values are 10 times larger than that in hole-doped three-dimensional (3D) CuFeS₂ (Ueff = -0.44 eV). For future calculations of the Tc and phase diagram by quantum Monte Carlo simulations, we propose the negative Ueff Hubbard model with the anti-bonding single π-band model for CuAlO₂, AgAlO₂ and AuAlO₂ by using the parameters obtained from abinitio electronic structure calculations. The behavior of Tc in the 2D Delafossite of CuAlO₂, AgAlO₂ and AuAlO₂ and 3D Chalcopyrite of CuFeS₂ shows the interesting chemical trend, i.e., Tc increases exponentially in the weak coupling regime |Ueff| < W (2 eV) (where W is the band width of Hubbard model) for the hole-doped CuFeS₂, and then Tc goes through a maximum when |Ueff| W (2.8 eV, 3.5 eV) for the hole-doped AgAlO₂ and AuAlO₂, and finally Tc decreases with increasing |Ueff| in the strong coupling regime, where |Ueff| > W (1.7 eV), for the hole-doped CuAlO₂. In this strong coupling regime, one can expect that Tc = 1,000 2,000 K by assuming the relation of the very strong coupling as 2Δ/ k BTc = 50 100 and the superconducting gap Δ |Ueff| = 4.53 eV 50,000 K.