Mott Transitions and d-wave Superconductivity in Half-Filled-Band   Hubbard Model on Square Lattice with Geometric Frustration
  Mechanisms of Mott transitions and dx2-y2-wave superconductivity (SC) are studied in the half-filled-band Hubbard model on square lattices with a diagonal hopping term (t'), using an optimization (or correlated) variational Monte Carlo method. In the trial wave functions, a doublon-holon binding effect is introduced in addition to the onsite Gutzwiller projection. We mainly treat a d-wave singlet state and a projected Fermi sea. In both wave functions, first-order Mott transitions without direct relevance to magnetic orders take place at U=Uc approximately of the bandwidth for arbitrary t'/t. These transitions originate in the binding or unbinding of a doublon to a holon. d-wave SC appears in a narrow range immediately below Uc. The robust d-wave superconducting correlation are necessarily accompanied by enhanced antiferromagnetic correlation; the strength of SC becomes weak, as t'/t increases. 
Hisatoshi Yokoyama, Masao Ogata and Yukio Tanaka