t'- and t"-dependence of the bulk-limit superconducting condensation   energy of the 2D Hubbard model
  The 2D Hubbard model having the 2nd- and 3rd-neighbor transfer energies t' and t" is investigated by use of the variational Monte Carlo method. At the nearly optimal doping with on-site Coulomb energy U=6 (energy unit is t) the condensation energy Econd for the d-wave superconductivity (SC) is computed for lattices of sizes from 10x10 to 28x28 with the aim to get its bulk-limit value. t" is fixed at -t'/2. Outside and in the neighborhood of the SDW region of -0.16=<t'=<-0.08 the SC Econd dominates over the SDW Econd. At t'=-0.05 and -0.10 we obtained a definitely finite bulk-limit SC Econd of the order of the experimental value for YBCO. At t'=0 Econd nearly vanishes. For t'=<-0.18, the SC Econd strongly oscillates as a function of the lattice size, when periodic boundary conditions (b.c.'s) are imposed to both axes. In the case of periodic and antiperiodic b.c.'s, a finite bulk-limit value is obtained at t'=-0.22. Econd tends to vanish with further decrease of t'. With our results the SC of LSCO is understandable with t'~ -0.10. The t' values of Hg1201, Tl2201 and Na-CCOC seem close to -0.20 so that they locate in the boundary zone of SC indicated in the present work. Slightly larger U improves the situation by increasing Econd. 
K. Yamaji, T. Yanagisawa, M. Miyazaki, R. Kadono