Filling-dependence of the zigzag Hubbard ladder for a   quasi-one-dimensional superconductor Pr_2Ba_4Cu_7O_{15-delta}
  We investigate filling dependence of the zigzag Hubbard ladder, using density matrix renormalization group method. We illustrate the chemical-potential vs. electron-density and spin gap vs. electron density curves, which reflect characteristic properties of the electron state.On the basis of the obtained phase diagram, we discuss the connection to a novel quasi-one-dimensional superconductor Pr$_2$Ba$_4$Cu$_7$O$_{15-\delta}$. 
Kouichi Okunishi