A mean-field theory for the spin-gap phase in the two-leg t-J ladder

Two-leg t-J ladders are investigated in the framework of a combination of t he phass string formulation and bond-operator representation. We develope a mean-field theory in the strong rung interaction regime, i.e. J(perpendicu lar to) >> J, t, which provides a unified description of the undoped insula ting phase and the low doping spin gap phase. The ground state of the doped phase is intrinsically a superconductor with a d-wave symmetry gap driven by the t- term. The ground-state energy is in good agreement with numerical results. Phase separation is shown to occur beyond some critical value of J/t for given doping concentration. The local structure of hole pairs as we ll as the spectra of various spin and charge modes are also analyzed.