Two-leg t-J ladder: A mean-field description

Two-leg t-J ladders are investigated in the Framework of a combination of t he phase string formulation and bond-operator representation. We develop a mean-field theory in the strong rung interaction regime, i.e., J(perpendicu lar to) much greater than J, t, which provides a unified description of the undoped insulating phase and the low-doping phase-the so-called C1S0 phase . Both of them are characterized by the resonating-valence-bond (RVB) order parameter, with gap opened up in all spin excitations. The ground state of the doped phase is intrinsically a superconductor with a d-wave symmetry, which is driven by the RVB correlations. The ground-state energy is in good agreement with numerical results. Phase separation is shown to occur beyon d some critical value of J/t for given doping concentration. The local stru cture of hole pairs as well as the spectra of various spin and charge modes are analyzed in comparison with other approaches. [S0163-1829(99)03440-2].