Quasi-excitations and superconductivity in the t-J model on a ladder

We study the t-J model on a ladder using the slave-fermion-CP 1 formalism which has been used successfully in studying lightly doped high-T C cuprates. Special at-tention is paid to the dynamics of composite gauge fields and the natures of quasi-excitations. The slave-fermion-CP 1 approach explains many aspects of the ladder system observed by experiments and numerical studies in a natural and coherent manner. We first obtain the low-energy effective model by integrating out half of the CP 1 variables (the Schwinger bosons) assuming a short-range antiferromagnetic order (SRAFO). The spin part of the effective model is the relativistic CP 1 model. In the single-chain case, there appears a topological θ -term with θ = π , as is well known. On the other hand, in the two-leg ladder case, we have θ = 2 π . The dynamics of the composite gauge boson strongly depends on the value of the coefficient of the θ -term. This fact explains why the quasi-excitations in a chain and ladders of even legs are different. For a ladder, the gauge dynamics realizes in the confinement phase, so the quasi-exciations are charge-neutral objects like spin triplet and electrons, etc. The effective model reveals attractive force between holes, which generates superconductivity in SRAFO. The symmetry of hole-pair condensation should be of the d-wave type.