QUASIEXCITATIONS AND SUPERCONDUCTIVITY IN THE T-J MODEL ON A LADDER

We study the t-J model on a ladder using the slave-fermion-CP1 formali sm that has been used successfully in studying lightly-doped high-T-c cuprates. We obtain a low-energy effective model assuming a short-rang e antiferromagnetic order (SRAFO), which includes gauge-field degrees of freedom because of the use of the slave-particle picture. We give a systematic study on the dynamics of composite gauge bosons to show th at the confinement phase is realized in a two-leg system whereas the d econfinement phase is realized in a chain system. This explains why qu asiexcitations are so different in the chain and ladder systems. The e ffective model also reveals attractive forces between holes, which gen erate d-wave-type superconductive correlations in SRAFO. The low-energ y excitations on a ladder are consistently described by the universali ty class of the Lurher-Emery liquid.