Comparison of quasiparticle energy dispersions and shadow peaks of doped two-leg ladder systems between various models including an SO(5) symmetry model

Considering various models including an SO(5) symmetry model, mean-field an d exact diagonalization studies on the quasiparticle excitation of two-leg ladder systems are reported. It is shown that the energy gap in the quasipa rticle excitation is caused by the formation of rung singlet states. We fin d that shadow peaks can occur above the Fermi surface with antiferromagneti c electron correlations involving only rungs in the spin ladder. [S0163-182 9(99)13005-4].