SU(2) theory of t-J two-leg ladders

We present a low-energy effective model for the charge degrees of freedom i n two-leg t-J ladders. Starting from the SU(2) mean-field theory, we find t hat, at low temperatures, the mean-field solution is the staggered-flux pha se. Our analysis is applicable at low doping and in the 'physical' range of parameters, t/J similar to 3, where there is one massless mode in the char ge sector and no massless modes in the spin sector. Low-lying charge excita tions are described as bipolarons. (C) 1998 Elsevier Science Ltd. All right s reserved.