The field-induced gap due to four-spin exchange in a spin ladder

The effect of the four-spin cyclic exchange interaction at each plaquette i n the S = 1/2 two-leg spin ladder is investigated at T = 0, focusing especi ally on the field-induced gap. The strong-rung-coupling approximation sugge sts that it yields a plateau at half of the saturation moment (m = 1/2) in the magnetization curve, which corresponds to a field-induced spin gap with a spontaneous breaking of the translational symmetry. A precise phase diag ram at m = 1/2 is also presented, based on the level spectroscopy analysis of the numerical data obtained by the Lanczos method. The boundary between the gapless and plateau phases is confirmed to be of the Kosterlitz-Thoules s (KT) universality class.