Abstract
The effect of the four-spin cyclic exchange interaction at eachplaquette in the S = 1/2 two-leg spin ladder is investigated atT = 0, focusing especially on the field-induced gap. Thestrong-rung-coupling approximation suggests that it yields a plateauat half of the saturation moment (m = 1/2) in the magnetizationcurve, which corresponds to a field-induced spin gap with aspontaneous breaking of the translational symmetry. A precise phasediagram at m = 1/2 is also presented, based on the levelspectroscopy analysis of the numerical data obtained by the Lanczosmethod. The boundary between the gapless and plateau phases isconfirmed to be of the Kosterlitz-Thouless (KT) universality class.
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