Magnetization plateau in the spin ladder with a four-spin exchange

The magnetization process of the S = 1/2 antiferromagnetic spin ladder with the four-spin cyclic exchange interaction at T = 0 is studied by the exact diagonalization of finite clusters and size scaling analyses. It is found that a magnetization plateau appears at half the saturation value if the ratio of the four- and two-spin exchange coupling constants J(4) is larger than the critical value J(4c) = 0.05 +/- 0.04. The phase transition with respect to J(4) at J(4c) is revealed to be the Kosterlitz-Thouless type. [S0163-1829(99)02226-2].