Magnetization plateau in the spin ladder with the 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\pm$0.04. The phase transition with respect to $J_4$ at $J_{4c}$ is revealed to be the Kosterlitz-Thouless-type.