Novel Spin Flop Transition of the S=1/2 Square-Kagome-Lattice Antiferromagnet

By means of the numerical diagonalization method, we study the S=1/2 Heisenberg antiferromagnet on the square-kagome lattice, similar to the kagome lattice. We examine the ground-state properties and the magnetization process of the model. It is found that a magnetization jump appears at the higher-field-side edge of the manetization plateau at the one-third height of the saturation. A spin-flop phenomenon is clearly observed at the jump even when the system is isotropic in the spin space.