Nontrivial ferrimagnetism of the Heisenberg model on the Union Jack strip lattice

We study the ground-state properties of the S=1/2 antiferromagnetic Heisenberg model on the Union Jack strip lattice by using the exact-diagonalization and density matrix renormalization group methods. We confirm a region of the intermediate-magnetization state between the Neel-like spin liquid state and the conventional ferrimagnetic state of Lieb-Mattis type. In the intermediate-state, we find that the spontaneous magnetization changes gradually with respect to the strength of the inner interaction. In addition, the local magnetization clearly shows an incommensurate modulation with long-distance periodicity in the intermediate-magnetization state. These characteristic behaviors lead to the conclusion that the intermediate-magnetization state is the non-Lieb-Mattis ferrimagnetic one. We also discuss the relationship between the ground-state properties of the S=1/2 antiferromagnetic Heisenberg model on the original Union Jack lattice and those on our strip lattice.