Spinon excitation and M\"obius boundary condition in S=1/2 antiferromagnetic Heisenberg spin ladder with zigzag structure

We investigate the low-lying excitation of the antiferromagnetic zigzag spin ladder with M\"obius boundary condition. Using the Lanczos and Householder diagonalization methods, we calculate the excitation spectrum of the zigzag ladder in the momentum space. We then show that the topological defect generated by the M\"obius boundary provides the clear evidence of the single spinon excitation. On the basis of the obtained single-spinon dispersion curve, we analyze the two-spinon scattering states under the usual periodic boundary condition. We further discuss the interaction effect between the spinons, and the connection to the cusp singularity in the magnetization process.