Magnetization Plateau of the Distorted Diamond Spin Chain

The frustrated quantum spin system on the distorted diamond chain lattice is investigated using the numerical diagonalization of finite-size clusters and the level spectroscopy analysis. In the previous work this system was revealed to exhibit the 1/3 magnetization plateau due to two different mechanisms depending on the coupling parameters, and the phase diagram at the 1/3 magnetization was obtained. In the present work it is found that the 1/3 magnetization plateau vanishes for sufficiently large $XY$-like coupling anisotropy. The phase diagram based on the level spectroscopy analysis is also presented.