Effect of monomer-monomer interactions on the phase diagrams of the S=1/2 distorted diamond type quantum spin chain

By use of mainly the exact diagonalization and the level spectroscopy method, we investigate the ground-state phase diagrams of the $S=1/2$ distorted diamond type quantum spin chain with the monomer-monomer interactions and/or ferromagnetic interactions for the zero magnetic field case, as well as the $M=M_{\rm s}/3$ case and the $M=(2/3)M_{\rm s}$ case, where $M$ is the total magnetization and $M_{\rm s}$ is the saturation magnetization. The magnetization plateau at $M=M_{\rm s}/3$ vanishes in the region where the ferromagnetic interaction is rather strong. The monomer-monomer interaction remarkably stabilizes the magnetization plateau at $M=(2/3)M_{\rm s}$.