Ground-State Phase Diagram of (1/2,1/2,1) Mixed Diamond Chains

Abstract: The ground-state phases of mixed diamond chains with ($S, \tau^{(1)}, \tau^{(2)})=(1/2,1/2,1)$, where $S$ is the magnitude of vertex spins, and $\tau^{(1)}$ and $\tau^{(2)}$ are those of apical spins, are investigated. The two apical spins in each unit cell are coupled by an exchange coupling $\lambda$. The vertex spins are coupled with the top and bottom apical spins by exchange couplings $1+\delta$ and $1-\delta$, respectively. Although this model has an infinite number of local conservation laws for $\delta=0$, they are lost for finite $\delta$. The ground-state phase diagram is determined using the numerical exact diagonalization and DMRG method in addition to the analytical approximations in various limiting cases. The phase diagram consists of a nonmagnetic phase and several kinds of ferrimagnetic phases. We find two different ferrimagnetic phases without spontaneous translational symmetry breakdown. It is also found that the quantized ferrimagnetic phases with large spatial periodicities present for $\delta=0$ are easily destroyed by small $\delta$ and replaced by a partial ferrimagnetic phase. The nonmagnetic phase is considered to be a gapless Tomonaga-Luttinger liquid phase based on the recently extended Lieb-Schultz-Mattis theorem to the site-reflection invariant spin chains and numerical diagonalization results.