Ground-State Phase Diagram of the Bond-Alternating $S=2$ Quantum Spin Chain with the $XXZ$ and On-Site Anisotropies -- Symmetry Protected Topological Phase versus Trivial Phase

We investigate the ground-state phase diagram of the bond-alternating $S=2$ quantum spin chain with the $XXZ$ and on-site anisotropies. For the on-site anisotropies, in addition to the popular $D_2 \sum_j (S_j^z)^2$ term, we consider the $D_4 \sum_j (S_j^z)^4$ term. Mainly we use the exact diagonalization and the level spectroscopy analysis. We show that the Haldane state, large-$D$ state and the Dimer2 state belong to the same trivial phase, by finding the existence of adiabatic paths directly connecting these states without the quantum phase transition. Similarly, we show that the intermediate-$D$ state and the Dimer1 state belong to the same symmetry protected topological phase.

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