Spin-singlet recombination responsible for the magnetization plateau in a tetrameric ladder-like system

The magnetic properties and quantum phase transitions (QPTs) of an anisotropic tetrameric ladder-like model compound Cu2CdB2O6, which is constructed by dimers embedded into two-leg ladder with weak coupling inside the leg, are investigated using Greenʹs function theory. It is shown that for weak dimer-chain (J2) coupling, the ground state lies in a gapless phase with a 1/2 magnetization plateau appearing in a magnetic field, which was observed in experiment. However, with J2 coupling ascending, the ground state goes into a gapped phase with an additional zero plateau emerging. It is found that the magnetization plateau states originate from the intra-dimer and dimer-chain singlet recombination, which are explicitly evidenced by the sublattice magnetization and two-site entanglement entropy, whose behaviors signal the QPTs clearly.