Fourfold degenerate columnar-dimer ground state in square lattice antiferromagnets
Abstract
We construct and study two frustrated quantum spin-1/2 models on square lattice, which are like the antiferromagnetic J1-J2 model with some additional four-spin exchange interactions. These models admit an exactly solvable case in which the ground state consists of four degenerate columnar-dimer singlet (CDS) configurations. Away from the exact case, we employ bond-operator mean-field theory to investigate the evolution of the ground state by varying the interaction parameters. The mean-field calculation reveals a quantum phase diagram in which the CDS phase undergoes a continuous phase transition to either N\'eel or collinear ordered antiferromagnetic phases.
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