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 $J_1$-$J_2$ 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 Néel or collinear ordered antiferromagnetic phases.

revtex4, 16 eps files; total 10 pages; accepted for publication in Phys. Rev. B