Orbital ordering driven spin dimer state in double-layered antiferromagnet K3Cu2O7

Magnetic, orbital and lattice structures of K3Cu2F7 are determined by cluster self-consistent field approach based on the spin-orbital-lattice Hamiltonian. Symmetry breaking and Jahn-Teller distortion of approximately isolated bilayer cause Cu2+ ions alternatively to occupy z2-x2〉/ z2-y2〉 orbitals in each layer. This orbital ordering occupation leads to the dominant intrabilayer antiferromagnetic coupling, which favors spin dimerization, and the weak intralayer ferromagnetic coupling. Due to absence of spin frustration resulting from the intralayer orbital arrangement and the weak ferromagnetic coupling satisfing Goodenough-Kanamori-Anderson (GKA)rule, the ground state is a stable spin dimer state. The spin singlet-triplet excitation gap obtained by bond-operator mean field method is about 326 K, which is close to the experimental value of 400 K. The present theory is also applicable to explaining the formation of spin dimer state in Cs3Cu2Cl4Br3.