Studies of the exactly dimerized bilayer Heisenberg model via strong coupling expansions

The one-triplet excitation spectra and thermodynamic properties for the dimerized phase of the frustrated bilayer Heisenberg model are studied using strong-coupling expansion theory. The model has an exact dimerized ground state as well as exact one-triplet excitations in a special case that the frustration J2 is equal to the in-plane coupling J1. We demonstrate that the models with and without frustrations have distinct excitation spectra, so their thermodynamic properties exhibit quite different behaviors. Especially, the low-temperature behaviors of the frustrated model with J1=J2 are independent of the inter-dimer couplings, due to the exact one-triplet excitations