Large-N SU(4) Schwinger boson theory for coupled-dimer antiferromagnets

We develop a systematic large-N expansion based on the Schwinger boson representation of SU(4) coherent states of dimers for the paradigmatic spin-1/2 bilayer square lattice Heisenberg antiferromagnet. This system exhibits a quantum phase transition between a quantum paramagnetic state and a Néel order state, driven by the coupling constant g=J'/J, which is defined as the ratio between the inter-dimer J' and intra-dimer J exchange interactions. We demonstrate that this approach accurately describes static and dynamic properties on both sides of the quantum phase transition. The critical coupling constant g_c ≈0.42 and the dynamic spin structure factor reproduce quantum Monte Carlo results with high precision. Notably, the 1/N corrections reveal the longitudinal mode of the magnetically ordered phase along with the overdamping caused by its decay into the two-magnon continuum. The present large-N SU(N) Schwinger boson theory can be extended to more general cases of quantum paramagnets that undergo a quantum phase transition into magnetically ordered states.