Valence-Bond Order in a Honeycomb Antiferromagnet Coupled to Quantum Phonons*

We use exact quantum Monte Carlo simulations to demonstrate that the Neel ground state of an antiferromagnetic SU(2) spin-1/2 Heisenberg model on the honeycomb lattice can be destroyed by a coupling to quantum phonons. We find a clear first-order transition to a valence-bond-solid state with Kekule order instead of a deconfined quantum critical point. However, quantum lattice fluctuations can drive the transition towards weakly first-order, revealing a tunability of the transition by the retardation of the interaction. In contrast to the one-dimensional case, our phase diagram in the adiabatic regime is qualitatively different from the frustrated J1-J2 model. Our results suggest that a coupling to bond phonons can induce Kekule order in Dirac systems.

[1] M. Weber, arXiv:2009.12393