Thermal critical points from competing singlet formations in fully frustrated bilayer antiferromagnets

We examine the ground-state phase diagram and thermal phase transitions in a plaquettized fully frustrated bilayer spin-1/2 Heisenberg model. Based on a combined analysis from sign-problem free quantum Monte Carlo simulations, perturbation theory, and free-energy arguments, we identify a first-order quantum phase transition line that separates two competing quantum-disordered ground states with dominant singlet formations on interlayer dimers and plaquettes, respectively. At finite temperatures, this line extends to form a wall of first-order thermal transitions, which terminates in a line of thermal critical points. From a perturbative approach in terms of an effective Ising model description, we identify a quadratic suppression of the critical temperature scale in the strongly plaquettized region. Based on free-energy arguments we furthermore obtain the full phase boundary of the low-temperature dimer-singlet regime, which agrees well with the quantum Monte Carlo data.

Automated summary

In this study, we examine the ground-state phase diagram and thermal phase transitions of a plaquettized fully frustrated bilayer spin-1/2 Heisenberg model. We find a first-order quantum phase transition line separating two competing quantum-disordered ground states, which have dominant singlet formations on interlayer dimers and plaquettes, respectively. At finite temperatures, this line extends to form a wall of first-order thermal transitions, terminating in a line of thermal critical points. A perturbative approach reveals a quadratic suppression of the critical temperature scale in the strongly plaquettized region. Based on free-energy arguments, we obtain the full phase boundary of the low-temperature dimer-singlet regime, which agrees well with quantum Monte Carlo data.