High-order study of the quantum critical behavior of a frustrated spin- 12 antiferromagnet on a stacked honeycomb bilayer

We study a frustrated spin-$\frac{1}{2}\phantom{\rule{0.16em}{0ex}}{J}₁\text{\ensuremath{-}}{J}₂\text{\ensuremath{-}}{J}₃\text{\ensuremath{-}}{J}₁^{\ensuremath{\perp}}$ Heisenberg antiferromagnet on an $AA$-stacked bilayer honeycomb lattice. In each layer we consider nearest-neighbor (NN), next-nearest-neighbor, and next-next-nearest-neighbor antiferromagnetic (AFM) exchange couplings ${J}₁,\phantom{\rule{0.16em}{0ex}}{J}₂$, and ${J}₃$, respectively. The two layers are coupled with an AFM NN exchange coupling ${J}₁^{\ensuremath{\perp}}\ensuremath{\equiv}\ensuremath{\delta}{J}₁$. The model is studied for arbitrary values of $\ensuremath{\delta}$ along the line ${J}₃={J}₂\ensuremath{\equiv}\ensuremath{\alpha}{J}₁$ that includes the most highly frustrated point at $\ensuremath{\alpha}=\frac{1}{2}$, where the classical ground state is macroscopically degenerate. The coupled cluster method is used at high orders of approximation to calculate the magnetic order parameter and the triplet spin gap. We are thereby able to give an accurate description of the quantum phase diagram of the model in the $\ensuremath{\alpha}\ensuremath{\delta}$ plane in the window $0\ensuremath{\le}\ensuremath{\alpha}\ensuremath{\le}1,\phantom{\rule{0.16em}{0ex}}0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}1$. This includes two AFM phases with N\'eel and striped order, and an intermediate gapped paramagnetic phase that exhibits various forms of valence-bond crystalline order. We obtain accurate estimations of the two phase boundaries, $\ensuremath{\delta}={\ensuremath{\delta}}_{{c}_i}(\ensuremath{\alpha})$, or equivalently, $\ensuremath{\alpha}={\ensuremath{\alpha}}_{{c}_i}(\ensuremath{\delta})$, with $i=1$ (N\'eel) and 2 (striped). The two boundaries exhibit an ``avoided crossing'' behavior with both curves being re-entrant. Thus, in this $\ensuremath{\alpha}\ensuremath{\delta}$ window, N\'eel order exists only for values of $\ensuremath{\delta}$ in the range ${\ensuremath{\delta}}_{{c}₁}^<(\ensuremath{\alpha})<\ensuremath{\delta}<{\ensuremath{\delta}}_{{c}₁}^>(\ensuremath{\alpha})$, with ${\ensuremath{\delta}}_{{c}₁}^<(\ensuremath{\alpha})=0$ for $\ensuremath{\alpha}<{\ensuremath{\alpha}}_{{c}₁}(0)\ensuremath{\approx}0.46(2)$ and ${\ensuremath{\delta}}_{{c}₁}^<(\ensuremath{\alpha})>0$ for ${\ensuremath{\alpha}}_{{c}₁}(0)<\ensuremath{\alpha}<{\ensuremath{\alpha}}₁^>\ensuremath{\approx}0.49(1)$, and striped order similarly exists only for values of $\ensuremath{\delta}$ in the range ${\ensuremath{\delta}}_{{c}₂}^<(\ensuremath{\alpha})<\ensuremath{\delta}<{\ensuremath{\delta}}_{{c}₂}^>(\ensuremath{\alpha})$, with ${\ensuremath{\delta}}_{{c}₂}^<(\ensuremath{\alpha})=0$ for $\ensuremath{\alpha}>{\ensuremath{\alpha}}_{{c}₂}(0)\ensuremath{\approx}0.600(5)$ and ${\ensuremath{\delta}}_{{c}₂}^<(\ensuremath{\alpha})>0$ for ${\ensuremath{\alpha}}_{{c}₂}(0)>\ensuremath{\alpha}>{\ensuremath{\alpha}}₂^<\ensuremath{\approx}0.56(1)$.