Liquid-gas analog multicriticality in a frustrated Ising bilayer

Abstract

We report the discovery of a multicritical point that extends the liquid-gas paradigm to systems with competing symmetry-breaking orders. Using large-scale Monte Carlo simulations of a frustrated bilayer Ising antiferromagnet with tunable couplings, we map out a rich finite-temperature phase diagram hosting three ordered phases separated by both continuous and first-order transitions. By tuning the couplings, a tricritical line and a critical end-point line converge into a single multicritical line. At all points along the multicritical line, symmetry-distinct phases exhibit identical leading critical behavior -- consistent with the tricritical Ising universality class -- while the subleading exponent exhibits a sharp shift from $y_g = 0.8$ to $y_g = 1$. This shift reflects an emergent $Z_2$ symmetry akin to that of the liquid-gas critical point, but realized here at a genuine multicritical point involving simultaneous microscopic symmetry breaking. Our results establish a universality scenario in which emergent symmetry preserves the leading class but reorganizes subleading scaling, providing a general mechanism for symmetry-enforced multicriticality.