Emergent Ising symmetry and supercritical fluids

The symmetry of Ising model questions any single crossover scenario for supercritical fluids. In this work, we firstly study a pair of thermodynamic crossovers L^± analytically for the Van der Waals class fluids. We uncover an emergent Z₂ symmetry in addition to the universal scalings in the scaling regime for this class fluids. By using the self-reciprocal property between coexistenct phases, we further establish that under suitable conditions, the Ising symmetry generally emerges in the scaling regime for a general universality class. As a consequence, the thermodynamic crossovers L^± generally exhibit an emergent Z₂ symmetry in the scaling regime. This partly resolves the symmetry puzzle raised by the Ising model. The results also imply that the physical importance of the Ising model in critical phenomenon is far beyond the scope of magentic transitions.