Criticality around the Spinodal Point of First-Order Quantum Phase Transitions

Universality and scaling are hallmarks of second-order phase transitions but are generally unexpected in first-order quantum phase transitions (FOQPTs). We present a microscopic theory showing that quantum criticality can emerge around the quantum spinodal point of FOQPTs where metastability disappears. We demonstrate that, at this instability, resonant local excitations dynamically decouple a Hilbert subspace characterized by an emergent discrete translational symmetry. Projecting the original Hamiltonian onto this subspace yields an effective Hamiltonian that exhibits a genuine second-order quantum phase transition (SOQPT) and the Kibble-Zurek scaling. We validate this framework in the tilted Ising chain which breaks $\mathbb{Z}_2$ symmetry, and predict the absence of criticality in the staggered-field PXP model. This work indicates that the dynamics of FOQPTs is usually governed by an emergent critical point around the quantum spinodal point. Our results uncover a hidden criticality in FOQPTs, reshaping the conventional understanding of FOQPTs beyond the mean-field theory.