Discontinuous Phase Transition in a Strongly Correlated Driven Lattice

Discontinuous (first-order) phase transitions and the associated metastability play fundamental roles in nature, from ferromagnetism in solids to the false-vacuum decay in the early Universe. However, their underlying mechanism is poorly understood, particularly in many-body systems.

Here, we realise a discontinuous quantum phase transition in an optical lattice with ultracold atoms. By shaking the optical lattice, we hybridise the lowest two bands, leading to a transition from a Mott insulator to a superfluid with staggered phase order, which is called a π-superfluid.

Crucially, the transition from the original Mott insulator in the lowest band to the resulting superfluid in the excited band can be first order, because the non-staggered order in the Mott insulator is incompatible with the staggered order of this superfluid – so the system has to choose one. We directly observed the metastability and hysteresis associated with this first-order transition by monitoring how fast one phase sweeps into another, or not.

Our results agree well with numerical simulations, and open a new avenue towards simulating false-vacuum decay as well as exploring the role of quantum fluctuations in strongly correlated systems.