AMO
In the literature: open
Nature of the striated and star transitions in square Rydberg arrays
In plain words
On a square grid, Rydberg atoms form several ordered patterns. The transitions into some of them may be smooth or abrupt, and this is not settled.
Precise statement
Square-lattice Rydberg arrays with $R_b/a = 1.2 \text{ to } 1.8$. For transitions from the disordered phase into the striated and star phases, determine whether they are continuous or first order and, if continuous, the universality class (exponents $\nu, \eta$). Answer: classification with exponents.
What would settle it
Quantum Monte Carlo or 2D tensor-network finite-size scaling, with Kibble-Zurek scaling on arrays of at least 256 atoms.