# A quantum clustering transition

Quantum particles interacting with a repulsive shoulder potential in one dimension experience a quantum phase transition from a strongly correlated liquid to a liquid of clusters. It is known, both classically and in lattice quantum models, that a repulsive flat potential at short distances favors clustering at high density, because this reduces the potential energy.

The Authors of a new contribution found that, even in the absence of an external one-dimensional lattice, an effective spin hamiltonian can be envisaged, where the spin is defined by a particle being closer to its left of right neighbors, namely by the **tendency to form a two-particle cluster** to the left or to the right. Such spins are transverse in the liquid phase, while they are aligned in the quasi-solid cluster phase. The transition in between is in the universality class of the two-dimensional Ising model, when the density is tuned so as to have two-particles clusters. An hallmark of this effective model is seen in a specific excited state, which becomes gapless at the transition. The results open the study of higher effective spin transitions at higher density, and could be relevant for future experiments with dressed Rydberg gases.

This study has been carried out by Gianluca Bertaina, Davide Galli, Davide Pini, and Stefano Rossotti at the Physics Department together with Martina Teruzzi at SISSA (Trieste) and has recently appeared in Physical Review Letters. The article file can be found at the link https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.215301