Quantum liquid of repulsively bound pairs of particles in a lattice

TL;DR

This paper derives an effective Hamiltonian for repulsively bound pairs of cold atoms in a lattice, revealing their formation of a quantum liquid with unique phase transition properties, useful for studying many-body physics.

Contribution

It introduces a new effective Hamiltonian for dimers in a lattice and explores their many-body dynamics and phase transitions.

Findings

Dimers form incompressible quantum droplets.

Strong on-site repulsion and nearest-neighbor attraction dominate at low temperatures.

First-order phase transition from droplet to gas phase observed.

Abstract

Repulsively interacting particles in a periodic potential can form bound composite objects, whose dissociation is suppressed by a band gap. Nearly pure samples of such repulsively bound pairs of cold atoms -- "dimers" -- have recently been prepared by Winkler et al. [Nature 441, 853 (2006)]. We here derive an effective Hamiltonian for a lattice loaded with dimers only and discuss its implications to the many-body dynamics of the system. We find that the dimer-dimer interaction includes strong on-site repulsion and nearest-neighbor attraction which always dominates over the dimer kinetic energy at low temperatures. The dimers then form incompressible, minimal-surface "droplets" of a quantum lattice liquid. For low lattice filling, the effective Hamiltonian can be mapped onto the spin-1/2 XXZ model with fixed total magnetization which exhibits a first-order phase transition from the "droplet" to a "gas" phase. This opens the door to studying first order phase transitions using highly controllable ultracold atoms.