Antiferromagnetic phase transition in a nonequilibrium lattice of   Rydberg atoms
  We study a driven-dissipative system of atoms in the presence of laser excitation to a Rydberg state and spontaneous emission. The atoms interact via the blockade effect, whereby an atom in the Rydberg state shifts the Rydberg level of neighboring atoms. We use mean-field theory to study how the Rydberg population varies in space. As the laser frequency changes, there is a continuous transition between the uniform and antiferromagnetic phases. The nonequilibrium nature also leads to a novel oscillatory phase and bistability between the uniform and antiferromagnetic phases. 
Tony E. Lee, H. H\"affner, M. C. Cross
cond-mat.quant-gas nlin.PS quant-ph