The Boson-Hubbard Model on a Kagome Lattice with Sextic Ring-Exchange   Terms
  High order ring-exchange interactions are crucial for the study of quantum fluctuations on highly frustrated systems. We present the first exact quantum Monte Carlo study of a model of hard-core bosons with sixth order ring-exchange interactions on a two-dimensional kagome lattice. By using the Stochastic Green Function algorithm, we show that the system becomes unstable in the limit of large ring-exchange interactions. It undergoes a phase separation at all fillings, except at 1/3 and 2/3 fillings for which the superfluid density vanishes and an unusual mixed valence bond and charge density ordered solid is formed. 
Val\'ery Rousseau, Ka-Ming Tam, Mark Jarrell, and Juana Moreno
cond-mat.quant-gas quant-ph