Exotic phase diagram of a cluster charging model of bosons on the kagome lattice

We study a model of hard-core bosons on the kagome lattice with short-range hopping (t) and repulsive interactions (V). This model directly maps onto an easy-axis S=1∕2 XXZ model on the kagome lattice and is also related, at large V∕t, to a quantum dimer model on the triangular lattice. Using quantum Monte Carlo numerics, we map out the phase diagram of this model at half-filling. At T=0, we show that this model exhibits a superfluid phase at small V∕t and an insulating phase at large V∕t, separated by a continuous quantum phase transition at V_c∕t≈19.8. The insulating phase at T=0 appears to have no conventional broken symmetries, and is thus a uniform Mott insulator (a “spin liquid” in magnetic language). We characterize this insulating phase as a uniform Z₂ fractionalized insulator from the topological order in the ground state and estimate its vison gap. Consistent with this identification, there is no apparent thermal phase transition upon heating the insulator. The insulating phase instead smoothly crosses over into the high temperature paramagnet via an intermediate cooperative paramagnetic regime. We also study the superfluid-to-normal thermal transition for V<V_c. We find that this is a Kosterlitz-Thouless transition at small V∕t but changes to a first order transition for V closer to V_c. We argue that this first order thermal transition is consistent with the presence of a nearby Z₂ insulating ground state obtained from the superfluid ground state by condensing double vortices.