ETH, Zurich

Title: Phases and phase transitions in hardcore bosons

Abstract: We employ the stochastic series expansion quantum Monte Carlo algorithm with loop-cluster updates to investigate quantum phase transitions and finite temperature phase transitions of hard core bosons on a square lattice. In models with predominantly nearest neighbor repulsion we find direct evidence from density histograms for a first order phase transition between a solid phase at half filling and a superfluid phase with lower density. No supersolid phase, a coexistence of solid and superfluid order is found in this case. For large next nearest neighbor interactions on the other hand, a supersolid phase and two quantum phase transitions are found. We will present the complete ground state and finite temperature phase diagrams, discuss phase separation and phase transitions at finite temperature between the solid, superfluid and disordered phases. We find an unusual melting line with reentrant behavior for thermal melting, three different types of transitions for doping-induced melting, and compare to phase diagrams of Helium.