Thermodynamic instability of a vacancy gas in solid Helium

The supersolid phase of matter, characterized by non-dissipative flow in a crystal, has been elusive for some 35 years. The recent discovery of a non-classical moment of inertia in solid $^4$He by Kim and Chan has provided the first piece of experimental evidence, although its interpretation in terms of supersolidity of the ideal crystal phase remains controversial. Using Quantum Monte Carlo methods, we investigate the long-standing question of vacany-induced superflow. We find that a uniform gas of vacancies is thermodynamically unstable against separation into two phases, an insulating, vacancy-free crystal and a liquid. We investigate the thermodynamics of other defects, such as edge dislocations.