Exchange Frequencies in Helium-4 Crystals with Defects

The torsional oscillator experiments by Kim and Chan indicate the presence of a superfluid flow in solid helium-4 [Science 305 (2004) 1941], which cannot be explained with the properties of a perfect h.c.p. lattice [Ceperley, Bernu, PRL 93 (2004) 155303]. Using \urllink{path integral Monte Carlo}{http://militzer.gl.ciw.edu} simulations, we study different types of defects and stacking faults in the crystal by analyzing the frequency of ring-exchanges along the defect lines. This focus lies on identifying a mechanism that could explain the observed 1\% superfluid fraction.