What makes He-4 supersolid? Nikolay Prokofiev, Professor of Physics, University of Massachusetts Amherst

First principle Monte Carlo simulations and recent experiments all point out that ideal, defect free, crystals of Helium-4 are not supersolid. Contrary to naive expectation that disorder inhibits suppresses superfluid response due to localization effects, disorder in quantum solids works in the opposite direction: if particles in the ideal crystal are already localized then structural defects can help to make the sample superfluid. The theory of superfluidity in lower dimensional defective structures embedded in a higher dimensional bulk has its own twists since such phenomena as superfluidity, roughening and defect mobility may be strongly linked, e.g. in the 3D network of dislocations. The possibilities for "designing" crystallographic defects are countless, and in the strongly correlated system each case (superfluid or not) has to be considered separately. We find that generic grain boundaries and the screw dislocation along the z-axis are superfluid, while special types of boundaries and most edge dislocations are insulating [1,2]. We also find that Helium-4 can form a metastable superfluid glass [3]. How these findings explain current experiments remains an open question. 1 L. Pollet, et al.  PRL  98, 135301 (2007). 2  M. Boninsegni et al.  PRL 99, 035301 (2007). 3 M. Boninsegni et al.  PRL 96, 105301 (2006).