Superfluidity of grain boundaries and supersolid behavior

We have found that, at the liquid-solid equilibrium pressure $P_m$, supersolid behavior is due to the superfluidity of grain boundaries in solid helium [1]. After describing this experiment and reviewing some of the related theoretical work [2], we discuss the possibility that , at larger pressure ($P > P_m$), grain boundaries could also explain the supersolid behavior which was observed with torsional oscillators [3-6]. \newline \newline [1] S. Sasaki, R. Ishiguro, F. Caupin, H.J. Maris, and S. Balibar, Science 313, 1098 (2006)\newline [2] E. Burovski, E. Kozik, A. Kuklov, N. Prokof'ev, and B. Svistunov, Phys. Rev. Lett. 94, 165301 (2005)\newline [3] E. Kim and M.H. Chan, Nature 427, 225 (2004)\newline [4] E. Kim and M.H. Chan, Science 305, 1941 (2004)\newline [5] A.S.C. Rittner and J.D. Reppy, Phys. Rev. Lett. 97, 165301 (2006)\newline [6] K. Shirahama, Bull. Am. Phys. Soc. 51, 302 (2006)