A supersolid state of matter, i.e. a solid with crystalline structure that exhibits frictionless superflow, was theoretically proposed to occur in solid 4He almost 40 years ago. The first experimental indication for the existence of supersolidity in helium was discovered in 2003 when Kim and Chan employed the torsional oscillator technique to observe nonclassical rotational inertia (NCRI). In order to determine if supersolidity is an intrinsic property of helium we have studied the effect of disorder on the NCRI. We find that the supersolid signal can be substantially reduced or sometimes eliminated by annealing of the helium crystal. Also, we have been able to increase the supersolid signals by three orders of magnitude by geometrically confining the sample space. We believe that the supersolid signal is strongly modified by varying the amount of crystalline disorder.

Laboratory of Atomic and Solid-state Physics, Cornell University