Search for supersolid 4He in neutron scattering experiments at ISIS

The observation of a non-classical rotation inertia (NCRI) fraction in bulk solid He-4 by M. Chan and E. Kim [1, 2] attracted significant interest as a possible manifestation of supersolid state of matter. Despite numerous experimental and theoretical studies inspired by this observation, an explicit explanation for this phenomenon is still missing. Neutron scattering experiments on solid helium may help to shed light on the physical grounds of NCRI and answer the question on whether this phenomenon could be caused by Bose-Einstein Condensation. In this paper we are going to discuss the results obtained in experiments involving neutron scattering on solid He-4. Microscopic quantitative data such as mean kinetic energy, mean square momentum and mean square displacement of helium atoms as well as the lattice parameter have been obtained for the first time for solid He-4 in temperature range 70mK - 500 mK. No change was seen in the single atom kinetic energy (within statistical error better than 1%) as well as change in the lattice parameter (within 0.03%). The mean square displacement did not change in the region of expected supersolid transition either. All these results suggest that the NCRI transition is quite different from the superfluid transition in liquid He-4.