Superfluidity and Bose-Einstein Condensation in solid helium
Searching for superfluidity in solids.
Superfluidity in solid helium was first reported in 2004. This remarkable behaviour is observed by oscillating the solid. A fraction of the solid ceases to oscillate with the rest of the solid, denoted the superfluid fraction. While superflow is observed in gases and liquids, it is unexpected in solids where the atoms are localised at lattice points. In fluids superfluidity is a consequence of Bose-Einstein Condensation (BEC), the condensation of a macroscopic fraction of the atoms into one single particle state. The BEC fraction can be observed using inelastic neutron scattering, a measurement ideally suited to the Mari instrument at ISIS. We have measured the BEC fraction in solid helium on Mari and find that it is zero within 1%, in other words no BEC was observed within the measurement precision, whereas it is found to be 7% in liquid helium. Thus BEC, the property most closely associated with superflow, has not yet been observed in solid helium and the search for corollaries of superflow continues.
SO Diallo, HR Glyde (University of Delaware), JV Pearce (National Physical Laboratory), RT Azuah (NIST Centre for Neutron Research), O Kirichek, JW Taylor (ISIS)