Ever since the seminal torsional oscillator (TO) measurements of Kim and Chan which suggested the existence of a phase transition in solid 4He, from normal to a ‘supersolid’ state below a critical temperature Tc= 200 mK, there has been an unprecedented amount of excitement and research activity aimed at better understanding this phase. Despite much work, this remarkable phase has yet to be independently confirmed by conventional scattering techniques, such as neutron scattering. We have carried out a series of neutron scattering measurements, which we here review, aimed at observing Bose-Einstein condensation (BEC) in solid 4He at temperatures below Tc. In bulk liquid 4He, the appearance of BEC below Tλ signals the onset of superfluidity. The observation of a condensate fraction in the solid would provide an unambiguous confirmation for ‘supersolidity’. Although, our measurements have not yet revealed a non-zero condensate fraction or algebraic off diagonal long-range order n0 in solid 4He down to 65 mK, i.e. n0=(0±0.3)%, our search for BEC and its corollaries continues with improved instrumentation.
BEC of excitations Supersolids Glasses and defects
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