Bose-Einstein Condensation and Kinetic energy of liquid ³He-⁴He Mixtures

الأصل · EN

We present neutron scattering measurements of the momentum distribution of liquid ³He-⁴He mixtures. The experiments were performed at wavevectors Q, 26 ≤ Q ≤ 29 Å⁻¹, on the MARI time-of-flight spectrometer at the ISIS Facility, Rutherford Appleton Laboratory, a spallation neutron source. Mixtures with ³He concentrations x between 0 and 20% were investigated both in the superfluid and normal phases. From the data, we extract the Bose-Einstein condensate fraction n₀ and the momentum distributions of ³He and ⁴He atoms. We find that n₀ increases somewhat above the pure ⁴He value when ³He is added; e.g from n₀=(7.25±0.75)% at x= 0 to (11±3)% at x= 15-20%. This agrees with predictions but is less than the only previous measurement. We find a ⁴He kinetic energy K₄ for pure ⁴He that agrees with previous determinations. K₄ decreases somewhat with increasing ³He concentration, less than observed previously and found in early calculations but in agreement with a more recent Monte Carlo calculation. The ³He response is not well reproduced by a Fermi gas momentum distribution, n(k). Rather an n(k) having a small step height at the Fermi surface and a substantial high momentum tail characteristic of a strongly interacting Fermi liquid provides a good fit. This n(k) is consistent with calculated n(k). Thus agreement between theory and experiment is obtained comparing n(k) in contrast to earlier findings based on comparing calculated and observed ³He kinetic energies.