Bose-Einstein Condensation and Kinetic energy of liquid ${}^3$He-${}^4$He Mixtures

Abstract

We present neutron scattering measurements of the momentum distribution of liquid ${}^3$He-${}^4$He mixtures. The experiments were performed at wavevectors $Q$, 26 $\le$ $Q$ $\le$ 29 {Å${}^{-1}$}, on the MARI time-of-flight spectrometer at the ISIS Facility, Rutherford Appleton Laboratory, a spallation neutron source. Mixtures with ${}^3$He 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 $n_0$ and the momentum distributions of ${}^3$He and ${}^4$He atoms. We find that $n_0$ increases somewhat above the pure ${}^4$He value when ${}^3$He is added; e.g from $n_0=(7.25\pm 0.75)$ at $x=$ 0 to $(11\pm 3)$ at $x=$ 15-20%. This agrees with predictions but is less than the only previous measurement. We find a ${}^4$He kinetic energy $K_4$ for pure ${}^4$He that agrees with previous determinations. $K_4$ decreases somewhat with increasing ${}^3$He concentration, less than observed previously and found in early calculations but in agreement with a more recent Monte Carlo calculation. The ${}^3$He response is not well reproduced by a Fermi gas momentum distribution, $n(\mathbf{k})$. Rather an $n(\mathbf{k})$ 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 $n(\mathbf{k})$ is consistent with calculated $n(\mathbf{k})$. Thus agreement between theory and experiment is obtained comparing $n(\mathbf{k})$ in contrast to earlier findings based on comparing calculated and observed ${}^3$He kinetic energies.