The authors present for the first time an effective interaction between two
4He atoms in a bath of liquid
3He at temperatures below 0.1 K. This interaction turns out to be, in its long-range limit, an oscillatory function of the
4He interatomic separation, the oscillations arising essentially from the sharp discontinuity in the momentum distribution at the Fermi surface. In addition, an expression is formulated for the oscillations in the density of the
3He bath induced by one
4He impurity, as well as a modified Friedel sum rule, which relates the relative amount of matter displaced by this impurity to the phase shifts pertaining to the impurity-
3He quasiparticle scattering at the Fermi surface. Finally, the dependence of the results on temperature and on the
3He superfluidity gap is discussed.
Dilute mixtures of
He3 in liquid
He4 are regarded as a set of
He3 atoms intereacting with elementary excitations of the
He4. The coupling between the
He3 and the
He4 is chosen phenomenologically and it is not weak but, for low temperatures, its main effects may be eliminated by means of two canonical transformations. The equations which determine the transformations describe the motion of a
He4 excitation in the presence of one or two
He3 atoms and, in the latter case, they resemble Faddeev's equations in the three-body problem. At low temperatures, there are few
He4 excitations and many of the properties of the mixture are determined by the effective
He3 Hamiltonian produced by the transformation. In this respect, the system behaves like a Fermi liquid. The
He3-
He3 potential is essentially momentum-independent and consists of the Van der Waals interaction together with an effectively repulsive potential induced by the
He4. It is suggested that a Fermi-liquid analysis is the best way of obtaining information about this potential, and the possible existence of a fermion superfluid phase transition is discussed from that point of view.
We present neutron-scattering measurements of the momentum distribution of liquid $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ mixtures. The experiments were performed at wave vectors
Q, $26\ensuremath{\leqslant}Q\ensuremath{\leqslant}29\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$, 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 ${n}_{0}=(7.25\ifmmode\pm\else\textpm\fi{}0.75)%$ at
x=0 to $(11\ifmmode\pm\else\textpm\fi{}3)%$ 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.
This book is about a certain class of superfluids, namely the high temperature superconductors. Perhaps the simplest of the superfluids is liquid He4. Since the very early days of quantum mechanics it has been known that the helium atom He4 is a boson; it is built of an even number of particles (two electrons, two neutrons and two protons), so that a wave function ^(ri,F2) must be symmetrical in FI — r? where FI, r-z are the electron's coordinates. This has the result that half the rotational states of the molecule H62 are absent and that the scattering of alpha particles by helium for low energies deviates from the Rutherford scattering law. It also has the remarkable result that this is not so for the light isotope He3, which shows that the number of neutrons in the nucleus affects properties depending otherwise only on the electrons and nuclear charge.