The magnitude, sign, and momentum dependence of the effective interaction between
He3 atoms in superfluid
He4 are derived from the experimental data. Calculations of the Fermi-liquid parameters for the normal state of
He3 in solution yield results in agreement with experiment; the temperature for a superfluid transition associated with the
He3 is estimated to be \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ \ifmmode^\circ\else\textdegree\fi{}K.
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.
The modifications of effective-range theory, necessitated by the long-range nature of interatomic dispersion and relativistic retardation forces, are discussed. A numerical comparison is given of parameters describing $^{3}\mathrm{\ensuremath{-}}^{3}$He, $^{3}\mathrm{\ensuremath{-}}^{4}$He, and $^{4}\mathrm{\ensuremath{-}}^{4}$He scattering at very low energies calculated with several recently published interaction potentials. The effect of relativistic retardation on these parameters and on the calculated binding energies of the $^{4}\mathrm{\ensuremath{-}}^{4}$He dimer is shown. It is demonstrated that all of these quantities are extremely sensitive to the potential. Scattering lengths and effective ranges for the lowest triplet interactions of H-H and D-D are also reported.
We predict
s-wave elastic cross-sections $\ensuremath{\sigma}$ for low-energy atom-molecule collisions with kinetic energies up to 40 mK for the
4He collision with weakly bound diatomic molecules formed by
4He with
7Li,6Li, and
23Na. Our scattering calculations are performed by using diatomic and triatomic molecular binding energies obtained from several available realistic models as input in a renormalized zero-range model as well as a finite-range one-term separable potential in order to quantify the relevance of range corrections to our predictions. Of particular relevance for possible experimental realization, we show the occurrence of a zero in $\ensuremath{\sigma}$ for the collision of cold
4He on a
4He23Na molecule below 20 mK. Also our results for the elastic collision
4He on
4He6,7Li molecules suggest that $\ensuremath{\sigma}$ varies considerably for the realistic models studied. As the chosen molecules are weakly bound and the scattering energies are very low, our results are interpreted on the light of the Efimov physics, which explains the model independence and robustness of our predictions despite some sensitivity on the potential range.
An energy-scaling rule for atom-atom scattering is derived. It is shown to be approximately accurate to third order in the parameter $\frac{\ensuremath{\Delta}M}{{M}_{\mathrm{av}}}$, where $\ensuremath{\Delta}M$ is the difference in the reduced masses between two different atomic pairs. The energy scaling depends upon the energy itself and on the interaction potential. The method is illustrated by the He-He scattering problem using the isotopes
He3 and
He4 in their three different combinations.
Nuclear magnetic susceptibility measurements of the
3He in dilute
3He-
4He mixture films are presented. At low
3He coverages (
d3<0.1 layer), the
3He behaves approximately as a 2D ideal Fermi gas. For larger submonolayer
3He coverages, the low-temperature susceptibility is enhanced, increasing with increasing amounts of
3He, contrary to the 2D ideal Fermi-gas model. No evidence for a phase transition in the
3He is seen.