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In the Raman spectrum of superfluid 4He under saturated vapour pressure at 1.51K, fine structures have been observed at energy shifts closely corresponding to twice the energies of the extremal points of the phonon-roton dispersion curve. From the observed cusps, twice the energies of the maximum and plateau regions of the dispersion curve have been determined to be 2 Delta 1=27.5+or-0.5K and 2 Delta 2=34.8+or-0.5K respectively. The two-roton bound state has been resolved as a pronounced single peak, and twice the roton energy and the attractive binding energy of the two-roton state have been determined to be 2 Delta 0=17.1+or-0.5K and EB=0.5+or-0.3K respectively. The observed fine structures clearly show the existence of the interactions between two elementary excitations having opposite and equal wavevectors.

Neutron inelastic scattering measurements have been made of the superfluid 4He 'one-phonon' dispersion relation at wavevector transfers between 2.7 AA-1 and 3.5 AA-1. These measurements were performed at a temperature of 1.1K and at pressures of 24.3 atm, 15.0 atm, and 1.4 atm. At the two higher pressures it was found that the energy of the 'phonon' was greater than 2 Delta , the value predicted by Pitaevskii, Delta being the excitation energy at the roton minimum. It is suggested that this discrepancy arises from the neglect of repulsive roton-roton interactions in the model. The magnitude of these interactions is estimated from the measurements and compared with that obtained using the Jackson-Feenberg theory of liquid helium.

The authors have investigated the temperature dependence of the rate nu at which negative ions nucleate vortex rings in isotopically pure superfluid 4He. Measurements of nu (ET) are presented for a pressure of 23 bar with electric fields E in 5.8

The total force between a vortex line and the rotons in rotating superfluid helium (mass 4) is calculated by summing -nabla(p.v5) over the entire roton fluid (rather than via the determination of roton-vortex collision diameters as in previous calculations). The corresponding values of the mutual friction parameters B and B' are determined using the Hall-Vinen theory (1956) and compared with experiment. Without a core one finds no longitudinal force, while the transverse force is of the wrong sign; these are results which are shown to be in close agreement with previous calculations when mistakes in the latter are corrected. With a roton absorbing core, very good agreement with experiment is achieved, except very close to Tlambda . The data show that the core radius is approximately 0.45 nm at 1.3K, increasing slowly with temperature until T approximately=1.7K and faster thereafter as Tlambda is approached.

It is shown that the neutron spectra of superfluid He in the energy range about 2 Delta can be well explained by assuming the roton-roton coupling constant g4 in the m=O channel to be repulsive. The endpoint of the single photon-roton curve is reconsidered by generalising Pitaevskii's theory to incorporate the finite lifetime of the single roton. The end point Qc is shown to depend on temperature. Theoretical estimation of Qc at 1.1K gives very good agreement with the experiment.

Magnetic counterflow is presented to account for the anomalous relaxation observed in superfluid helium-3. The difference between the A and B phases are explained as due to the vanishing critical spin velocity in the B phase. The importance of dissipation is emphasized and an experiment to check the validity of the model is proposed.

It is argued that an earlier calculation by Hille (see ibid., vol.14, p.4027, 1981) for the force between a vortex and the roton gas in He II was incomplete in that the emission of rotons by the vortex core was ignored. It turns out that the values previously deduced for the force parameters C and C' were in fact values of D and D'. The corrected calculation is in reasonable agreement with the mutual friction data for T

Various alternatives to the Balian-Werthamer model of the B phase of superfluid 3He are considered. The linear NMR properties of a certain f-wave state of axial symmetry are shown to be very similar to those required by experiment.

A new technique has been developed for measuring the nucleation rate nu of negatively charged vortex rings in HeII when the temperature T, pressure P and electric field E are such that the bare-ion drift velocity is limited principally by roton emission. For P=25 bar, E=7.9*105 V m-1 and 0.7

The second-order Raman scattering spectrum of superfluid helium has been examined with a grating spectrometer providing a resolution of 1K. Extensive studies at SVP in the region of the two-roton peak have not revealed the fine structure recently reported by other authors employing lower resolution. Possible reasons for this discrepancy are outlined. Measurements at higher pressures up to 15 atm show good agreement of the roton pressure shift with neutron scattering data.

The surface density as a function of temperature for H down arrow adsorbed on superfluid helium film is studied for several gas densities. The theory used is exact in the low-density limit and predicts Kosterlitz-Thouless transition temperatures lower than those predicted by previous calculations, and also predicts lower saturation density than predicted previously. The results confirm the possibility of a 2D superfluid transition under conditions close to those achieved experimentally.

A set of boson models, incorporating an excitation spectrum which produces a linear relation between the temperature and the superfluid density in narrow channels at low temperatures as seen by pobell et al. (see abstr. A25131 of 1972), are presented. These energy states are interpreted as arising from adatom-substrate interactions resulting from the heterogeneity of the surface.

It is shown that due to the hybridisation between the one-particle excitation and the two-roton states in superfluid 4He the interaction between the rotons is characterised by an affective roton-roton coupling g4eff instead of the bare roton-roton coupling. The magnitude of this interaction, estimated from the neutron scattering measurements at momentum transfer between 2.4 AA-1 and 3.5 AA-1, is g4eff approximately=-1*10-38 erg cm3 and therefore the neutron scattering measurements show the existence of the two-roton bound state even at large momentum values.

The frequency dependence of phonon transmission through a solid-liquid 4He interface is studied on the basis of a simple model of the solid-liquid coupling. For single-quantum processes the transmission coefficient is calculated using a T-matrix formalism and shows extremely sharp peaks, where the transmission probability becomes unity, at frequencies near the roton minimum and phonon maximum of the superfluid dispersion curve. Multi excitation processes in the helium are also analysed but are found to be small. The conditions required for observation of transmission peaks are discussed with reference to recent experimental data on high-frequency phonon transmission and modifications of the model are proposed.

The motion of negative ions in superfluid 3He when acted on by a field is calculated using a model in which the cross-section is taken to be constant. The theory is developed for both the A and B-phase It is argued that the scattering processes can be treated as being elastic for temperatures close to the transition temperature. The theory is in good agreement with experimental results for the B-phase of superfluid 3He for velocities in excess of the critical velocity. It is possible to estimate the energy gap from the data. The theory is not in agreement with experimental results for lower velocities. This implies that the momentum transfer cross-section is smaller than in the normal phase for quasiparticle scattering processes.

The energies of excitations in the region of the minimum in the dispersion curve for superfluid liquid 4He have been determined at SVP and T=0.75K using neutron inelastic scattering. The value of Delta /k is 8.618+or-0.009K. Comparison with recent Raman-scattering measurements gives a value of the roton-roton binding energy of 0.027+or-0.04K at Q=0.

A high resolution study has been made of the two-roton Raman scattering from liquid 4He at T=0.6K and SVP. This experiment shows conclusively that roton-roton interactions are necessary to explain the spectrum. From the shape of the two-roton peak the two-roton binding energy is estimated to be 0.22+or-0.07K. An intrinsic zero temperature linewidth of the pair equal to 0.11+or-0.01K has been found.

The theory is based on the calculation of moments of response functions using dense random packing of spheres as the model for the lattice. The response functions themselves are obtained by fitting the moments to a continued fraction expansion. The method appears well suited to the study of amorphous materials, and results are obtained for both the density of states and the spectral density of a simple model system. A possible explanation for the so-called 'roton minimum' observed in some neutron scattering measurements is also indicated.

The Raman scattering from liquid helium has been calculated by use of the measured dispersion relation for the excitations and a model for the coupling between the light and these excitations. A comparison of the results with experimental measurements suggests that the attractive interaction between the excitations is much weaker than presently supposed, and that the binding energy of a two-roton bound state, if it exists at all, is less than 0.1 K.

The mobilities of positive and negative ions have been measured in liquid helium in the temperature range 0.9-1.0 K as a function of electric field. Within the limits of accuracy of 3%, the mobilities have been fitted to a hydrodynamic theory for the interaction of the ions with rotons in the normal fluid component of He II. A drop in mobility with velocity is predicted. Both the results and those of Schwartz (1970) can be fitted to the theory. The theory contains one adjustable constant. This constant links the variation of ion drift velocity to the velocity field of the superfluid which is responsible for producing an excess roton distribution around the ion, and it relates a macroscopically observed velocity to the microscopic processes of momentum loss by the ion. The results show also no clear evidence for periodic discontinuities in mobility and the theory does not require their existence.

An apparent discrepancy between the binding energy of a roton pair determined by measuring the position of the peak in the light-scattering cross section and that determined by fitting the shape of the peak is resolved. By using a theory which includes the finite lifetime of the bound roton pair, the peak shape is fitted with a binding energy of 0.37K, which agrees with the peak position measurement.