[en] Short communication. 4 refs

[en] Short communication. 2 refs

[en] Using the Bohm-Pines hamiltonian the authors present a method for calculating the effective interaction between two plasmons by solving the equation of motion of a double-time retarded commutator defined to represent the propagation of two plasmons. This is achieved by using a decoupling procedure for the higher-order plasmon Green functions which is based on the conservation of frequency moments. The effective interaction between two plasmons is found to be repulsive, in contrast with the results obtained earlier. (Auth.)

[en] A nonperturbative approach developed by Tripathy and Rao to the problem of charge transfer in proton-hydrogen collision has been applied to the formation of positronium in the ground state by shooting a beam of positrons over a gas of atomic hydrogen. The outgoing scattered wave-function in the incident channel is found to satisfy a Schroedinger equation where the potential field includes the Coulomb distortion effects and those due to the polarization of the target atom. The capture cross section has been calculated by neglecting the polarization effect in the energy range 10 eV to 780 eV for the incident positron. It is found that the cross section is higher than the Born result of Massey and Mohr at energies upto 650 eV and then gradually merges with their values for energies higher than this. The large discrepancy of the results at lower energies is expected to be due to the neglect of the polarization potential although some difference from the Born result at these energies is conceivable. The results have also been compared with those of a few recent researchers. (K.B.)

[en] Published in summary form only

[en] In a recent work it has been shown that even in an electron gas there is a T³lnT kind of contribution to its low temperature specific heat arising out of the temperature dependence of interparticle correlations. Extending it to liquid ³He it was possible to obtain a T³lnT contribution to its low temperature specific heat

[en] Using the Bohm-Pines hamiltonian the problem of the bound state of two plasmons is investigated following two different decoupling procedures where the energy transfer Ω = 0 or 2ωsub(pl). The effective interaction g₄ between the two plasmons is found to be repulsive for Ω = 2ωsub(pl) and attractive when Ω = 0. The case of Ω = 2ωsub(pl) is shown to be analogous to the situation under which the critical coupling g₄sup(c) is determined. (Auth.)

[en] The Bohm-Pines method for the collective description of the Coulomb interactions in a dense electron gas has been generalized to calculate the ground state energy of liquid ³He in its normal phase. The present calculation is based on the picture where one visualizes liquid ³He as an interacting system of two fields corresponding to the helium atoms and the quanta of its zero-sound mode. This gives a value of -2.55 K for the average binding energy per particle, which is found to be in excellent agreement with the experimental result -2.47 ± 0.01 K. The present theory gives rise to a value of 1.41 angstrom ^-1 for the critical wave vector k_c up to which the zero-sound modes exist as well-defined collective excitations of the system. This matches extremely well with the experimental result obtained by Skoeld et al

[en] Using the model Hamiltonian which has been derived by us recently for the study of liquid ³He, we have succeeded in reproducing a value of 204.2 m/s for the velocity of the zero-sound in liquid ³He at temperature T=0, by following a self-consistent method. This is close to the experimental result of 194.4 m/s as obtained by Abel et al. An exact agreement with the experimental value is obtained by introducing a term into our theory to account for the local field corrections to the bare particle-hole propagator beyond the random phase approximation (RPA) in a parametric way. By considering the effects of damping of the zero-sound mode, we have been able to explain fairly well the variation of the zero-sound velocity with temperature. The present theory seems to also indicate the propagation of a new kind of sound wave in liquid ³He having a velocity of 153.7m/s. (orig.)

[en] An attempt has been made to explain the second branch of excitations observed by Cowley and Woods in the recent neutron scattering experiments from superfluid ⁴He by evaluating the two phonon contribution, X₂(vector k,ω), of the dynamic structure function. The theory gives a good fit with the intensity distribution curve of the scattered neutrons in the deep inelastic scattering region. Using a model short range potential, it is shown that for Bose systems like superfluid ⁴He, the dominant contribution to X₂(vector k, ω) in the limit of small momentum and large energy transfer is of the order (ksup(2)/ωsup(5/2)) as against the result (ksup(4)/ωsup(7/2)), obtained in earlier theories. An interesting feature of the present theory is that it is able to reproduce a peak in the intensity versus ω curve for k=0.8 Asup(-1) around ω approximately equal to 22.1deg K which is close to the value of ω a.oproximately equal to 24.5deg K at the position of the experimentally observed second peak. (author)