8. The Quasi-Particles

Philippe Nozières, David Pines

The wave propagation might be appropriately called quasi-particle sound, since in the long wave-length limit the only density fluctuations of importance are those produced by exciting a single quasi-particle from the condensate. Quasi-particle sound has essentially the same physical origin as zero sound in a Fermi liquid. The quasi-particle sound velocity will be temperature dependent, since the energy of a given quasi-particle depends on the thermal excitation of other quasi-particles. Superfluid hydrodynamics is richer than its Fermi liquid counterpart because one can have relative motion of the thermal quasi-particles and the condensate. This chapter describes the propagation of first and second sound in the hydrodynamic limit and then go on to a consideration of quasi-particle sound in the collisionless regime. A wide variety of irreversible phenomena which have as their physical origin collisions between the thermally-excited quasi-particles.

Brigitte Falkenburg

Philippe Nozières, David Pines

The density fluctuation excitation spectrum is continuous, since all particle transitions allowed by the Pauli principle will contribute. This chapter shows that the quasi-particle, as a bare particle moving together with the surrounding distortion brought about by particle interactions. The importance, or indeed the existence, of the mechanism, depends on the quasi-particle energies and the phase space available for the decay process. Higher order collision processes can also limit the lifetime of a quasi-particle. In contrast to the single quasi-particle excitations, the multi-particle excitations will be spread over a continuous range of energies, as one allows the momentum of any single quasi-particle component to vary. For an arbitrary particle interaction, there is no a priori region of momenta for which quasi-particles exist as well-defined elementary excitations of the system. In contrast to the single quasi-particle excitations, the multi-particle excitations will be spread over a continuous range of energies, as one allows the momentum of any single quasi-particle component to vary.

J. Högaasen-Feldman

Yanxian Zhang

The concept of quasi-particles is analyzed briefly in this paper,and then the application of laws of quantum statistics abided by the system of quasi-particles is discussed systematically.On the basis of the analysis and the discussion,the general methods to deal with the system of quasi-particles are presented finally.

Kenneth W. Ford