Quasi-particles at finite temperatures

Peter A. Henning, Hiroomi Umezawa

Kulveer Kaur

Nicolaas Marinus Hugenholtz

Yuji Nakawaki, Azuma Tanaka, K. Ozaki

To investigate validity of non-scattering of quasi-particles ( S = 1) at finite temperature, which Narnhofer et al. claimed, we assume asymptotic completeness at zero temperature and consider whether out-going particles are well-defined or not at finite temperature in Fock space of in-coming particles. It is found in nonlinear Schrödinger model that contrary to the anticipation of Narnhofer et al., not only one particle operator but also continuous background operator contribute to construct in-coming and out-going particles, which are not identical. However Fock spaces of them become inequivalent so that non-scattering of quasi-particles at finite temperature remains not to be denied. It is also noted that LSZ reduction formula for S -matrix does not apply at finite temperature.

Philippe Nozières, David Pines

We now consider the nature of the excited states of a Bose liquid at finite temperatures. In Chapters 2 and 3 , we have discussed the quasi-particle excitations at finite temperatures. We here wish to concentrate on the nature of the excited states in the long wave-length limit . As in the previous chapters, we shall restrict our attention to states for which the condensate is very nearly uniform.

Robert Arvieu, Vincenzo Manfredi