ONE-PARTICLE AND COLLECTIVE ELECTRON SPECTRA IN HOT AND DENSE QED AND THEIR GAUGE DEPENDENCE

The one-particle electron spectrum is found for hot and dense QED and its properties are investigated in comparison with the collective spectrum. It is shown that the one-particle spectrum (in any case its zero momentum limit) is gauge-invariant, but the collective spectrum, being qualitatively different, is always gauge-dependent. The exception is the case m,μ=0 for which the collective spectrum long wavelength limit demonstrates the gauge invariance as well.

SINGLE-PARTICLE EXCITATIONS IN SUPERFLUID HELIUM

We show theoretically that the collective spectrum of a Bose liquid contains, besides the well-known phonon-roton branch, single-particle excitations existing below the roton-stability boundary k r , but above a threshold kt. The existence of these single-particle excitations is not an experimental artifact as has been supposed earlier. Calculations made for superfluid helium give the threshold momentum k t ≅1 Å−1. This value and the attenuation found of the single-particle excitations are in very good agreement with experimental data.