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On the basis of a moment method, general solutions of a linearized Boltzmannequation for a normal Fermi system are investigated. In particular, we studythe sound velocities and damping rates as functions of the temperature and thecoupling constant. In the extreme limits of collisionless and hydrodynamicregimes, eigenfrequency of sound mode obtained from the moment equationsreproduces the well-known results of zero sound and first sound. In addition,the moment method can describe crossover between those extreme limits at finitetemperatures. Solutions of the moment equations also involve a thermaldiffusion mode. From solutions of these equations, we discuss excitationspectra corresponding to the particle-hole continuum as well as collectiveexcitations. We also discuss a collective mode in a weak coupling case.

Zero and First Sound in Normal Fermi Systems