On a New Decoupling Scheme for the Retarded Green Function and Its Application to the Heisenberg Ferromagnet

Abstract

The problem on the decoupling of the retarded Green function is discussed in a vector space by making use of the method of orthogonal operators expansion. Usual operators are expressed by vectors and a tetradic operator such as the Liouville operator is expressed by a matrix, respectively. With these representations a new and general decoupling scheme is contrived. The utility of the scheme is testified by applying to the Heisenberg ferromagnet. At low temperatures the theory gives the correct spin wave spectrum and at higher temperatures yields a reasonable solution.