Functional Integral Approach to the Bound State Due to thes-dExchange Interaction

The Stratonovich-Hubbard transformation is applied to the partition function of the s-d exchange system to bring it into a functional average form over flucutuating fictitious s-d admixtures in order to study the singlet bound state. The static approximation is examined and effects of fluctuating components are taken into account by a perturbational method from the static approximation in the approximation of the most divergent terms. The correct binding energy is derived at 0°K. Calculations at low temperatures or at weak magnetic fields are achieved, on an assumption, to lead the normal behavior of the system. Green functions are also transformed similarly and their general property is discussed.