Many Body Interactions – Introduction

Abstract

In this chapter, we briefly introduced method of second quantization and Hartree–Fock approximation to describe the ferromagnetism of a degenerate electron gas and spin density wave states in solids. Equation of motion method is considered for density matrix to describe gauge invariant theory of linear responses in the presence of the most general electromagnetic disturbance. Behavior of Lindhard dielectric functions and static screening effects are examined in detail. Oscillatory behavior of the induced electron density in the presence of point charge impurity and an anomaly in the phonon dispersion relation are also discussed.