ABSTRACT
The random phase approximation predicts the energies of the single-particles like excitations to be identical with corresponding excitation energies in the non-interacting system, the "wave functions" of the excitations are markedly different because of correlations existing between the excited electron and the background electrons. H. V. Bohm and D. Pines were the first to discover these excitations in metals and named them plasmons. Screening also plays an important role in determining the dressed phonon frequencies and the dressed electron–phonon interaction, since the long-range Coulomb forces between ions and between ions and electrons are screened out by the conduction electrons. The resolution of the singularities leads to a qualitatively new state of the system which exhibits an energy gap in the elementary excitation spectrum of the electrons and corresponds to the superconducting state of the metal.
