A diagram dictionary for electrons, which are FERMIons, and phonons, which are
Bosons, are shown in Table E. Diagrams for electrons are in
coordinate-time space, while phonon diagrams are in momentum-energy space. As
described in Section 3.1.1, the GREEN's function can be interpreted
as the creation of a particle at
in space-time, the
propagation of the corresponding perturbation to the point
in
space-time, where the particle is annihilated. Hence, the full GREEN's
function is represented by a double line joining these two points. The free
GREEN's function is characterized by a single line.
The COULOMB potential is represented by a wavy line with two inputs and outputs
which can be coupled together to describe a self-interaction. The COULOMB
interaction is assumed to be instantaneous. It is convenient to consider the
inter-particle potential as a static instantaneous potential proportional to a
delta function
.
Intermediate variables describe events taking place between the
two space-time arguments of the
GREEN's function, but without any constraints for exact time or place.
The overall amplitude involves an integration over these variables. Each time
a FERMIon loop appears, the perturbation expression corresponding to this
FEYNMAN diagram must be multiplied by a factor .
Electrons can also interact with phonons. For phonons it is more convenient
to work in the momentum-energy rather than in the space-time domain. Diagrams
concerning a free phonon GREEN's functions and the interaction between
electrons and phonons are also shown in Table E. The factor
refers to the electron-phonon interaction matrix elements.