ABSTRACT
Screening of the local moment below Tk is the essence of the Kondo problem. A much less exotic example of screening takes place when a positive charge Q is introduced into an electron gas. To restore charge neutrality at the site of the positive charge, the electrons will screen the charge, thereby decreasing its net electric field. In fact, screening occurs regardless of the sign of the test charge. This state of affairs arises because as an electron moves through the electron gas, it does not have to push all the other electrons out of the way. The mutual repulsions among all the electrons in the electron gas help to clear a path for an electron to move. As a result, the effective interaction between the electrons is diminished from the long-range Coulomb 1/r to a much more short-ranged interaction. At the level of Thomas-Fermi, the new interaction falls off exponentially. While this interaction overestimates the effect of screening, it does illustrate how efficiently repulsive interactions screen electrons in an electron gas.
