Current in normal and superconductors
In this paper the center of mass momentum operator is derived from its generating function. It is emphasized that this operator describes current in normal conductors. A state which is an eigenstate of this operator has a finite Drude weight and is also delocalized. An external potential is a necessary but not sufficient condition for insulation. Whether a model corresponds to an insulator or conductor depends on the ratio of the the external potential to the energy scale of the underlying interacting system. It is also shown that a normal conductor of finite size exhibits flux quantization, and it is only in the thermodynamic limit that the flux becomes continuous. The Hubbard model is analyzed, and it is demonstrated that it can not exhibit insulating behaviour. Both the large $U$ and small $U$ states are perfect conducting states, the difference is that the former does not exhibit flux quantization whereas the latter does.