Coulomb Gap in the Density of States of Disordered Metals in Two   Dimensions
  We calculate the effect of Coulomb interactions on the average density of states nu (omega) of two-dimensional disordered electrons. It is shown that for weak disorder the most singular terms in the perturbative expansion of nu (omega) can be summed by means of a simple gauge transformation, which also establishes a relation between the low-frequency behavior of nu (omega) and the average conductivity sigma (omega). Using this relation, we show that if lim_{omega rightarrow 0} sigma (omega) is finite, then nu (omega) approaches C | omega | / e^4 for omega rightarrow 0, where C is a dimensionless constant and e is the charge of the electron. This implies that a normal metallic state of disordered electrons in two dimensions is not a Fermi liquid. 
Peter Kopietz