Coulomb Blocking of Tunneling: From Zero-bias Anomaly to Coulomb Gap
  We present an effective action approach for the problem of Coulomb blocking of tunneling. The method is applied to the ``strong coupling'' problem arising near zero bias, where perturbation theory diverges. By a semiclassical argument, we obtain electrodynamics in imaginary time, and express the anomaly through exact conductivity of the system $\sigma(\omega, q)$ and exact interaction. The calculation is tested by comparing with the known perturbation theory result for the diffusive anomaly. Also, we use the method to study the anomaly enhancement due to external magnetic field, and the effect of screening by electrodes. 
L. S. Levitov (MIT) and A. V. Shytov (Landau Inst.)