Quantum fluctuations of the charge near the Coulomb-blockade threshold

In the vicinity of the Coulomb-blockade threshold, quantum fluctuations of the charge strongly influence the energy spectrum of small metallic junctions. We develop a nonperturbative calculation of the junction ground-state energy and evaluate the junction charge in the limit of a small junction conductance. Close to the Coulomb-blockade threshold this charge is effectively screened due to intensive virtual electron tunneling. We also describe level splitting close to the edges of the Brillouin zone and show that the electron tunneling rate can be substantially suppressed due to quantum fluctuations of the charge. As a result the junction current-voltage characteristic becomes non-Ohmic near the Coulomb gap.