Coulomb blockade and nonperturbative ground-state properties of ultrasmall tunnel junctions

We present a nonperturbative calculation of the ground-state energy of a normal tunnel junction. The junction with a large conductance shows Coulomb blockade of tunneling provided the external charge ${\mathit{Q}}_{\mathit{x}}$ is less than e/4. For ${\mathit{Q}}_{\mathit{x}}$>e/4, the band is flat and the junction behaves like an Ohmic resistor. We predict a phase transition between insulating and conducting behavior of the junction controlled by an external Ohmic resistance. We plot the corresponding zero-temperature phase diagram and calculate the junction effective resistance.