Single electron tunnelling in small tunnel junctions

Electrons can be hindered to tunnel at voltages less than a threshold value and they can be made to tunnel periodically, one by one through a small tunnel junction. For this, the junction capacitance has to be sufficiently small such that the charging energy becomes larger than the thermal energy, the junction resistance larger than the quantised resistance value, and the impedance of the junction environment high. The current transport for a small island that is connected to the outside by two high-resistance tunnel junctions can be controlled by charge, less than 0.0001e, supplied by a gate (single charge tunnel transistor). The periodic variation of the current on gate charge may switch from a period given by e to either e- or 2e-periodic behaviour if the island becomes superconducting, depending upon the relative size of the superconducting gap and the charging energies. This may be used to probe the gap of high-T-c superconductors. Coulomb blockade phenomena are complementary to Josephson effects and there are several interesting analogies in properties and applications. Phase transitions may occur in networks of high-resistance Josephson junctions, and correlated transport may be seen in coupled arrays of junctions.