Single-electron tunneling transistors incorporating Cooper pair processes

A superconducting single-electron tunneling transistor composed of two ultrasmall capacitance Al/Al/sub 2/O/sub 3//Al tunnel junctions and a capacitively coupled gate electrode has been fabricated. Transistor operation is based on single electron and Cooper pair charging effects. This three-terminal device exhibits novel I-V characteristics not seen in either conventional superconducting tunnel junctions or normal metal Coulomb blockage devices. Current peaks appear above V approximately 2 Delta /e which originate from combined Cooper pair/quasi-particle tunneling processes. These peaks show e-periodic modulation with respect to the gate-induced charge. At lower voltages, the I-V curve shows features which are 2e-periodic. In a magnetic field, it is found that the 2e-periodicity changes into e-periodicity above a crossover line, T*(H). The data strongly suggest the existence of a free energy difference between states with an even versus an odd number of electrons on the metal island between the two junctions.<>