MAGNETIC-FIELD-INDUCED CROSSOVER FROM 2E TO E-PERIODICITY IN THE SUPERCONDUCTING SINGLE-ELECTRON TRANSISTOR

We present extensive experimental data on the gate-charge-periodic cur rent I(Q(o)) through a single-electron transistor as a function of the applied magnetic field. This device consists of a mesoscopic supercon ducting island which is coupled to two macroscopic superconducting lea ds through small tunnel junctions and to a capacitive gate. The behavi or of the system exhibits two separate transitions as the magnetic fie ld is increased. In the first, the I-Q(o) curves cross from 2e to e pe riodicity in Q(o) while two-electron tunneling is still the dominant c harge transport mechanism. In a second transition that occurs at highe r magnetic fields, single-electron tunneling becomes the dominant mech anism. This transition from two-electron to single-electron tunneling shifts the maxima of the I-Q(o) curves by e/2 in Q(o), and results in a significant increase in the current magnitude. Both transitions can be understood by considering how superconductivity in the leads and th e island is affected by an increasing magnetic field.