MAGNETIC-FLUX AND GATE VOLTAGE MODULATION OF THE CURRENT IN A SUPERCONDUCTING LOOP OF ULTRA-SMALL TUNNEL-JUNCTIONS

We have measured the current-voltage (I-V) characteristics of a loop o f four superconducting tunnel junctions at low temperatures (T almost- equal-to 50mK). In each branch of the loop, tunneling occurs through a small capacitance island (C(SIGMA) almost-equal-to 1fF). In this geom etry we can vary the magnetic flux PHI in the loop and the voltage V(g ) on gate electrodes, which have capacitance C(g) to the small islands . We find that both PHI and V(g) modulate the I-V curve.