Direct demonstration of circulating currents in a controllable π-SQUID generated by a 0 to π transition of the weak links -: art. no. 224513

A controllable pi-SQUID is a DC SQUID (direct-current superconducting quantum interference device) with two controllable pi junctions as weak links. A controllable pi junction consists of a superconducting-normal metal-superconducting Josephson junction with two additional contacts to the normal region of the junction. By applying a voltage V-c over these contacts it is possible to control the state of the junction, i.e., a conventional (0) state or a pi state, depending on the magnitude of V-c. We demonstrate experimentally that, by putting one junction into a pi state, a screening current is generated around the SQUID loop at an integer external flux. To be able to do this, we have fabricated controllable pi junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at 1.4 K only the Ag-Nb device shows the transition to a pi state as a function of V-c consistent with theoretical predictions. In a controllable pi-SQUID based on Nb-Ag we observe, apart from a screening current at integer external flux, a phase shift of pi of the V-SQUID-B oscillations under a suitable current bias, depending on the magnitude of V-c.