Direct demonstration of circulating currents in a controllable π-SQUID generated by a 0 to π transition of the weak links

A controllable {π}-SQUID is a DC SQUID (direct-current superconducting quantum interference device) with two controllable {π} junctions as weak links. A controllable {π} 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 Vc over these contacts it is possible to control the state of the junction, i.e., a conventional (0) state or a {π} state, depending on the magnitude of Vc. We demonstrate experimentally that, by putting one junction into a {π} 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 π 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 {π} state as a function of Vc consistent with theoretical predictions. In a controllable {π}-SQUID based on Nb-Ag we observe, apart from a screening current at integer external flux, a phase shift of {π} of the VSQUID-B oscillations under a suitable current bias, depending on the magnitude of Vc.