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

Abstract

A controllable $π$-SQUID is a DC SQUID 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 $V_c$ over these contacts it is possible to control the sate of the junction, i.e. a conventional (0) state or a $π$-state, depending on the magnitude of $V_c$. We demonstrate experimentally that, by putting one junction into a $π$-state, a screening current is generated around the SQUID loop at 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 Nb-Ag device shows the transition to a $π$-state as a function of $V_c$ consistent with theoretical predictions. In a controllable $π$ SQUID based on Nb-Ag we observe, a part from a screening current at integer external flux, a phase shift of $π$ of the $V_{SQUID}-B$ oscillations under suitable current bias, depending on the magnitude of $V_c$.