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

TL;DR

This paper demonstrates the experimental control of circulating currents in a $ ext{pi}$-SQUID by inducing a $ ext{0}$ to $ ext{pi}$ transition in weak links, confirming theoretical predictions and showing phase shifts in SQUID oscillations.

Contribution

It introduces a new controllable $ ext{pi}$-junction geometry with Cu-Nb or Ag-Nb, and experimentally demonstrates the $ ext{0}$ to $ ext{pi}$ transition and associated circulating currents.

Findings

Only Nb-Ag devices show $ ext{pi}$-state transition at 1.4 K.

A phase shift of $ ext{pi}$ in SQUID oscillations is observed.

Circulating currents are generated at integer external flux.

Abstract

A controllable $\pi$-SQUID is a DC SQUID 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 sate 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 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 Nb-Ag 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, a part from a screening current at integer external flux, a phase shift of $\pi$ of the $V_{SQUID}-B$ oscillations under suitable current bias, depending on the magnitude of $V_c$.