Direct Observation of the Transition from the Conventional Superconducting State to the π State in a Controllable Josephson Junction

We measure the full supercurrent-phase relation of a controllable π junction around the transition from the conventional 0 state to the π state. We show that around the transition the Josephson supercurrent-phase relation changes from IscIcsin(φ) to IscIcsin(2φ). This implies that, around the transition, two minima in the junction free energy exist, one at φ=0 and one at φ=π, whereas only one minimum exists at the 0 state (at φ=0) and at the π state (at φ=π). Theoretical calculations based on the quasiclassical theory are in good agreement with the observed behavior.