Direct observation of the transition from the conventional superconducting state to the π state in a controllable Josephson junction -: art. no. 207002

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