Observation of a controllable $\pi$ junction in a 3-terminal Josephson device

Recently Baselmans et al. [Nature, (London) $397,$ 43 (1999)] showed that the direction of the supercurrent in a superconductor/normal/superconductor Josephson junction can be reversed by applying, perpendicularly to the supercurrent, a sufficiently large control current between two normal reservoirs. The unusual behavior of their 4-terminal device (called a controllable $\pi$ junction) arises from the nonequilibrium electron energy distribution established in the normal wire between the two superconductors. We have observed a similar supercurrent reversal in a 3-terminal device, where the control current passes from a single normal reservoir into the two superconductors. We show theoretically that this behavior, although intuitively less obvious, arises from the same nonequilibrium physics present in the 4-terminal device. Moreover, we argue that the amplitude of the $\pi$-state critical current should be at least as large in the 3-terminal device as in a comparable 4-terminal device.