We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase $\psi =\pm \phi$. The value of $\phi$ can be chosen by design in the interval $0<\phi <\pi$. The junctions used in our experiments are fabricated as 0-$\pi$ Josephson junctions of moderate normalized length with asymmetric 0 and $\pi$ regions. We show that (a) these $\phi$ Josephson junctions have two critical currents, corresponding to the escape of the phase $\psi$ from $-\phi$ and $+\phi$ states, (b) the phase $\psi$ can be set to a particular state by tuning an external magnetic field, or (c) by using a proper bias current sweep sequence. The experimental observations are in agreement with previous theoretical predictions.

• Received 13 June 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.107002