Direct Observation of the Current-Phase Relation of an Adjustable Superconducting Point Contact

By placing a mechanically controllable breakjunction in a superconducting loop the current-phase relation is measured over the complete phase range for atomic-size quantum point contacts with direct conductivity. The current-phase relations show at low temperatures a clear nonsinusoidal behavior with an extremum at phase differences between $\pi /2$ and $\pi$. When the self-inductance of the loop is reduced the measured current-phase relations approach the predictions of the theory for (quantum) point contacts in the ballistic limit.