The current-phase relation of a mechanically controllable break junction

In order to determine the current-phase relation (CPR) of a mechanically controllable break (MCB) junction an adjustable SQUID has been developed. A ring, interrupted by a MCB junction, is cut out of a thin niobium foil using laser cutting techniques. In this SQUID the critical current can be varied continuously by adjusting the contact size of the junction. A new technique has been developed which enables us to measure directly the CPR. Superconducting current leads are attached on either side of the contact in order to determine the selfinductance of the ring. The CPR has been investigated at 4.2 K and at 1.3 K in a range of normal resistances which is estimated to be between 0.5 and 8 kΩ. A deviation from the pure sinusoidal CPR is found at 4.2 K while the CPR at 1.3 K seems to be nearly sinusoidal. It is striking that the CPRs at 4.2 K have a maximum in the current at values slightly smaller than π/2, while theories predict it to be between π/2 and π for superconducting pointcontacts with three-dimensional banks in this temperature range.