THE CURRENT-PHASE RELATION OF A MECHANICALLY CONTROLLABLE BREAKJUNCTION

In order to determine the current-phase relation (CPR) of a mechanical ly controllable break (MCB) junction an adjustable SQUID has been deve loped. A ring, interrupted by a MCB junction, is cut out of a thin nio bium foil using laser cutting techniques. In this SQUID the critical c urrent can be varied continuously by adjusting the contact size of the junction. A new technique has been developed which enables us to meas ure directly the CPR. Superconducting current leads are attached on ei ther side of the contact in order to determine the selfinductance of t he ring. The CPR has been investigated at 4.2 K and at 1.3 K in a rang e of normal resistances which is estimated to be between 0.5 and 8 k O mega. 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 t he CPRs at 4.2 K have a maximum in the current at values slightly smal ler than pi/2, while theories predict it to be between pi/2 and pi for superconducting pointcontacts with three-dimensional banks in this te mperature range.