Magnetic field behavior of vertical stacks of Josephson junctions with large idle regions

We report first measurements of the magnetic field dependence of the critical currents of vertical stacks of two Nb/Al-AlO/sub x//Nb Josephson junctions with large idle regions. We kept constant the width of the lateral idle region and changed the ratio of the longitudinal idle part to the junction length. To provide a close coupling of the two junctions of the stack, the thickness of the intermediate Nb electrode was smaller than the London penetration depth. The effect of the idle region on the magnetic behavior of the stack is discussed. In same experiments, a clear and nearly periodic current locking effect has been observed, that was independent of the dimensions of the idle region.