Experimental study on stacked Josephson tunnel junction arrays under microwave irradiation

We investigated the behavior of vertically stacked tunnel junctions in a circuit suitable for voltage standards. Such three-dimensional (3D) circuits might offer a considerable reduction of chip size when compared with conventional, two-dimensional circuits. Up to 84 series connected stacks with four junctions in a stack were incorporated into a microstripline. In the current-voltage characteristic, we measured quantized current steps until 100 mV upon applying microwave irradiation in the W band. We observed both chaotic and nonchaotic behavior. Whether chaos or nonchaotic behavior is observed under microwave radiation depends on the coupling strength which is defined by the intermediate niobium layer thickness between the junctions of a stack. Large thickness corresponds to weak coupling and absence of chaos. We showed that 3D circuits are promising for voltage standard application.