chapter  4
19 Pages

DC SQUIDs

A dc squid consists of a ring of superconductor, interrupted by two Josephson junctions, preferably with at least similar properties (see figure 4.1). The introduction of a second weak link has the important effect of allowing a finite, time-averaged, direct voltage difference to be established across the junctions by a direct bias currenŧ. This arises because there is no strongly coupled superconducting short circuit across them, unlike the case of the single junction in an rf squid. (Although in principle the single junction could be dc voltage biased by a linear flux ramp applied to the ring, this is not a technique which has been much used in practice). Traditionally the operation of dc squids has been described in a phenomenological way and we shall first give such a description. For many years rf squids have dominated the applications field and although this has been mainly due to the greater difficulty of producing two similar junctions in a single loop it is also, one suspects, the result of a less than complete understanding of the dc device. It is only relatively recently that a numerical model of the two-junction system has been solved and even now a simple physical explanation of its operation is difficult to find, a situation which section 4.2 of this chapter attempts to remedy. Schematic circuit diagram of <sc>dc squid</sc>