Moat-guarded Josephson SQUIDs

We report experimental investigations of a simple structure, called a moat, which significantly reduces the probability of flux-trapping in Josephson SQUIDs. Proper operation of Josephson logic and memory circuits requires that the SQUIDs be free of stray magnetic flux that may become trapped in the superconducting groundplane upon cooling through the critical temperature. The problem is particularly severe for so-called holey SQUIDs which rely on holes in the groundplane to obtain suitably large device inductances. Moats are rectangular channels in the groundplane surrounding the SQUID´s which provide preferred sites for trapping flux, thus preventing such flux from coupling to the SQUID. We have measured the effectiveness of moats by monitoring the flux trapped in the moats and comparing it to the flux trapped in the associated SQUID as a function of applied field. The number of flux quanta in the moat is determined by measuring the shift of the threshold curve of a two-junction SQUID coupled to the moat. The data indicates that at fields on the order of a mG, moats reduce the sensitivity of holey SQUIDs to trapped flux by at least several orders of magnitude. As the chips are cooled through Tc, transient magnetic fields are produced in the metallic sample holder parts surrounding the chip by thermal-gradient-induced EMF´s. The effects of such magnetic fields on the flux trapping behaviour of the SQUIDs are also reported.