Observation of and noise reduction by vortex lattice matching in YBa2Cu3O7 thin films and rf SQUIDs with regular arrays of antidots

Abstract

Abstract We report on the detection of matching effects between antidot and Abrikosov vortex lattices for fields far below the matching field Bm using rf SQUIDs and demonstrate, that antidots can strongly reduce the low-frequency l/f noise in active devices in unshielded environment. Square lattices of submicron holes (antidots) with diameters of 250–450 nm and lattice parameters ranging from 0.5–5 μm are patterned into optimized sputtered YBa2Cu3O7 thin films on CeO2-buffered sapphire substrates without deterioration of superconducting properties. A special experimental set-up consisting of an antidot lattice in flip-chip configuration with a bicrystal rf-SQUID has been used for the measurements. Thus, two different matching conditions are exposed upon the vortex lattice defined by the vortex–antidot interaction and the vortex–grain boundary interaction at the grain boundary in the washer. Vortices within the vicinity of the grain boundary of the washer can either be trapped (reduction of l/f noise) or exposed to a kind of double potential (increase of l/f noise) depending upon the magnitude of the applied magnetic induction and the geometrical arrangement between antidot array and grain boundary in the washer. Matching effects between vortex and antidot lattice could be observed in the form of minima in the noise spectrum for magnetic inductions much smaller then the matching field, e.g. for B*=(1/18)2Bm. The maxima in the low-frequency noise are a result of the existence of two matching conditions, i.e. caused by the presence of the grain boundary in the SQUID washer. The experiments demonstrate that thermally activated hopping of vortices can strongly be reduced by antidots in the superconducting layer. The resulting reduction of the low frequency excess noise bears a great potential for applications of active high-Tc superconducting devices if other matching conditions are avoided, e.g. by using step-edge type rf-SQUIDs.