OBSERVATION OF AND NOISE-REDUCTION BY VORTEX LATTICE MATCHING IN YBA2CU3O7 THIN-FILMS AND RF SQUIDS WITH REGULAR ARRAYS OF ANTIDOTS

We report on the detection of matching effects between antidot and Abr ikosov vortex lattices for fields far below the matching field B-m usi ng rf SQUIDs and demonstrate, that antidots can strongly reduce the lo w-frequency l/f noise in active devices in unshielded environment. Squ are lattices of submicron holes (antidots) with diameters of 250-450 n m and lattice parameters ranging from 0.5-5 mu m are patterned into op timized sputtered YBa2Cu3O7 thin films on CeO2-buffered sapphire subst rates without deterioration of superconducting properties. A special e xperimental set-up consisting of an antidot lattice in flip-chip confi guration with a bicrystal rf-SQUID has been used for the measurements. Thus, two different matching conditions are exposed upon the vortex l attice defined by the vortex-antidot interaction and the vortex-grain boundary interaction at the grain boundary in the washer. Vortices wit hin the vicinity of the grain boundary of the washer can either be tra pped (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 m agnetic induction and the geometrical arrangement between antidot arra y and grain boundary in the washer. Matching effects between vortex an d 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)B-2(m). The maxima in the low-frequency noise are a result of the existence of two matching conditions, i.e. caused by t he 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 re duction of the low frequency excess noise bears a great potential for applications of active high-T-c superconducting devices if other match ing conditions are avoided, e.g. by using step-edge type rf-SQUIDs. (C ) 1998 Published by Elsevier Science Ltd. All rights reserved.