Meandering of the grain boundary and d-wave effects inhigh-Tc bicrystal Josephson junctions

Abstract

We have produced submicrometre wide bicrystal Josephson junctions on the basis ofYBa2Cu3O7−δ (YBCO)films and investigated their microstructure and superconducting properties. A 300 nm thick buffer layerof SrTiO3 onthe SrTiO3 bicrystal substrates has helped to improve the quality and reproducibility of the junctions.The surface of the films at the grain boundary (GB) was investigated by high-resolutionscanning electron microscopy (HRSEM). This has revealed one unit cell steps on theYBCO film surface and meandering of the GB in the YBCO film. Planar transmissionelectron microscopy investigations of the junction have confirmed that the GB inYBCO had a linear deviation from the underlying GB in the substrate up to∼0.4 µm. The observed spread of the angles of the deviation of the GB in the film from the direction ofthe GB in the substrate on the slot- and other defect-free parts of the GB was below10°. Due to the d-wave symmetry of the order parameter in YBCO thereduction of the curvature of the GB leads to an improvement of thehomogeneity of the current flow and a doubling of the critical currentIc of the junction. Theobserved IcRn product ofsymmetric 20° bicrystaljunctions was ∼400 µV at 77.4 K and ∼1 mV at 63 K. Integration of such junctions in SQUIDs with a square 16 mm multilayer fluxtransformer has allowed us, for the first time, to reach a field resolution for thehigh-Tc magnetometers of ∼3.5 fT Hz−1/2 at 77.4 K.