HIGH-QUALITY YBCO DC SQUIDS BASED ON BICRYSTAL SUBSTRATES

An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of a c-axis-oriented YBCO film on a SrTiO3 bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle theta = 20 degrees are reported. The SQUID shows a critical temperature T-c approximate to 89 K and a high critical current density J(c) approximate to 3.10(6) A/cm(2) at T = 4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependence J(c)(T) shows a linear behaviour at small reduced temperatures and a depressed J(c) value close to T-c. High-quality flux-voltage curves have been found up to T = 87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of theta make YBCO bicrystal GBJs very useful for applications in electronics.