This invention relates to an oxide superconductor Josephson junction element having a Josephson junction using an oxide superconductor and a process of producing same. Specifically, the present invention pertains to a Josephson junction using only an oxide superconductor having a junction of (110) plane and (001) plane of an oxide superconductor which is a d-wave superconductor.
In Josephson junctions which represent a basic oxide superconductor electric device, it is reported in, for example, an article of Y. Ishimaru et al., Jpn. J. Appl. Phys. 34 (1995) L1123 and Solid Static Physics, June (1966) p541-548, that a Josephson junction using an oxide superconductor having a (110)-(001) junction of a d-wave superconductor using only an oxide superconductor, for example, a junction in which a needle-like a-axis (or b-axis) oriented oxide superconductor film is sandwiched between c-axis oriented oxide super conductor films to use, as a weak link, the needle-like a-axis (or b-axis) oriented portion or the crystal boundary between the c-axis oriented portion and the a-axis (or b-axis) oriented portion, exhibits excellent characteristics such as relatively high IcRn (critical current x normal conductive resistance (Rn)) and current voltage characteristics of a clear weak link which may be explained by the RSJ model.
In a junction using a needle-like, a-axis (or b-axis) oriented, oxide superconductor film region which has been grown such that the direction of the c-axis thereof is inclined at an angle of 45 degrees with respect to the (100) or (010) direction of a substrate and a c-axis oriented oxide superconductor film, phenomena specific to copper-type oxide superconductors, such as a phenomenon that the Ic is minimum in a zero magnetic field, are reported, as described in the above article of Y. Ishimaru et al., Jpn. J. Appl. Phys. 34 (1995) L1532.
In the above prior art, the fabrication of the junction is limited to a method in which needle-like a-axis (or b-axis) oriented portions, which are formed in happenstance in a c-axis oriented film, are utilized. The formation of such needle-like a-axis (or b-axis) oriented portions is considered to be attributed to, for example, the influence of the temperature of the substrate or the atmosphere in which the oxide superconductor film has been formed and the roughness of the surface of the substrate. It is, therefore, difficult to control the formation of such portions. It is completely impossible to form such portions at desired positions of the substrate with good reproducibility. Thus, the above technique is not suited for purposes other than the study of the Josephson junctions per se. There is, accordingly, a problem that the practical applications to, for example, superconducting quantum interference devices (SQUID) are not possible.