No abstract

Nb-Nb0,-Pb tunnel junctions with areas of about 10 x 10 pm2 and high current densities are prepared by cleaning the niobium surface in a rf glow discharge. The dependence of the critical Josephson currents on magnetic field is investigated. To a first approximation the behaviour of the critical current is described by the theory of one-dimensional junctions. Critical current densities up t~ 2 x 105 A/cma are reached. Nb-NbO,-Pb-Tiinnelverbindui~genmit Fliichen von etwe 10 X 10 ym2 und mit hohen Strorndichten werden durch Reinigen der Nioboberflache in einer HF-Glimmentladung hergestellt. Die Abhingigkeit des kritischen Josephsonstromes vom Magnetfeld wird untersucht. In erster Niiherung wird das Verhalten des kritischen Stromes durch die Theorie fur eindimensionale Tunnelverbindungen beschrieben. Es werden kritische Stromdichten bis zii 2 x 105 A/cm2 erreicht. F. DETTMAXN and P. B. WEBER where y is the phase difference between the two superconductors and [8 to 101is the Josephson penetrat*ion depth, 4 and 1, are the London penetration depths, dl and d, the thicknesses of the electrode films, t is the thickness of the barrier, jo the maximum current density, Go the flux quantum, and pcco the free space permeability. The current density in bhe barrier is given by

Superconducting quantum interference devices with tunnel junctions were reported by Clarke et al. /l/, Ketchen et al. /2, 3/, and Ehnholm et al. /4/ Using a cylindrical thin film SQUID in which'is placed a wire wound input coil Clarke et al. have reached the highest sensitivity up to now. Disadvantages in further enhancement of the sensitivity of this construction are firstly the cylindrical shape which leads to an undesirable influence by external magnetic perturbation fields and secondly the wire wound input coil. This coil allows only relatively small values for the coupling factor between coil and SQUID, especially in the case of high sensitive devices with low inductances. The thin film SQUID with thin film gradiometer antenna reported in /2, 3/ is not extremely sensitive because of the mismatch between the high gradiometer inductance and the low inductance of the SQUID. We have solved the problem of coupling the input signal into the SQUID with a construction which uses consequently the flat thin film technique. A sufficiently low SQUID inductance and number of windings of the input coils required for matching are realizable by means of the modarn thin film technology only, if a superconducting screening film is used. A schematic of our device is shown in Fig. 1. The SQUID cons i s t s of a first superconducting film which forms the contacts 1 to 6, the input coils represented by the dashed lines in the figure, and the ground electrode of the tunnel junctions 7 in the middle part of the device. A second superconducting film insulated from the f i r s t with the exception of the windows for the tunnel junctions f o r m s a double hole loop which is closed in the middle part by the junctions and the first superconducting film. The tunnel junctions are shunted by two normal conducting thin film stripes 8 connecting the second superconducting film with the lower contact 6 in Fig. 1. The arrangement is completed by a second insulating film 9 and a superconducting screening film 10 which cover the whole SQUID with exception of the contacts.

The temperature variation of the dc critical Josephson current of NbNbOxPb tunnel junctions is investigated experimentally. The experimental data are compared with the temperature dependence of the critical Josephson current derived by Ambegaokar and Baratoff. Deviations giving an evidence of a normal surface layer on niobium are not found.

No abstract