摘要:

Thin‐film niobium‐niobium oxide‐lead Josephson junctions were fabricated by use of getter sputtering for the Nb films, thermal oxidation of the Nb and evaporation of the Pb top films. These devices have characteristics comparable to good Pb&sngbnd;PbO&sngbnd;Pb devices. Temperature cycling is not harmful to the devices although some drift with time is seen in the characteristics. This drift is in the form of a conductance increase which has been measured as something between a factor of 1.1 to a factor of 20 over a period of months with the devices stored at room temperature.

ISSN:0021-8979    

DOI:10.1063/1.1657931

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

High background currents, often observed in tunnel junctions when the barrier is formed by oxidizing niobium, probably stem from the fact that one oxide of niobium (NbO) is not an insulator. The fabrication process described in this paper uses an active layer of gas adsorbed on the niobium surface that reacts with the upper film (e.g., lead) and forms the junction barrier. This technique avoids the problem above and results in tunnel junctions with low background currents.

ISSN:0021-8979    

DOI:10.1063/1.1657932

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The maximum tunneling supercurrent through Josephson barriers as a function of barrier dimensions and external magnetic fields has been investigated experimentally. A versatile sample preparation technique allows a two‐dimensional distribution of the tunneling current. The results confirm earlier theoretical predictions qualitatively. At zero applied field the maximum tunneling supercurrent initially increases linearly with the barrier length, then saturates due to self‐limiting. At nonzero applied field the maximum tunneling supercurrent decreases linearly with increasing magnetic field. It is shown that this holds also for structures in which the field is generated by crossed‐film control supercurrents. The Josephson penetration depth of the samples has been 9–22 &mgr;m.

ISSN:0021-8979    

DOI:10.1063/1.1657933

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The barrier‐layer thickness of cross‐type square Pb&sngbnd;PbO&sngbnd;Pb thin‐film Josephson junctions has been varied systematically and reproducibly by employing a mask‐substrate changer and a low‐pressure‐glow‐discharge oxidation technique. A series of junctions has been fabricated in which the ratio of film widthwto the Josephson penetration depth &lgr;Jis varied from &lgr;J≫wto &lgr;J≪wwhile the width of both films is kept constant at 0.3 mm. The magnetic‐field dependence of the maximum zero‐voltage current in these junctions has been measured as a function of the barrier‐layer thickness. For non‐negligible self‐fields of the tunneling currents, the field dependence follows a Fresnel diffraction pattern rather than a Fraunhofer diffraction pattern. For very thin barrier layers, the maximum zero‐voltage current is eventually limited by the critical current of the Pb films. Relatively thick barrier layers, on the other hand, tend to be shorted at the edges, as can be seen from the magnetic‐field dependence. The stability of Pb&sngbnd;PbO&sngbnd;Pb Josephson junctions is compared with that of Nb&sngbnd;NbOx&sngbnd;Pb Josephson junctions.

ISSN:0021-8979    

DOI:10.1063/1.1657934

出版商:AIP    

年代:1969

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1657936

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A magnet system capable of guiding a ferromagnetic‐tipped catheter through vessels in the body would greatly facilitate many medical treatments by providing access to parts of the body otherwise inaccessible except by major surgery. A rotatable iron magnet which has been employed in preliminary experiments with laboratory animals has indicated the feasibility of this technique, but a more advanced system is required for sophisticated medical applications. This paper describes a proposed design for a superconducting magnet system, with a 10 in. room‐temperature access, which can exert an arbitrarily directed force of 200 times that of gravity on a tip anywhere within the head. A larger but otherwise similar system could be used for cathetarization of any part of the body.

ISSN:0021-8979    

DOI:10.1063/1.1657937

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Guidance of high speed, 5000 m/sec rockets in evacuated tubes may be accomplished by eddy‐current repulsion between moving superconducting coils (on the rocket) and stationary sheets of copper or aluminum. A method for determining lift and drag forces of moving rectangular coils over an infinite plane sheet is presented. The lift force saturates at high speedsv, the drag forces decrease asv−1/2at high speeds, while the power dissipated increases asv1/2at high speeds. Methods of determining lift and drag forces from scale models of finite systems are discussed. The forces as obtained from impedance measurements on scale models in circular conducting tubes are given.

ISSN:0021-8979    

DOI:10.1063/1.1657938

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Short, steep pulses of high electrical energy are required to drive systems such as pulsed transmitters, light sources and plasma discharges. A low impedance stationary coil will generate such pulses when the magnetic flux threading the coil undergoes large and rapid changes. The rapid change can be achieved by a ``flux displacer,'' a conductor driven at high speed past the stationary coil. The applied field is provided in the present work by a superconducting magnet in order to produce a magnetic flux of the necessary magnitude and density with small operating loss, bulk and weight. The stationary coil together with the load and the flux displacer were not superconducting and were placed outside the Dewar. Two series of experiments with flux displacers driven by a rotary prime mover are discussed. The first series used relatively small, isolated specimens. These experiments served to determine the effect of displacer shape, speed and conductivity on output. The second series used relatively large displacers formed by pairs of cylindrical shell segments which are electrically and mechanically connected by end plates. These experiments give information on the influence of various system parameters on the energy, and on the duration and shape of the pulses. The results are presented in dimensionless form, so that the performance of higher‐energy systems can be predicted. Ultimately, the rotary prime mover may be a lightweight explosive drive or gas turbine or, in a vehicle or aircraft, one of the propulsion engines.

ISSN:0021-8979    

DOI:10.1063/1.1657939

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Superconducting critical current‐field measurements were made on a series of Nb&sngbnd;Ti&sngbnd;N thin film alloys (1000–8000 Å) prepared by reactive sputtering. Samples were measured in dc transverse and parallel fields. A pronounced maximum in the critical field occurs at approximately 20 mole % TiN. The extrapolated zero current value ofHc2for this composition at 4.2°K was greater than 180 kG. Critical current densities in parallel fields are comparable to those of other high‐field bulk superconductors (104A/cm2at 150 kG). Films 2000 Å thick and 0.1 in. wide carried a few tenths of an ampere in fields as high as 120 kG in the transverse fields and as high as 140 kG in parallel fields. In the transverse orientation the critical current density appears to be independent of thickness. Possible application of these materials to miniature thin‐film solenoids is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1657940

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Thin films of the rocksalt structure (B1) compounds of the transition metals can be prepared by the method of reactive sputtering. By a refinement of this technique we have produced, for the first time, thin films of niobium nitride (NbN) with transition temperatures similar to those of the bulk materials (≳ 15°K). Films have been prepared on both hastelloy and fused silica substrates. Measurements of the normal state resistivity &rgr;nyield values in excess of 200 &mgr;&OHgr;·cm. Preliminary studies of the resistive transitions in these materials nearTcindicate thatHr(J), the field at which &rgr;/&rgr;n=½, is independent of current density below 200 A/cm2for films 1000–4000 Å thick. IdentifyingHr(J) (J<50 A/cm2) with the upper critical fieldHc2(t), the measurements indicate thatHc2(0) is significantly in excess of 200 kOe, and thus is substantially higher than the values obtained for the bulk material. These results are discussed in terms of the current theories of high field superconductivity with particular reference to the importance of Pauli spin paramagnetism and spin‐orbit scattering in these materials.

ISSN:0021-8979    

DOI:10.1063/1.1657941

出版商:AIP    

年代:1969

数据来源: AIP