Superconducting devices

The bicrystal single grain boundary Josephson junction is fabricated by depositing an epitaxial c-axis oriented cuprate film on a substrate bicrystal 

Here dm is the effective magnetic thickness of the junction, 0o the magnetic flux quantum, and My) the maximum Josephson current density. A complete oscillation of the pair current density is called a Josephson vortex and contains one magnetic flux quantum. This behavior is shown schematically in Fig. 5.10 

During electron beam scanning the maximum Josephson current density 

Here dJg(y) is the beam-induced change of the supercurrent density at the coordinate point y. A typical LTSEM voltage image obtained in this way is shown in Fig. 5.11 for a bicrystal YBa2Cu307 5 grain boundary junction of 23 pm width. The image is recorded at 83 K and displays the pair current density distribution of the 4-5 vortex state. (It is this state which is also shown 

For simplieity, we have assumed in our discussion that the extension of the Josephson weak link along the y-direction does not exceed the Josephson penetration depth Aj. Further, we have ignored the non-local effects arising from the beam-induced change in the phase difference function (p(y). Discussions of the deviations from these simplifying assumptions are given elsewhere [5.6, 5.7, 5.20, 5.28]. 

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Superconductivity is the loss of all electrical resistance when a substance is cooled below a certain characteristic transition temperature (Ts). It is thought that the low temperatures are required to reduce the effect of the vibrations of the atoms in their crystalline lattice. Superconductivity was first observed in 1911 in mercury, for which Ts = 4 K. Over the years, many other metallic superconductors were identified, some having transition temperatures as high as 23 K. However, low-temperature superconductors need to be cooled with liquid helium, which is very expensive. To use superconducting devices on a large scale, higher transition temperatures would be required. 

Two-phase flows containing other types of fluids of interest are those of helium and refrigerants. The former fluid is used for cooling different superconductivity devices, while the latter are used in the refrigeration industry. The pressure drop in a two-phase flow of helium in a tube of 1.6 mm (0.06 in.) I.D. under adiabatic conditions and with heat supply were reported by Deev et al. (1978). They indicated that although the actual measured AP (at P = 1.0 to 1.8 X 10s N/m2) differed... 

T.van Duzer, C.W. Turner Principles of Superconducting Devices and Circuits, Edwards Arnold (1981)... 

Superconducting dc electromagnets, 23 854 Superconducting devices, transition metals for, 23 828-829... 

Thin-fiim technology has also played an important role in developing Josephson superconducting devices, which offer outstanding advantages in constructing ultrahigh-speed computers, These are tunnel-junction type devices. 

Research chemists found that they could modify the conducting properties of solids by doping them, a process commonly used to control the properties of semiconductors (see Section 3.13). In 1986, a record-high Ts of 35 K was observed, surprisingly not for a metal, but for a ceramic material (Section 14.24), a lanthanum-copper oxide doped with barium. Then early in 1987, a new record T, of 93 K was set with yttrium-barium-copper and a series of related oxides. In 1988, two more oxide series of bismuth-strontium-calcium-copper and thallium-barium-calcium-copper exhibited transition temperatures of 110 and 125 K, respectively. These temperatures can be reached by cooling the materials with liquid nitrogen, which costs only about 0.20 per liter. Suddenly, superconducting devices became economically viable. 

Products closest to commercial production are SQUlDs - " and a microwave passive device. A SQUID is a superconducting device containing one or two... 

Figure 10. Mini pulse tube cryocooler used for cooling infrared sensors or superconducting devices in space. Courtesy TRW/NGST.

The long-term interest, as far as the applied physicist is concerned, lies in the possible uses of supermolecular assemblies for memory storage, molecular switching, and superconducting devices. However, at the present time, potential improvement areas are where monomolecular films show most promise and where the prospects of commercial exploitation seem reasonable in the medium term. A few of these areas are described in this section most of the illustrations are based on work carried out in my research laboratories. These potential improvement areas are grouped into three categories, but most of the emphasis is placed on utilizing the nonlinear properties of LB films. 

Another major use of helium is in low-temperature cooling systems. This application is based on the fact that liquid helium—at —454°F (—270°C)—is cold enough to cool anything else. For example, it is used in superconducting devices. 

Technium was the first synthetic element, although a naturally occurring isotope was discovered in minute quantities in 1962. The metal is silvery grey. It has not been used as a commercial product but may have uses in specialty steel and superconducting devices. 

Josephson junctions are superconducting devices that are capable of generating voltage oscillations of extraordinarily high frequency, typically 1O —10" cycles... 

Van Duzer, T., and Turner, C. W. (1981) Principles of Superconductive Devices and Circuits (Elsevier, New York). 

W.J. Gallagher, Three-Terminal Superconducting Devices, IEEE Trans. MAG-21,709 (1985). 

T. Nishino, M. Miyake, Y. Harada, and U. Kawabe, Three-Terminal Superconducting Device using a Silicon Single-Crystal Film, IEEE Electron Dev. Lett., EDL6, 297 (1985). 

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