Superconductor

Before 1986, although a variety of superconductors had been discovered and synthesized, all had critical temperatures at or below the boiling point of He (e.g., Pb at -446.5°F [-265.8°C, 7.19K] and Nb₃Sn at -426.91°F [-254.95°C, 18.05K]). Since expensive refrigeration units are required to produce liquid He, this strictly limited the circumstances under which it was economical to apply superconductivity.

A small cube magnet hovering over a nitrogen-cooled specimen of a superconducting ceramic demonstrates the Meissner effect. Walther Meissner observed in 1933 that a magnetic field does not penetrate a superconductor. Magnetic lines of force loop around them, so the magnet is neither attracted nor repelled but instead floats above. Because the effect is so distinctive, it is used as a test for identifying superconducting materials. Photograph by Yoav Levy. Phototake NYC. Reproduced by permission.

The first superconductor to be discovered having T_c > -320.8°F (-196.0°C, 77K) (the boiling point of liquid N₂, which is much cheaper to produce than liquid He) was YBa₂Cu₃O₇ (T_c ~ -294°F [-181°C, 92K]). The Y-Ba-Cu-O compound was discovered by U.S. physicist C. W. Chu (1948–), working with a graduate student, in 1987 following the 1986 discovery by German physicists G. Bednorz (1950–) and K. A. Müller (1927–) of the La-Ba-Cu-O oxide superconductor (T_c = -394.6°F [-237.0°C, 36K). One year later, in 1988, bismuth-based (e.g., (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀, T_c = -261.4°F [-163.0°C, 110K) and thallium-based (e.g.,Tl₂Ba₂Ca₂Cu₃O₁₀, T_c = -234.4°F [-148.0°C, 125K]) superconductors were successfully synthesized; their T_c's were some 20K higher than that of Y-Ba-Cu-O. Very recently, mercury-based cuprates (HgBa₂Ca_n-1Cu_nO2_n+2+Δ) have been shown to have T_c values higher than 130K. These oxide superconductors are now classified as the high-temperature (or high-T_c) superconductors (HTSCs).

All HTSCs so far discovered have an atomic structure that consists of thin planes of atoms, many of which consist of the compound copper dioxide (CuO₂). This compound is, so far, uniquely important to producing the property of high-temperature conductivity. Ironically, CuO₂ is a Mott insulator, meaning that at temperatures approaching absolute zero it begins to behave as an insulator (a substance having very high resistance) rather than as a conductor: yet at higher temperatures, embedded in an appropriate crystal matrix, it is key to the production of zero resistance (superconduction).

Current flow in the CuO₂ family of HTSCs has directional properties. That is, the critical current density, J_c—the largest current density that a superconductor can carry without lapsing into finite resistivity—along the CuO₂ plane direction is orders of magnitude higher than at right angles to it. For HTSCs to carry a large amount of current, this implies that individual crystalline grains in bulk conductors (e.g., wires or tapes) of HTSC material should be well aligned with the current transport direction. Significant grain misorientation and chemistry inhomogeneity at grain boundaries can form weak links between neighboring grains and thus lower local J_c values. Several bulk material manufacture technologies, such as the melt-powder-melt-growth (MPMG) method for Y-Ba-Cu-O and the oxide-powder-in-tube (OPIT) process for Bi- and Tlbased superconductors, have been demonstrated to develop textured bulk structures. J values of 10⁶–10⁸ c amperes per square centimeter at 77K have been achieved over small distances. For thin-film growth, dual ion beam sputtering (DIBS), molecular beam epitaxy (MBE), pulsed-laser deposition (PLD), and metal-organic chemical vapor deposition (MOCVD) have been shown to be successful methods. By optimizing processing temperature and pressure and using proper buffer layers, epitaxial HTSC films can be deposited on templates of other crystalline materials such as Si and MgO. With the integration of Si-based microelectronics processes, HTSC thin-film devices can be fabricated for a variety of applications.