Superconductors

The resistance to flow of electrical current of a normal metallic conductor decreases smoothly with temperature but never vanishes. However, certain solids known as superconductors conduct electricity without resistance below a critical temperature, Tc. Following the discovery in 1911 that mercury is a superconductor below 4.2 K, the boiling point of liquid helium, physicists and chemists made slow but steady progress in the discovery of superconductors with higher values of Tc. Metals, such as tungsten, mercury, and lead, tend to have Tc values below about 10 K. Intermetallic compounds, such as Nb3X (X = Sn, Al, or Ge), and alloys, such as Nb/Ti and Nb/Zr, have intermediate Tc values ranging between 10 K and 23 K. In 1986, high-temperature super conductors (HTSC) were discovered. Several ceramics, inorganic powders that have been fused and hardened by heating to a high temperature, containing oxo uprate motifs, CumOn, are now known with Tcvalues well above 77 K, the boiling point of the inexpensive refrigerant liquid nitrogen. For example, HgBa₂Ca₂Cu₂O₈ has Tc = 153 K. Superconductors have unique magnetic properties as well. Some superconductors, classed as Type I, show abrupt loss of superconductivity when an applied magnetic field exceeds a critical value Hc characteristic of the material. It is observed that the value of Hc depends on temperature and Tc as

Hc(T) =Hc(0) (1-

where Hc(0) is the value of Hc as T → 0. Type I superconductors are also completely diamagnetic below Hc, meaning that no magnetic field lines penetrate into the material. This complete exclusion of a magnetic field in a material is known as the Meissner effect, which can be visualized by the levitation of a superconductor above a magnet. Type II superconductors, which include the HTSCs, show a gradual loss of superconductivity and diamagnetism with increasing magnetic field. There is a degree of periodicity in the elements that exhibit superconductivity. The metals iron, cobalt, nickel, copper, silver, and gold do not display superconductivity, nor do the alkali metals. It is observed that, for simple metals, ferromagnetism and superconductivity never coexist, but in some of the oxocuprate superconductors ferromagnetism and superconductivity can coexist. One of the most widely studied oxocuprate superconductors YBa2Cu3O7 (informally known as ‘123’ on account of the proportions of the metal atoms in the compound) has the structure shown in Fig. 20.67. The square-pyramidal CuO5 units arranged as two-dimensional layers and the square planar CuO4 units arranged in sheets are common structural features of oxo uprate HTSCs. The mechanism of superconduction is well-understood for low-temperature materials but there is as yet no settled explanation of high-temperature superconductivity. The central concept of low-temperature superconduction is the existence of a Cooper pair, a pair of electrons that exists on account of the indirect electron–electron interactions fostered by the nuclei of the atoms in the lattice. Thus, if one electron is in a particular region of a solid, the nuclei there move toward it to give a distorted local structure (Fig. 20.68). Because that local distortion is rich in positive charge, it is favourable for a second electron to join the first. Hence, there is a virtual attraction between the two electrons, and they move together as a pair. The local distortion can be easily disrupted by thermal motion of the ions in the solid, so the virtual attraction occurs only at very low temperatures. A Cooper pair undergoes less scattering than an individual electron as it travels through the solid because the distortion caused by one electron can attract back the other electron should it be scattered out of its path in a collision. Because the Cooper pair is stable against scattering, it can carry charge freely through the solid, and hence give rise to superconduction. The Cooper pairs responsible for low-temperature superconductivity are likely to be important in HTSCs, but the mechanism for pairing is hotly debated. There is evidence implicating the arrangement of CuO5 layers and CuO4sheets in the mechanism of high-temperature superconduction. It is believed that movement of electrons along the linked CuO4 units accounts for superconductivity, whereas the linked CuO5 units act as ‘charge reservoirs’ that maintain an appropriate number of electrons in the superconducting layers. Superconductors can sustain large currents and, consequently, are excellent materials for the high-field magnets used in modern NMR spectroscopy (Chapter 15). However, the potential uses of superconducting materials are not limited to the field to chemical instrumentation. For example, HTSCs with Tc values near ambient temperature would be very efficient components of an electrical power transmission system, in which energy loss due to electrical resistance would be minimized. The appropriate technology is not yet available, but research in this area of materials science is active.

Fig. 20.67 Structure of the YBa₂Cu₃O₇ superconductor. (a) Metal atom positions. (b) The polyhedra show the positions of oxygen atoms and indicate that the metal ions are in square-planar and square pyramidal coordination environments.

Fig. 20.68 The formation of a Cooper pair. One electron distorts the crystal lattice and the second electron has a lower energy if it goes to that region. These electron–lattice interactions effectively bind the two electrons into a pair.

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مواضيع ذات صلة

Strong electrolytes

Conductance and conductivity

Induced magnetic moments

Magnetic susceptibility

Nonlinear optical phenomena

Light emission by solid-state lasers and light-emitting diodes

Light absorption by molecular solids, metallic conductors, and semiconductors

Insulators and semiconductors

The occupation of orbitals

The formation of bands

Molecular solids and covalent networks

Energetics