More than a dozen years after the discovery of high temperature superconductivity, the microscopic mechanism responsible for this phenomenon is still mysterious. An international group of researchers led by the Max Planck Institute of Solid State Research in Stuttgart (Germany) and Princeton University (USA) reports neutron scattering experiments providing incisive new information about the behavior of electron spins that will be crucial for models of high temperature superconductivity (Nature, April 15, 1999). Superconductivity was discovered in 1911 and occurs in many ordinary metals such as lead and aluminum at very low temperatures (below about 5 degrees above absolute zero, or about 290 degrees below room temperature). In the superconducting state, electrons can flow through the material without any resistance. Electrical currents in a superconductor therefore do not decay by heating up the material, as they would in a nonsuperconducting metal. In principle, superconductors can therefore be used to transmit currents without any losses. Despite these advantages, traditional superconductors have found few practical applications, as a lot of energy has to be invested to cool them down to the required temperatures. The discovery in 1986 of chemical compounds that superconduct at much higher temperatures has therefore caused much excitement. The current record (at ambient pressure) stands at about 135 degrees above absolute zero (or about 160 degrees below room temperature), so that much less cooling is required to initiate superconductivity. In these "high temperature" superconductors, the chemical elements copper and oxygen are arranged in a layered structure, with other elements sandwiched between the layers. The complex chemistry and materials physics of high temperature superconductors has slowed down the development of technological applications. Nevertheless, promising applications ranging from radio-frequency filters and magnetic field sensors to electrical motors are now beginning to emerge. What is missing so far is a theoretical understanding of the origin of high temperature superconductivity in the copper oxides. The theory of low temperature superconductivity in ordinary metals was developed in 1956 and is now well accepted. Electrons which normally move through the material individually and lose energy by colliding with impurities and other electrons are paired up in the superconducting state. Electrons also carry a tiny magnetic moment (the so-called "spin"), but the spins of two electrons in a pair are oriented in an antiparallel fashion so that the pair is actually nonmagnetic. Such electron pairs, which can move through the material without dissipating energy, also exist in high temperature superconductors. The "glue" that keeps the pairs together in the copper oxides, however, is still mysterious. Most theorists now agree that the mechanism that leads to pairing in traditional superconductors, vibrations of the atomic nuclei, cannot be responsible for high temperature superconductivity. The experiment reported in Nature provides important clues to what may take the place of atomic vibrations in pairing up the electrons. The researchers used neutrons produced in research reactors in Saclay and Grenoble, France, to excite and detect fluctuations of the electron spins in a particular high temperature superconductor of chemical formula Bi2Sr2CaCu2O8. Since neutrons carry a magnetic moment and penetrate deeply into most materials, neutron scattering is a powerful probe of magnetism in solids. The neutron scattering experiment revealed a "collective" spin excitation in Bi2Sr2CaCu2O8, that is, in the superconducting state all of the electron spins suddenly begin to move in unison. Such collective spin excitations are normally found only in magnetically ordered materials such as iron (see figure). The fact that similar excitations also exist in high temperature superconductors points towards a magnetic pairing mechanism. Efforts to develop a theoretical description of such a mechanism are still controversial, but the neutron experiments are an important step forward. It is to be hoped that a comprehensive theory of high temperature superconductivity will lead to the design of materials which superconduct at even higher temperatures, perhaps eventually room temperature.
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