Science made simple: What is superconductivity?

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A cube of magnetic material floats on a superconductor. The magnetic field induces currents in the superconductor that generate an equal and opposite field, precisely balancing the gravitational force on the cube. Credit: Image courtesy of Oak Ridge National Laboratory

In what most people think of as “normal” temperatures, all materials have some electrical resistance. This means that they resist the flow of electricity in the same way that a narrow pipe resists the flow of water. Due to resistance, some energy is lost while heat is generated when electrons move through electronics in our devices, such as computers or cell phones. For most materials, this resistance remains even if the material is cooled to very low temperatures. Exceptions are superconducting materials. Superconductivity is the property of certain materials to conduct direct current (DC) electricity without loss of energy when they are cooled below a critical temperature (referred to as Tc) These materials also expel magnetic fields as they pass into the superconducting state.

Superconductivity is one of nature’s most intriguing quantum phenomena. Wax discovered more than 100 years ago in cooled mercury at the temperature of liquid helium (about -452 ° F, just a few degrees above absolute zero) At first, scientists could explain what happened to superconductivity, but why and how superconductivity was a mystery for almost 50 years.

In 1957, three physicists at the University of Illinois used quantum mechanics to explain the microscopic mechanism of superconductivity. They proposed a radically new theory of how negatively charged electrons, which normally repel each other, form in pairs under Tc. These paired electrons are held together by atomic-level vibrations known as phonons, and collectively pairs can move through material without resistance. For their discovery, these scientists received the Nobel Prize in Physics in 1972.

After the discovery of superconductivity in mercury, the phenomenon was also observed in other materials at very low temperatures. Materials included several metals and one alloy of niobium and titanium that can easily be made into wire. Wires led to a new challenge for the search for superconductors. The lack of electrical resistance in superconducting wires means that they can support very high electric currents, but above a “critical current” the electron pairs break down and the superconductivity is destroyed. Technologically, wires opened up completely new uses for superconductors, including wound coils to create powerful magnets. In the 1970s, scientists used superconducting magnets to generate the high magnetic fields needed to develop magnetic resonance imaging (MRI) imaging machines. Recently, scientists introduced superconducting magnets to direct electron beams at synchrotrons and accelerators in users’ scientific facilities.

In 1986, scientists discovered a new class of copper oxide materials that exhibited superconductivity but at temperatures much higher than metals and metal alloys by the turn of the century. These materials are known as high temperature superconductors. While still to be cooled, they are superconducting at much warmer temperatures – some of them at temperatures above liquid nitrogen (-321 ° F). This discovery kept the promise of revolutionary new technologies. He also suggested that scientists may be able to find materials that are superconducting at or near room temperature.

Since then, many new high-temperature superconducting materials have been discovered using educated conjectures combined with trial and error experiments, including a class of iron-based materials. However, it also became clear that the microscopic theory describing superconductivity in metals and metal alloys does not apply to most of these new materials, so once again the mystery of superconductivity is challenging the scientific community. Recent experiments on hydrogen-based materials under extremely high pressure confirmed a theoretical prediction of superconductivity at temperatures approaching room temperature.

Facts of superconductivity

  • Superconductivity was discovered in 1911 by Heike Kamerlingh-Onnes. For this discovery, helium liquefaction and other achievements, he won the Nobel Prize in 1913 in Physics.
  • Five Nobel Prizes in Physics have been awarded for research in superconductivity (1913, 1972, 1973, 1987 and 2003).
  • Approximately half of the elements in the periodic table exhibit low-temperature superconductivity, but superconductivity applications often use easier-to-use or less cost-effective connections. For example, MRI machines use an alloy of niobium and titanium.

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