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UNITED KINGDOM - APRIL 03: These crucibles, made from yttria, a ceramic, are used to make samples of single crystal superconductors. One of the crucibles has been used and has been turned green by the production process. Superconductors are materials which offer zero resistance to the passage of an electric current when cooled to below a certain critical temperature. With early superconductors this was only a few degrees Kelvin (K), meaning that they had to be cooled using liquid helium, which is expensive and difficult to handle. In the 1980s, so-called high-temperature superconductors were discovered, allowing the use of liquid nitrogen instead of helium. Today, superconductors with critical temperatures as �high� as 150 K (-123 C) have been developed. (Photo by SSPL/Getty Images)