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Superconductivity is a property displayed by certain materials at very low temperatures. Materials found to be superconductive include metals and their alloys (tin, aluminum, and others), some semiconductors, and certain ceramics known as cuprates which contain copper and oxygen atoms. A superconductor conducts electricity without resistance, a unique property. It also repels magnetic fields perfectly in a phenomenon known as the Meissner effect, losing any internal magnetic field it might have had before being cooled to a critical temperature. Because of this effect, certain superconductors can be made to float endlessly above a strong magnetic field.

For most superconducting materials, the critical temperature is below about 30K (30°C above absolute zero). But some materials, called high-temperature superconductors, make the phase transition to superconductivity at much higher critical temperatures, typically higher than 70K and sometimes as high as 138K. These materials are almost always cuprate-perovskite ceramics. They display slightly different properties than other superconductors, and the way they transition to superconductivity has still not been entirely explained. Sometimes they are called Type II superconductors to distinguish them from conventional Type I superconductors.

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