High-temperature superconductivity
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The term High-temperature superconductor was initially employed to designate the new family of cuprate-perovskite ceramic materials discovered by J.G. Bednorz and K.A. Müller in 1986. These materials are characterized by presenting superconductivity at a higher temperature than conventional superconductors (which require temperatures a few degrees above absolute zero (−273.15 °C or −459.67 °F)), and by other unconventional features. So-called high-temperature superconductors are generally considered to be those that demonstrate superconductivity at or above the temperature of liquid nitrogen, or −196 °C (77 K).
Recently, other unconventional superconductors have been discovered. Some of them also have unusually high values of the critical temperature Tc, and hence they are sometimes also called high-temperature superconductors, although the record is still held by a cuprate perovskite material (Tc=138 K, that is −135 °C). Nevertheless it is widely believed that if room temperature superconductivity is ever achieved it will be in a different family of materials.
Despite its name, high-temperature superconductivity still occurs at cryogenic temperatures. The main difference from low-temperature superconductivity is usually that 'high-Tc' superconductors can use liquid nitrogen (at 77 K) as a coolant.
Most prominent materials in the high-Tc range are the so-called cuprates, such as La1.85Ba0.15CuO4, YBCO (Yttrium-Barium-Copper-Oxide) and related substances.
All known high-Tc superconductors are so-called Type-II superconductors. A Type-II superconductor allows magnetic field to penerate its interior in the units of flux quanta, creating 'holes' (or tubes) of normal metallic regions in the superconducting bulk. This property makes high-Tc superconductors capable of sustaining much higher magnetic fields.