Superconductivity is a phenomenon in which a material's resistance suddenly drops to zero as its temperature is lowered past a certain point. This point, called the critical temperature, is different for each material. Superconductors are divided broadly into two classes, "low temperature" and "high temperature", based on their critical temperature. The high temperature superconductors, discovered more recently, are not completely understood; their behavior is one of the major unsolved problems of modern physics. Superconductors currently have significant use in extremely high-field electromagnets and wires which carry very large currents, seen in their role as an essential part of the Large Hadron Collider.
The low temperature superconductors were first discovered in 1911 when Heike Kammerlingh Onnes cooled solid mercury past 4.2 Kelvin, leading to an abrupt loss of resistance. Many materials like this, most individual metals and all with similarly low critical temperatures, have been found. It took until 1957 for a model which explained the behavior of these materials to emerge; this model is known as the BCS theory after its authors Bardeen, Cooper, and Schrieffer. The BCS theory explained the very low critical temperatures and predicted that they could not rise above 30 K.
In 1986 the first high-temperature superconductor, a complex copper-oxide material, was discovered with a critical temperature of 35 K. Karl Müller and Johannes Bednorz won the Nobel Prize in Physics in 1987 for this breakthrough. New high-temperature superconductors were rapidly discovered and by 1988 the highest critical temperature had grown to 120 K .
The superconducting transition's rapid loss of resistance is accompanied by an expulsion of magnetic field from the material. This phenomenon is called the Meissner effect. This expulsion has the effect of pushing away permanent magnets, which allows a superconductor to levitate a magnet.