Superconductivity is the complete disappearance of electrical resistance in certain materials when cooled below a characteristic temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes in 1911, who found that the electrical resistivity of mercury dropped abruptly to zero when cooled below 4.2 kelvins (−268.9 °C). The effect was quickly reproduced in other laboratories and studied intensively from all perspectives, both fundamental and applied, including its potential for technological application. In 1957 John Bardeen, Leon Cooper, and John Robert Schrieffer developed the BCS theory of superconductivity, which won them the Nobel Prize in Physics in 1972. The first high-temperature superconductor (HTS), capable of operating at temperatures above 77 K (−196 °C), was discovered by Georg Bednorz and Karl Müller in 1986; they were awarded the Nobel Prize in Physics in 1987 “for their decisive contributions to the discovery of high temperature superconductivity”. YBa_2Cu_3O_7−x (YBCO) is currently the best known HTS material; it can operate up to 133 K (−140 °C). Many types of superconductors have been discovered since 1986 and are being exploited commercially; however, most have critical temperatures that are too low for many applications.
The phenomenon of superconductivity arises from a complex interplay between the electronic properties of solids and their ability to cool to extremely low temperatures. It is characterized by three key features: zero electrical resistivity, expulsion of magnetic fields (the Meissner effect), and perfect diamagnetism. All three effects arise from a quantum mechanical phenomenon known as electron pairing: at low enough temperatures some electrons bind together into pairs such that their individual behavior becomes indistinguishable from that of bosons—particles with integer spin that obey Bose–Einstein statistics instead of Fermi–Dirac statistics. Under these conditions electrons can flow freely through the material without scattering off lattice vibrations or other electrons, leading to zero electrical resistance; moreover, they exclude magnetic fields from their interior (the Meissner effect) due to a cancelling out of currents flowing in opposite directions within each pair (known as Josephson coupling); finally they exhibit perfect diamagnetism because any magnetic field applied externally creates circulating currents within each pair that exactly cancel out its effects (a property known as flux quantization).
Pairing requires not only very low temperatures but also special arrangements of atoms within solid crystals so that overlapping electron wavefunctions can occur—a condition typically met only near “dumbbell”-shaped defects called lattice imperfections or impurities. Such impurities are commonly introduced deliberately during crystal growth for this purpose; alternatively an external pressure may be applied which deforms the crystal lattice slightly so as to generate locally advantageous arrangements of atoms without introducing overall disorder into the sample. In any case it is generally necessary for there to be more than one kind (“species”)of charge carrier participating in order for pairing interactions between them to take place; this explains why superconductivity generally occurs only in metals or metallic alloys containing more than one element—e.g., lead–tin alloys or niobium–titanium alloys—and not in pure elemental metals such as copper or silver which contain just a single speciesof charge carrier .
In addition to these two essential ingredients—low temperature and specially arranged atoms—several others are often important depending on the particular system under consideration: e.g., strong repulsive Coulomb interactions between electrons may needto be suppressed if they areto undergo attractive interactions responsible for binding them into pairs; phonon-mediated attractions may playa role if conventional BCS theory does not adequately describe pairing interactions; certain kinds offorbidden energy level transitions maybe involved if exotic mechanisms suchas phonon-induced Cooper pairingor resonatingvalence bond theoryare invoked .