History of superconductivity
Superconductivity was discovered on April 8, 1911 by Heike Kamerlingh
Onnes, who was studying the resistance of
solid mercury atcryogenic temperatures using the recently
produced liquid helium as a refrigerant. At the temperature of 4.2 K, he
observed that the resistance abruptly disappeared.[15] In the same
experiment, he also observed the superfluid transition of helium at
2.2 K, without recognizing its significance. The precise date and
circumstances of the discovery were only reconstructed a century later,
when Onnes's notebook was found.[16] In subsequent decades,
superconductivity was observed in several other materials. In
1913, lead was found to superconduct at 7 K, and in 1941 niobium
nitride was found to superconduct at 16 K.
Great efforts have been devoted to finding out how and why
superconductivity works; the important step occurred in 1933,
when Meissnerand Ochsenfeld discovered that superconductors
expelled applied magnetic fields, a phenomenon which has come to be
known as theMeissner effect.[17] In 1935, Fritz and Heinz
London showed that the Meissner effect was a consequence of the
minimization of the electromagnetic free energy carried by
superconducting current.[18]
London theory[edit]
The first phenomenological theory of superconductivity was London
theory. It was put forward by the brothers Fritz and Heinz London in
1935, shortly after the discovery that magnetic fields are expelled from
superconductors. A major triumph of the equations of this theory is
their ability to explain the Meissner effect,[17] wherein a material
exponentially expels all internal magnetic fields as it crosses the
superconducting threshold. By using the London equation, one can
obtain the dependence of the magnetic field inside the superconductor
on the distance to the surface.[19]
There are two London equations:
The first equation follows from Newton's second law for
superconducting electrons.
Conventional theories (1950s)[edit]
During the 1950s, theoretical condensed matter physicists arrived at
an understanding of "conventional" superconductivity, through a
pair of remarkable and important theories: the
phenomenological Ginzburg-Landau theory (1950) and the
microscopic BCS theory (1957).[20][21]
In 1950, the phenomenological Ginzburg-Landau theory of
superconductivity was devised by Landau and Ginzburg.[22] This
theory, which combined Landau's theory of second-order phase
transitions with a Schrödinger-like wave equation, had great success
in explaining the macroscopic properties of superconductors. In
particular, Abrikosov showed that Ginzburg-Landau theory predicts
the division of superconductors into the two categories now
referred to as Type I and Type II. Abrikosov and Ginzburg were
awarded the 2003 Nobel Prize for their work (Landau had received
the 1962 Nobel Prize for other work, and died in 1968). The four-
dimensional extension of the Ginzburg-Landau theory, theColeman-
Weinberg model, is important in quantum field
theory and cosmology.
Also in 1950, Maxwell and Reynolds et al. found that the critical
temperature of a superconductor depends on the isotopic mass of
the constituent element.[23][24] This important discovery pointed to
the electron-phonon interaction as the microscopic mechanism
responsible for superconductivity.
The complete microscopic theory of superconductivity was finally
proposed in 1957 by Bardeen, Cooper and Schrieffer.[21] This BCS
theory explained the superconducting current as
a superfluid of Cooper pairs, pairs of electrons interacting through
the exchange of phonons. For this work, the authors were awarded
the Nobel Prize in 1972.
The BCS theory was set on a firmer footing in 1958, when N. N.
Bogolyubov showed that the BCS wavefunction, which had originally
been derived from a variational argument, could be obtained using a
canonical transformation of the electronic Hamiltonian.[25] In
1959, Lev Gor'kov showed that the BCS theory reduced to the
Ginzburg-Landau theory close to the critical temperature.[26][27]
Generalizations of BCS theory for conventional superconductors
form the basis for understanding of the phenomenon
of superfluidity, because they fall into the lambda
transitionuniversality class. The extent to which such generalizations
can be applied to unconventional superconductors is still
controversial.
Further history
The first practical application of superconductivity was developed in
1954 with Dudley Allen Buck's invention of the cryotron.[28] Two
superconductors with greatly different values of critical magnetic
field are combined to produce a fast, simple switch for computer
elements.
Soon after discovering superconductivity in 1911, Kamerlingh Onnes
attempted to make an electromagnet with superconducting
windings but found that relatively low magnetic fields destroyed
superconductivity in the materials he investigated. Much later, in
1955, G.B. Yntema [29] succeeded in constructing a small 0.7-tesla
iron-core electromagnet with superconducting niobium wire
windings. Then, in 1961, J.E. Kunzler, E. Buehler, F.S.L. Hsu, and J.H.
Wernick [30] made the startling discovery that, at 4.2 kelvin, a
compound consisting of three parts niobium and one part tin, was
capable of supporting a current density of more than 100,000
amperes per square centimeter in a magnetic field of 8.8 tesla.
Despite being brittle and difficult to fabricate, niobium-tin has since
proved extremely useful in supermagnets generating magnetic fields
as high as 20 tesla. In 1962 T.G. Berlincourt and R.R.
Hake [31][32] discovered that alloys of niobium and titanium are
suitable for applications up to 10 tesla. Promptly thereafter,
commercial production of niobium-titanium supermagnet wire
commenced at Westinghouse Electric Corporation and at Wah
Chang Corporation. Although niobium-titanium boasts less-
impressive superconducting properties than those of niobium-tin,
niobium-titanium has, nevertheless, become the most widely used
“workhorse” supermagnet material, in large measure a consequence
of its very-high ductility and ease of fabrication. However, both
niobium-tin and niobium-titanium find wide application in MRI
medical imagers, bending and focusing magnets for enormous high-
energy-particle accelerators, and a host of other applications.
Conectus, a European superconductivity consortium, estimated that
in 2014, global economic activity for which superconductivity was
indispensable amounted to about five billion euros, with MRI
systems accounting for about 80% of that total.
In 1962, Josephson made the important theoretical prediction that a
supercurrent can flow between two pieces of superconductor
separated by a thin layer of insulator.[33] This phenomenon, now
called the Josephson effect, is exploited by superconducting devices
such as SQUIDs. It is used in the most accurate available
measurements of the magnetic flux quantum Φ0 = h/(2e), where h is
the Planck constant. Coupled with the quantum Hall resistivity, this
leads to a precise measurement of the Planck constant. Josephson
was awarded the Nobel Prize for this work in 1973.
In 2008, it was proposed that the same mechanism that produces
superconductivity could produce a superinsulator state in some
materials, with almost infinite electrical resistance.[34]
High-temperature superconductors
Until 1986, physicists had believed that BCS theory forbade
superconductivity at temperatures above about 30 K. In that
year, Bednorz and Müller discovered superconductivity in
a lanthanum-based cuprate perovskite material, which had a
transition temperature of 35 K (Nobel Prize in Physics, 1987).[5] It
was soon found that replacing the lanthanum
with yttrium (i.e., making YBCO) raised the critical temperature to
92 K.[35]
This temperature jump is particularly significant, since it allows liquid
nitrogen as a refrigerant, replacing liquid helium.[35] This can be
important commercially because liquid nitrogen can be produced
relatively cheaply, even on-site. Also, the higher temperatures help
avoid some of the problems that arise at liquid helium
temperatures, such as the formation of plugs of frozen air that can
block cryogenic lines and cause unanticipated and potentially
hazardous pressure buildup.[36][37]
Many other Cuprate superconductors have since been discovered,
and the theory of superconductivity in these materials is one of the
major outstanding challenges of theoretical condensed. There are
currently two main hypotheses – the resonating-valence-bond
theory, and spin fluctuation which has the most support in the
research community.[ The second hypothesis proposed that
electron pairing in high-temperature superconductors is mediated
by short-range spin waves known as paramagnons.
In 2008, holographic superconductivity, which uses holographic
duality or AdS/CFT correspondence theory, was proposed by Gubser,
Hartnoll, Herzog, and Horowitz, as a possible explanation of high-
temperature superconductivity in certain materials.[42]
Since about 1993, the highest-temperature superconductor has
been a ceramic material consisting of mercury, barium, calcium,
copper and oxygen (HgBa2Ca2Cu3O8+δ) with Tc = 133–138 K.[43][44] The
latter experiment (138 K) still awaits experimental confirmation,
however.
In February 2008, an iron-based family of high-temperature
superconductors was discovered.[45][46] Hideo Hosono, of the Tokyo
Institute of Technology, and colleagues found lanthanum oxygen
fluorine iron arsenide (LaO1−xFxFeAs), an oxypnictide that super
conducts below 26 K. Replacing the lanthanum in LaO1−xFxFeAs
with samarium leads to superconductors that work at 55 K.[47]
In May 2014, hydrogen sulfide (H
2S) was predicted to be a high-temperature superconductor with a
transition temperature of 80 K at 160 gigapascals of pressure.[48] In
2015, H
2Shas been observed to exhibit superconductivity at below 203 K but
at extremely high pressures — around 150 gigapascals.[49]
In 2018, a research team from the Department of
Physics, Massachusetts Institute of Technology,
discovered superconductivity in bilayer graphene twisted at an angle
of approximately 1.1 degrees with cooling and applying a small
electric charge. Even if the experiments were not carried out in a
high-temperature environment, the results are correlated less to
classical but high temperature superconductors, given that no
foreign atoms need to be introduced.[50]
Applications
Superconducting magnets are some of the most
powerful electromagnets known. They are used
in MRI/NMR machines, mass spectrometers, the beam-steering
magnets used in particle accelerators and plasma confining magnets
in some tokamaks. They can also be used for magnetic separation,
where weakly magnetic particles are extracted from a background of
less or non-magnetic particles, as in the pigment industries.
In the 1950s and 1960s, superconductors were used to build
experimental digital computers using cryotron switches. More
recently, superconductors have been used to make digital
circuits based on rapid single flux quantum technology and RF and
microwave filters formobile phone base stations.
Superconductors are used to build Josephson junctions which are
the building blocks of SQUIDs (superconducting quantum
interference devices), the most sensitive magnetometers known.
SQUIDs are used in scanning SQUID
microscopes and magnetoencephalography. Series of Josephson
devices are used to realize the SI volt. Depending on the particular
mode of operation, a superconductor-insulator-
superconductor Josephson junction can be used as a
photon detector or as a mixer. The large resistance change at the
transition from the normal- to the superconducting state is used to
build thermometers in cryogenic micro-
calorimeter photon detectors. The same effect is used in
ultrasensitive bolometers made from superconducting materials.
Other early markets are arising where the relative efficiency, size
and weight advantages of devices based on high-temperature
superconductivity outweigh the additional costs involved. For
example, in wind turbines the lower weight and volume of
superconducting generators could lead to savings in construction
and tower costs, offsetting the higher costs for the generator and
lowering the total LCOE.[51]
Promising future applications include high-performance smart
grid, electric power transmission, transformers, power storage
devices, electric motors (e.g. for vehicle propulsion, as
in vactrains or maglev trains), magnetic levitation devices, fault
current limiters, enhancing spintronic devices with superconducting
materials,[52] and superconducting magnetic refrigeration. However,
superconductivity is sensitive to moving magnetic fields so
applications that use alternating current (e.g. transformers) will be
more difficult to develop than those that rely upon direct current.
Compared to traditional power lines superconducting transmission
lines are more efficient and require only a fraction of the space,
which would not only lead to a better environmental performance
but could also improve public acceptance for expansion of the
electric grid.[53]
