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High-temperature superconductors (abbreviated high-
c or HTS) are defined as materials that behave as
superconductors at temperatures above , the boiling point of
liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
.
The adjective "high temperature" is only in respect to previously known superconductors, which function at even colder temperatures close to absolute zero. In absolute terms, these "high temperatures" are still far below ambient, and therefore require cooling. The first high-temperature superconductor was discovered in 1986, by IBM researchers
Bednorz and
Müller,
who were awarded the
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
in 1987 "for their important break-through in the discovery of superconductivity in ceramic materials". Most high-
c materials are
type-II superconductor
In superconductivity, a type-II superconductor is a superconductor that exhibits an intermediate phase of mixed ordinary and superconducting properties at intermediate temperature and fields above the superconducting phases.
It also features the ...
s.
The major advantage of high-temperature superconductors is that they can be cooled by using
liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
,
[ as opposed to the previously known superconductors which require expensive and hard-to-handle coolants, primarily ]liquid helium
Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity.
At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...
. A second advantage of high-c materials is they retain their superconductivity in higher magnetic fields than previous materials. This is important when constructing superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire has no electrical resistance and therefore can conduct mu ...
s, a primary application of high-c materials.
The majority of high-temperature superconductors are ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
materials, as opposed to the previously known metallic materials. Ceramic superconductors are suitable for some practical uses but they still have many manufacturing issues. For example, most ceramics are brittle
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...
which makes the fabrication of wires from them very problematic. However, overcoming these drawbacks is the subject of considerable research, and progress is ongoing.
The main class of high-temperature superconductors is copper oxides combined with other metals, especially the Rare-earth barium copper oxide
Rare-earth barium copper oxide (also referred to as ReBCO) is a family of chemical compounds known for exhibiting high temperature superconductivity (HTS). ReBCO superconductors have the potential to sustain stronger magnetic fields than other su ...
s (REBCOs) such as Yttrium barium copper oxide
Yttrium barium copper oxide (YBCO) is a family of crystalline chemical compounds that display high-temperature superconductivity; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen ...
(YBCO). The second class of high-temperature superconductors in the practical classification is the iron-based compounds.
Magnesium diboride
Magnesium diboride is the inorganic compound with the formula MgB2. It is a dark gray, water-insoluble solid. The compound has attracted attention because it becomes superconductor, superconducting at 39 K (−234 °C). In terms of its ...
is sometimes included in high-temperature superconductors: It is relatively simple to manufacture, but it superconducts only below 43 K, which makes it unsuitable for liquid nitrogen cooling (approximately 30 K below nitrogen triple point temperature). Some extremely-high pressure superhydride
Lithium triethylborohydride is the organoboron compound with the formula Li Et3 BH. Commonly referred to as LiTEBH or Superhydride, it is a powerful reducing agent used in organometallic and organic chemistry. It is a colorless or white liquid bu ...
compounds are usually categorized as high-temperature superconductors. In fact, many articles on high-temperature superconductors can be found on this research on high pressure gases, which are not suitable for practical applications. The current c record holder is carbonaceous sulfur hydride
Carbonaceous sulfur hydride is a purported room-temperature superconductor that was announced in October 2020. The material is claimed to have a maximal superconducting transition temperature of at a pressure of 267 gigapascals (GPa), though th ...
, beating the previous record held by lanthanum decahydride by nearly 30 K. The superconductivity in these compounds, however, has recently come under question.
History
Superconductivity was discovered by Kamerlingh Onnes in 1911, in a metal solid. Ever since, researchers have attempted to observe superconductivity at increasing temperatures with the goal of finding a room-temperature superconductor
A room-temperature superconductor is a material that is capable of exhibiting superconductivity at operating temperatures above , that is, temperatures that can be reached and easily maintained in an everyday environment. , the material with the h ...
.
By the late 1970s, superconductivity was observed in several metallic compounds (in particular Nb-based, such as NbTi, Nb3Sn, and Nb3Ge) at temperatures that were much higher than those for elemental metals and which could even exceed .
In 1986, at the IBM research lab near Zurich, in Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
, Bednorz and Müller were looking for superconductivity in a new class of ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s: the ''copper oxides'', or ''cuprates''.
Bednorz encountered a particular copper oxide whose resistance dropped to zero at a temperature around .[ Their results were soon confirmed by many groups, notably Paul Chu at the ]University of Houston
The University of Houston (UH) is a Public university, public research university in Houston, Texas. Founded in 1927, UH is a member of the University of Houston System and the List of universities in Texas by enrollment, university in Texas ...
and Shoji Tanaka at the University of Tokyo
, abbreviated as or UTokyo, is a public research university located in Bunkyō, Tokyo, Japan. Established in 1877, the university was the first Imperial University and is currently a Top Type university of the Top Global University Project by ...
.[
]
In 1987, Anderson
Anderson or Andersson may refer to:
Companies
* Anderson (Carriage), a company that manufactured automobiles from 1907 to 1910
* Anderson Electric, an early 20th-century electric car
* Anderson Greenwood, an industrial manufacturer
* Anderson Ra ...
gave the first theoretical description of these materials, based on the resonating valence bond theory In condensed matter physics, the resonating valence bond theory (RVB) is a theoretical model that attempts to describe high-temperature superconductivity, and in particular the superconductivity in cuprate compounds. It was first proposed by an Amer ...
,[
] but a full understanding of these materials is still developing today. These superconductors are now known to possess a ''d''-wave pair symmetry. The first proposal that high-temperature cuprate superconductivity involves ''d''-wave pairing was made in 1987 by Bickers, Scalapino Scalapino is a surname. Notable people with the surname include:
*Douglas James Scalapino (born 1933), American physicist
*Leslie Scalapino (1944–2010), American poet, experimental prose writer, playwright, essayist, and editor
*Robert A. Scalapi ...
and Scalettar,[
] followed by three subsequent theories in 1988 by Inui, Doniach, Hirschfeld and Ruckenstein,[
] using spin-fluctuation theory, and by Gros
Gros may refer to:
People
*Gros (surname)
* Gross (surname), the German variant of Gros
* Le Gros, the Norman variant of Gros
Other uses
* Gros (coinage), a type of 13th-century silver coinage of France
* Gros (grape), another name for Elbling, ...
, Poilblanc, Rice and Zhang,[
] and by Kotliar and Liu identifying ''d''-wave pairing as a natural consequence of the RVB theory.[
] The confirmation of the ''d''-wave nature of the cuprate superconductors was made by a variety of experiments, including the direct observation of the ''d''-wave nodes in the excitation spectrum through Angle Resolved Photoemission Spectroscopy, the observation of a half-integer flux in tunneling experiments, and indirectly from the temperature dependence of the penetration depth, specific heat and thermal conductivity.
As of 2021,[ the superconductor with the highest transition temperature at ambient pressure is the cuprate of mercury, barium, and calcium, at around 133 K.][
] There are other superconductors with higher recorded transition temperaturesfor example lanthanum superhydride at 250 K, but these only occur at very high pressures.[
]
The origin of high-temperature superconductivity is still not clear, but it seems that instead of ''electron-phonon
In physics, a phonon is a collective excitation in a periodic, Elasticity (physics), elastic arrangement of atoms or molecules in condensed matter physics, condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phon ...
'' attraction mechanisms, as in conventional superconductivity, one is dealing with genuine ''electronic'' mechanisms (e.g. by antiferromagnetic correlations), and instead of conventional, purely ''s''-wave pairing, more exotic pairing symmetries are thought to be involved (''d''-wave in the case of the cuprates; primarily extended ''s''-wave, but occasionally ''d''-wave, in the case of the iron-based superconductors).
In 2014, evidence showing that fractional particles can happen in quasi two-dimensional magnetic materials, was found by EPFL scientists lending support for Anderson's theory of high-temperature superconductivity.
Selected list of superconductors
Properties
Unfortunately, the "High-temperature" superconductor class has had many definitions in the context of superconductivity.
The label high-c should be reserved for materials with critical temperatures greater than the boiling point of liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
. However, a number of materialsincluding the original discovery and recently discovered pnictide superconductorshave critical temperatures below 77 K but nonetheless, are commonly referred to in publications as high-c class.[
][
]
A substance with a critical temperature above the boiling point of liquid nitrogen, together with a high critical magnetic field and critical current density (above which superconductivity is destroyed), would greatly benefit technological applications. In magnet applications, the high critical magnetic field may prove more valuable than the high c itself. Some cuprates have an upper critical field of about 100 tesla. However, cuprate materials are brittle ceramics which are expensive to manufacture and not easily turned into wires or other useful shapes. Furthermore, high-temperature superconductors do not form large, continuous superconducting domains, rather clusters of microdomains within which superconductivity occurs. They are therefore unsuitable for applications requiring actual superconductive currents, such as magnets for magnetic resonance
Magnetic resonance is a process by which a physical excitation (resonance) is set up via magnetism.
This process was used to develop magnetic resonance imaging and Nuclear magnetic resonance spectroscopy technology.
It is also being used to ...
spectrometers. For a solution to this (powders), see HTS_wire.
There has been considerable debate regarding high-temperature superconductivity coexisting with magnetic ordering
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
in YBCO,[
] iron-based superconductor
Iron-based superconductors (FeSC) are iron-containing chemical compounds whose superconducting properties were discovered in 2006.
In 2008, led by recently discovered iron pnictide compounds (originally known as oxypnictides), they were in the firs ...
s, several ruthenocuprates and other exotic superconductors, and the search continues for other families of materials. HTS are Type-II superconductor
In superconductivity, a type-II superconductor is a superconductor that exhibits an intermediate phase of mixed ordinary and superconducting properties at intermediate temperature and fields above the superconducting phases.
It also features the ...
s, which allow magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s to penetrate their interior in quantized units of flux, meaning that much higher magnetic fields are required to suppress superconductivity. The layered structure also gives a directional dependence to the magnetic field response.
All known high-c superconductors are Type-II superconductors. In contrast to Type-I superconductor
The interior of a bulk superconductor cannot be penetrated by a weak magnetic field, a phenomenon known as the Meissner effect. When the applied magnetic field becomes too large, superconductivity breaks down. Superconductors can be divided int ...
s, which expel all magnetic fields due to the Meissner effect
The Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. This expulsion will repel a n ...
, Type-II superconductors allow magnetic fields to penetrate their interior in quantized units of flux, creating "holes" or "tubes" of normal metallic regions in the superconducting bulk called vortices
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
. Consequently, high-c superconductors can sustain much higher magnetic fields.
Cuprates
Cuprates are layered materials, consisting of superconducting layers of copper oxide Copper oxide is a compound from the two elements copper and oxygen.
Copper oxide may refer to:
* Copper(I) oxide (cuprous oxide, Cu2O)
* Copper(II) oxide (cupric oxide, CuO)
* Copper peroxide (CuO2)
* Copper(III) oxide (Cu2O3)
* Copper(IV) oxide
...
, separated by spacer layers.
Cuprates generally have a structure close to that of a two-dimensional material. Their superconducting properties are determined by electrons moving within weakly coupled copper-oxide (CuO2) layers. Neighbouring layers contain ions such as lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...
, barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
, strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
, or other atoms which act to stabilize the structure and dope electrons or holes onto the copper-oxide layers. The undoped "parent" or "mother" compounds are Mott insulator
Mott insulators are a class of materials that are expected to conduct electricity according to conventional band theories, but turn out to be insulators (particularly at low temperatures). These insulators fail to be correctly described by band ...
s with long-range antiferromagnetic order at sufficiently low temperatures. Single band
Band or BAND may refer to:
Places
*Bánd, a village in Hungary
*Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran
*Band, Mureș, a commune in Romania
* Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, I ...
models are generally considered to be enough to describe the electronic properties.
The cuprate superconductors adopt a perovskite structure. The copper-oxide planes are checkerboard
A checkerboard (American English) or chequerboard (British English; see spelling differences) is a board of checkered pattern on which checkers (also known as English draughts) is played. Most commonly, it consists of 64 squares (8×8) of altern ...
lattice
Lattice may refer to:
Arts and design
* Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material
* Lattice (music), an organized grid model of pitch ratios
* Lattice (pastry), an ornam ...
s with squares of O2− ions with a Cu2+ ion at the centre of each square. The unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
is rotated by 45° from these squares. Chemical formulae of superconducting materials generally contain fractional numbers to describe the doping required for superconductivity. There are several families of cuprate superconductors and they can be categorized by the elements they contain and the number of adjacent copper-oxide layers in each superconducting block. For example, YBCO and BSCCO can alternatively be referred to as "Y123" and Bi2201/Bi2212/Bi2223 depending on the number of layers in each superconducting block (). The superconducting transition temperature has been found to peak at an optimal doping value (=0.16) and an optimal number of layers in each superconducting block, typically =3.
Possible mechanisms for superconductivity in the cuprates continue to be the subject of considerable debate and further research. Certain aspects common to all materials have been identified. Similarities between the antiferromagnetic
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...
the low-temperature state of undoped materials and the superconducting state that emerges upon doping, primarily the x2-y2 orbital state of the Cu2+ ions, suggest that electron-electron interactions are more significant than electron-phonon interactions in cupratesmaking the superconductivity unconventional. Recent work on the Fermi surface In condensed matter physics, the Fermi surface is the surface in reciprocal space which separates occupied from unoccupied electron states at zero temperature. The shape of the Fermi surface is derived from the periodicity and symmetry of the cryst ...
has shown that nesting occurs at four points in the antiferromagnetic Brillouin zone
In mathematics and solid state physics, the first Brillouin zone is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigner–Seitz cells in the real lattice, the reciprocal lattice i ...
where spin waves exist and that the superconducting energy gap is larger at these points. The weak isotope effects observed for most cuprates contrast with conventional superconductors that are well described by BCS theory.
Similarities and differences in the properties of hole-doped and electron doped cuprates:
* Presence of a pseudogap phase up to at least optimal doping.
* Different trends in the Uemura plot relating transition temperature to the superfluid density. The inverse square of the London penetration depth
In superconductors, the London penetration depth (usually denoted as \lambda or \lambda_L) characterizes the distance to which a magnetic field penetrates into a superconductor and becomes equal to e^ times that of the magnetic field at the surface ...
appears to be proportional to the critical temperature for a large number of underdoped cuprate superconductors, but the constant of proportionality is different for hole- and electron-doped cuprates. The linear trend implies that the physics of these materials is strongly two-dimensional.
* Universal hourglass-shaped feature in the spin excitations of cuprates measured using inelastic neutron diffraction.
* Nernst effect
In physics and chemistry, the Nernst effect (also termed first Nernst–Ettingshausen effect, after Walther Nernst and Albert von Ettingshausen) is a thermoelectric (or thermomagnetic) phenomenon observed when a sample allowing electrical conducti ...
evident in both the superconducting and pseudogap phases.
The electronic structure of superconducting cuprates is highly anisotropic (see the crystal structure of YBCO
Yttrium barium copper oxide (YBCO) is a family of crystalline chemical compounds that display high-temperature superconductivity; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen ...
or BSCCO
Bismuth strontium calcium copper oxide (BSCCO, pronounced ''bisko''), is a type of cuprate superconductor having the generalized chemical formula Bi2 Sr2 Ca''n''−1 Cu''n'' O2''n''+4+''x'', with ''n'' = 2 being the most commonly stud ...
). Therefore, the Fermi surface In condensed matter physics, the Fermi surface is the surface in reciprocal space which separates occupied from unoccupied electron states at zero temperature. The shape of the Fermi surface is derived from the periodicity and symmetry of the cryst ...
of HTSC is very close to the Fermi surface of the doped CuO2 plane (or multi-planes, in case of multi-layer cuprates) and can be presented on the 2‑D reciprocal space
In physics, the reciprocal lattice represents the Fourier transform of another lattice (usually a Bravais lattice). In normal usage, the initial lattice (whose transform is represented by the reciprocal lattice) is usually a periodic spatial fu ...
(or momentum space) of the CuO2 lattice. The typical Fermi surface within the first CuO2 Brillouin zone
In mathematics and solid state physics, the first Brillouin zone is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigner–Seitz cells in the real lattice, the reciprocal lattice i ...
is sketched in Fig. 1 (left). It can be derived from the band structure
In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...
calculations or measured by angle resolved photoemission spectroscopy
Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in th ...
(ARPES
Angle-resolved photoemission spectroscopy (ARPES) is an experimental technique used in condensed matter physics to probe the allowed energies and momenta of the electrons in a material, usually a crystalline solid. It is based on the photoele ...
). Fig. 1 (right) shows the Fermi surface of BSCCO measured by ARPES
Angle-resolved photoemission spectroscopy (ARPES) is an experimental technique used in condensed matter physics to probe the allowed energies and momenta of the electrons in a material, usually a crystalline solid. It is based on the photoele ...
. In a wide range of charge carrier concentration (doping level), in which the hole-doped HTSC are superconducting, the Fermi surface is hole-like (''i.e.'' open, as shown in Fig. 1). This results in an inherent in-plane anisotropy of the electronic properties of HTSC. In 2018, the full three dimensional Fermi surface structure was derived from soft x-ray ARPES.
Iron-based
Iron-based superconductors contain layers of iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
and a pnictogen
A pnictogen ( or ; from grc, πνῑ́γω "to choke" and -gen, "generator") is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the ele ...
such as arsenic
Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...
or phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
or a chalcogen
The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioact ...
. This is currently the family with the second highest critical temperature, behind the cuprates. Interest in their superconducting properties began in 2006 with the discovery of superconductivity in LaFePO at 4 K[
] and gained much greater attention in 2008 after the analogous material LaFeAs(O,F)[
] was found to superconduct at up to 43 K under pressure.[
]
The highest critical temperatures in the iron-based superconductor family exist in thin films of FeSe,[
][
][
] where a critical temperature in excess of 100 K was reported in 2014.[
]
Since the original discoveries several families of iron-based superconductors have emerged:
* LnFeAs(O,F) or LnFeAsO1−x (Ln=lanthanide) with c up to 56 K, referred to as 1111 materials.[ A ]fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typ ...
variant of these materials was subsequently found with similar c values.[
]
*(Ba,K)Fe2As2 and related materials with pairs of iron-arsenide layers, referred to as 122 compounds. c values range up to 38 K.[
][
] These materials also superconduct when iron is replaced with cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
.
* LiFeAs and NaFeAs with c up to around 20 K. These materials superconduct close to stoichiometric composition and are referred to as 111 compounds.[
][
][
]
* FeSe with small off-stoichiometry
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
or tellurium
Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
doping.[
]
Most undoped iron-based superconductors show a tetragonal-orthorhombic structural phase transition followed at lower temperature by magnetic ordering, similar to the cuprate superconductors.[
] However, they are poor metals rather than Mott insulators and have five band
Band or BAND may refer to:
Places
*Bánd, a village in Hungary
*Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran
*Band, Mureș, a commune in Romania
* Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, I ...
s at the Fermi surface In condensed matter physics, the Fermi surface is the surface in reciprocal space which separates occupied from unoccupied electron states at zero temperature. The shape of the Fermi surface is derived from the periodicity and symmetry of the cryst ...
rather than one.[
]
The phase diagram emerging as the iron-arsenide layers are doped is remarkably similar, with the superconducting phase close to or overlapping the magnetic phase. Strong evidence that the c value varies with the As-Fe-As bond angles has already emerged and shows that the optimal c value is obtained with undistorted FeAs4 tetrahedra.[
] The symmetry of the pairing wavefunction is still widely debated, but an extended ''s''-wave scenario is currently favoured.
Magnesium diboride
Magnesium diboride
Magnesium diboride is the inorganic compound with the formula MgB2. It is a dark gray, water-insoluble solid. The compound has attracted attention because it becomes superconductor, superconducting at 39 K (−234 °C). In terms of its ...
is occasionally referred to as a high-temperature superconductor because its c value of 39 K is above that historically expected for BCS superconductors. However, it is more generally regarded as the highest c conventional superconductor, the increased c resulting from two separate bands being present at the Fermi level
The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''µ'' or ''E''F
for brevity. The Fermi level does not include the work required to remove ...
.
Carbon-based
In 1991 Hebard et al discovered Fulleride superconductors,[
] where alkali-metal atoms are intercalated into C60 molecules.
In 2008 Ganin et al demonstrated superconductivity at temperatures of up to 38 K for Cs3C60.[
]
P-doped Graphane
Graphane is a two-dimensional polymer of carbon and hydrogen with the formula unit (CH)n where ''n'' is large. Partial hydrogenation results in hydrogenated graphene, which was reported by Elias et al in 2009 by a TEM study to be "direct evidence ...
was proposed in 2010 to be capable of sustaining high-temperature superconductivity.
Nickelates
In 1999, Anisimov et al. conjectured superconductivity in nickelates, proposing nickel oxides as direct analogs to the cuprate superconductors. Superconductivity in an infinite-layer nickelate, Nd0.8Sr0.2NiO2, was reported at the end of 2019 with a superconducting transition temperature between 9 and 15 K. This superconducting phase is observed in oxygen-reduced thin films created by the pulsed laser deposition of Nd0.8Sr0.2NiO3 onto SrTiO3 substrates that is then reduced to Nd0.8Sr0.2NiO2 via annealing the thin films at in the presence of CaH2. The superconducting phase is only observed in the oxygen reduced film and is not seen in oxygen reduced bulk material of the same stoichiometry, suggesting that the strain induced by the oxygen reduction of the Nd0.8Sr0.2NiO2 thin film changes the phase space to allow for superconductivity.
Of important is further to extract access hydrogen from the reduction with CaH2, otherwise topotactic hydrogen may prevent superconductivity.
Cuprates
The structure of cuprate
Cuprate loosely refers to a material that can be viewed as containing anionic copper complexes. Examples include tetrachloridocuprate ( uCl4sup>2−), the superconductor YBa2Cu3O7, and the organocuprates (e.g., dimethylcuprate u(CH3)2sup> ...
s which are superconductors are often closely related to perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known as ...
structure, and the structure of these compounds has been described as a distorted, oxygen deficient multi-layered perovskite structure. One of the properties of the crystal structure of oxide superconductors is an alternating multi-layer of CuO2 planes with superconductivity taking place between these layers. The more layers of CuO2, the higher c. This structure causes a large anisotropy in normal conducting and superconducting properties, since electrical currents are carried by holes induced in the oxygen sites of the CuO2 sheets. The electrical conduction is highly anisotropic, with a much higher conductivity parallel to the CuO2 plane than in the perpendicular direction. Generally, critical temperatures depend on the chemical compositions, cations substitutions and oxygen content. They can be classified as superstripes; i.e., particular realizations of superlattices at atomic limit made of superconducting atomic layers, wires, dots separated by spacer layers, that gives multiband and multigap superconductivity.
Yttrium–barium cuprate
An yttrium–barium cuprate, YBa2Cu3O7−x (or Y123), was the first superconductor found above liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
boiling point. There are two atoms of Barium for each atom of Yttrium.
The proportions of the three different metals in the YBa2Cu3O7 superconductor are in the mole ratio of 1 to 2 to 3 for yttrium to barium to copper, respectively: this particular superconductor has also often been referred to as the 123 superconductor.
The unit cell of YBa2Cu3O7 consists of three perovskite unit cells, which is pseudocubic, nearly orthorombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a ...
. The other superconducting cuprates have another structure: they have a tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square ...
cell.
Each perovskite cell contains a Y or Ba atom at the center: Ba in the bottom unit cell, Y in the middle one, and Ba in the top unit cell. Thus, Y and Ba are stacked in the sequence a–Y–Baalong the c-axis. All corner sites of the unit cell are occupied by Cu, which has two different coordinations, Cu(1) and Cu(2), with respect to oxygen. There are four possible crystallographic sites for oxygen: O(1), O(2), O(3) and O(4). The coordination polyhedra of Y and Ba with respect to oxygen are different. The tripling of the perovskite unit cell leads to nine oxygen atoms, whereas YBa2Cu3O7 has seven oxygen atoms and, therefore, is referred to as an oxygen-deficient perovskite structure. The structure has a stacking of different layers: (CuO)(BaO)(CuO2)(Y)(CuO2)(BaO)(CuO). One of the key feature of the unit cell of YBa2Cu3O7−x (YBCO) is the presence of two layers of CuO2. The role of the Y plane is to serve as a spacer between two CuO2 planes. In YBCO, the Cu–O chains are known to play an important role for superconductivity. c is maximal near 92 K when ''x'' ≈ 0.15 and the structure is orthorhombic. Superconductivity disappears at ''x'' ≈ 0.6, where the structural transformation of YBCO occurs from orthorhombic to tetragonal.[
]
Other cuprates
The preparation of other cuprates is more difficult than the YBCO preparation.
They also have a different crystal structure: they are tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square ...
where YBCO is orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
.
Problems in these superconductors arise because of the existence of three or more phases having a similar layered structure.
Moreover, the crystal structure of other tested cuprate superconductors are very similar. Like YBCO, the perovskite-type feature and the presence of simple copper oxide Copper oxide is a compound from the two elements copper and oxygen.
Copper oxide may refer to:
* Copper(I) oxide (cuprous oxide, Cu2O)
* Copper(II) oxide (cupric oxide, CuO)
* Copper peroxide (CuO2)
* Copper(III) oxide (Cu2O3)
* Copper(IV) oxide
...
(CuO2) layers also exist in these superconductors. However, unlike YBCO, Cu–O chains are not present in these superconductors. The YBCO superconductor has an orthorhombic structure, whereas the other high-c superconductors have a tetragonal structure.
There are three main classes of superconducting cuprates: bismuth-based, thallium-based and mercury-based.
The second cuprate by practical importance is currently BSCCO
Bismuth strontium calcium copper oxide (BSCCO, pronounced ''bisko''), is a type of cuprate superconductor having the generalized chemical formula Bi2 Sr2 Ca''n''−1 Cu''n'' O2''n''+4+''x'', with ''n'' = 2 being the most commonly stud ...
, a compound of Bi–Sr–Ca-Cu-O. The content of bismuth
Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...
and strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
creates some chemical issues.
It has three superconducting phases forming a homologous series as Bi2Sr2Ca''n''−1Cu''n''O4+2''n''+''x'' (''n''=1, 2 and 3).
These three phases are Bi-2201, Bi-2212 and Bi-2223, having transition temperatures of 20, 85 and 110 K, respectively, where the numbering system represent number of atoms for Bi Sr, Ca and Cu respectively. The two phases have a tetragonal structure which consists of two sheared crystallographic unit cells. The unit cell of these phases has double Bi–O planes which are stacked in a way that the Bi atom of one plane sits below the oxygen atom of the next consecutive plane. The Ca atom forms a layer within the interior of the CuO2 layers in both Bi-2212 and Bi-2223; there is no Ca layer in the Bi-2201 phase. The three phases differ with each other in the number of cuprate planes; Bi-2201, Bi-2212 and Bi-2223 phases have one, two and three CuO2 planes, respectively. The ''c'' axis lattice constants of these phases increases with the number of cuprate planes (see table below). The coordination of the Cu atom is different in the three phases. The Cu atom forms an octahedral coordination with respect to oxygen atoms in the 2201 phase, whereas in 2212, the Cu atom is surrounded by five oxygen atoms in a pyramidal arrangement. In the 2223 structure, Cu has two coordinations with respect to oxygen: one Cu atom is bonded with four oxygen atoms in square planar configuration and another Cu atom is coordinated with five oxygen atoms in a pyramidal arrangement.[
]
Cuprate of Tl–Ba–Ca: The first series of the Tl-based superconductor containing one Tl–O layer has the general formula TlBa2Ca''n''-1Cu''n''O2''n''+3, whereas the second series containing two Tl–O layers has a formula of Tl2Ba2Ca''n''-1Cu''n''O2''n''+4 with ''n'' =1, 2 and 3. In the structure of Tl2Ba2CuO6 (Tl-2201), there is one CuO2 layer with the stacking sequence (Tl–O) (Tl–O) (Ba–O) (Cu–O) (Ba–O) (Tl–O) (Tl–O). In Tl2Ba2CaCu2O8 (Tl-2212), there are two Cu–O layers with a Ca layer in between. Similar to the Tl2Ba2CuO6 structure, Tl–O layers are present outside the Ba–O layers. In Tl2Ba2Ca2Cu3O10 (Tl-2223), there are three CuO2 layers enclosing Ca layers between each of these. In Tl-based superconductors, c is found to increase with the increase in CuO2 layers. However, the value of c decreases after four CuO2 layers in TlBa2Ca''n''-1Cu''n''O2''n''+3, and in the Tl2Ba2Ca''n''-1Cu''n''O2''n''+4 compound, it decreases after three CuO2 layers.
Cuprate of Hg–Ba–Ca The crystal structure of HgBa2CuO4 (Hg-1201), HgBa2CaCu2O6 (Hg-1212) and HgBa2Ca2Cu3O8 (Hg-1223) is similar to that of Tl-1201, Tl-1212 and Tl-1223, with Hg in place of Tl. It is noteworthy that the c of the Hg compound (Hg-1201) containing one CuO2 layer is much larger as compared to the one-CuO2-layer compound of thallium (Tl-1201). In the Hg-based superconductor, c is also found to increase as the CuO2 layer increases. For Hg-1201, Hg-1212 and Hg-1223, the values of c are 94, 128, and the record value at ambient pressure 134 K,[
] respectively, as shown in table below. The observation that the c of Hg-1223 increases to 153 K under high pressure indicates that the c of this compound is very sensitive to the structure of the compound.
Preparation and manufacturing
The simplest method for preparing ceramic superconductors is a solid-state thermochemical reaction involving mixing, calcination
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), genera ...
and sintering
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
.
The appropriate amounts of precursor powders, usually oxides and carbonates, are mixed thoroughly using a Ball mill
A ball mill is a type of grinder used to grind or blend materials for use in mineral dressing processes, paints, pyrotechnics, ceramics, and selective laser sintering. It works on the principle of impact and attrition: size reduction is done ...
. Solution chemistry processes such as coprecipitation
In chemistry, coprecipitation (CPT) or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. Analogously, in medicine, coprecipitation is specifically the precipitation of an unbound " ...
, freeze-drying
Freeze drying, also known as lyophilization or cryodesiccation, is a low temperature dehydration process that involves freezing the product and lowering pressure, removing the ice by sublimation. This is in contrast to dehydration by most conve ...
and sol–gel methods are alternative ways for preparing a homogeneous mixture. These powders are calcined
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), genera ...
in the temperature range from 800–950 °C for several hours. The powders are cooled, reground and calcined again. This process is repeated several times to get homogeneous material. The powders are subsequently compacted to pellets and sintered. The sintering environment such as temperature, annealing time, atmosphere and cooling rate play a very important role in getting good high-c superconducting materials. The YBa2Cu3O7−''x'' compound is prepared by calcination and sintering of a homogeneous mixture of Y2O3, BaCO3 and CuO in the appropriate atomic ratio. Calcination is done at 900–950 °C, whereas sintering is done at 950 °C in an oxygen atmosphere. The oxygen stoichiometry in this material is very crucial for obtaining a superconducting YBa2Cu3O7−''x'' compound. At the time of sintering, the semiconducting tetragonal YBa2Cu3O6 compound is formed, which, on slow cooling in oxygen atmosphere, turns into superconducting YBa2Cu3O7−''x''. The uptake and loss of oxygen are reversible in YBa2Cu3O7''−x''. A fully oxygenated orthorhombic YBa2Cu3O7−''x'' sample can be transformed into tetragonal YBa2Cu3O6 by heating in a vacuum at temperature above 700 °C.
The preparation of Bi-, Tl- and Hg-based high-c superconductors is more difficult than the YBCO preparation. Problems in these superconductors arise because of the existence of three or more phases having a similar layered structure. Thus, syntactic intergrowth and defects such as stacking faults occur during synthesis and it becomes difficult to isolate a single superconducting phase. For Bi–Sr–Ca–Cu–O, it is relatively simple to prepare the Bi-2212 (c ≈ 85 K) phase, whereas it is very difficult to prepare a single phase of Bi-2223 (c ≈ 110 K). The Bi-2212 phase appears only after few hours of sintering at 860–870 °C, but the larger fraction of the Bi-2223 phase is formed after a long reaction time of more than a week at 870 °C. Although the substitution of Pb in the Bi–Sr–Ca–Cu–O compound has been found to promote the growth of the high-c phase, a long sintering time is still required.
Ongoing research
The question of how superconductivity arises in high-temperature superconductors is one of the major unsolved problems of theoretical condensed matter physics
Condensed matter physics is the field of physics that deals with the macroscopic and microscopic physical properties of matter, especially the solid and liquid phases which arise from electromagnetic forces between atoms. More generally, the sub ...
. The mechanism that causes the electrons in these crystals to form pairs is not known. Despite intensive research and many promising leads, an explanation has so far eluded scientists. One reason for this is that the materials in question are generally very complex, multi-layered crystals (for example, BSCCO
Bismuth strontium calcium copper oxide (BSCCO, pronounced ''bisko''), is a type of cuprate superconductor having the generalized chemical formula Bi2 Sr2 Ca''n''−1 Cu''n'' O2''n''+4+''x'', with ''n'' = 2 being the most commonly stud ...
), making theoretical modelling difficult.
Improving the quality and variety of samples also gives rise to considerable research, both with the aim of improved characterisation of the physical properties of existing compounds, and synthesizing new materials, often with the hope of increasing c. Technological research focuses on making HTS materials in sufficient quantities to make their use economically viable and optimizing their properties in relation to applications
Application may refer to:
Mathematics and computing
* Application software, computer software designed to help the user to perform specific tasks
** Application layer, an abstraction layer that specifies protocols and interface methods used in a c ...
.
Metallic Hydrogen
Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.
At high pressure and temperatures, metallic hydrogen c ...
has been proposed as a room-temperature superconductor, some experimental observations have detected the occurrence of the Meissner effect
The Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. This expulsion will repel a n ...
Theoretical models
There have been two representative theories for high-temperature or unconventional superconductivity
Unconventional superconductors are materials that display superconductivity which does not conform to either the conventional BCS theory or Nikolay Bogolyubov's theory or its extensions.
History
The superconducting properties of CeCu2Si2, a ty ...
.
Firstly, weak coupling theory suggests superconductivity emerges from antiferromagnetic spin fluctuations in a doped system.[
] According to this theory, the pairing wave function of the cuprate HTS should have a ''d''x2-y2 symmetry. Thus, determining whether the pairing wave function has ''d''-wave symmetry is essential to test the spin fluctuation mechanism. That is, if the HTS order parameter
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...
(a pairing wave function like in Ginzburg–Landau theory
In physics, Ginzburg–Landau theory, often called Landau–Ginzburg theory, named after Vitaly Ginzburg and Lev Landau, is a mathematical physical theory used to describe superconductivity. In its initial form, it was postulated as a phenomenol ...
) does not have ''d''-wave symmetry, then a pairing mechanism related to spin fluctuations can be ruled out. (Similar arguments can be made for iron-based superconductors but the different material properties allow a different pairing symmetry.) Secondly, there was the interlayer coupling model, according to which a layered structure consisting of BCS-type (''s''-wave symmetry) superconductors can enhance the superconductivity by itself.[
] By introducing an additional tunnelling interaction between each layer, this model successfully explained the anisotropic symmetry of the order parameter as well as the emergence of the HTS. Thus, in order to solve this unsettled problem, there have been numerous experiments such as photoemission spectroscopy
Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in th ...
, NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
, specific heat
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
measurements, etc. Up to date the results were ambiguous, some reports supported the ''d'' symmetry for the HTS whereas others supported the ''s'' symmetry. This muddy situation possibly originated from the indirect nature of the experimental evidence, as well as experimental issues such as sample quality, impurity scattering, twinning, etc.
This summary makes an implicit assumption
A tacit assumption or implicit assumption is an assumption that underlies a logical argument, course of action, decision, or judgment that is not explicitly voiced nor necessarily understood by the decision maker or judge. These assumptions may be ...
: superconductive properties can be treated by mean-field theory
In physics and probability theory, Mean-field theory (MFT) or Self-consistent field theory studies the behavior of high-dimensional random (stochastic) models by studying a simpler model that approximates the original by averaging over degrees of ...
. It also fails to mention that in addition to the superconductive gap, there is a second gap, the pseudogap
In condensed matter physics, a pseudogap describes a state where the Fermi surface of a material possesses a partial energy gap, for example, a band structure state where the Fermi surface is gapped only at certain points. The term pseudogap wa ...
. The cuprate layers are insulating, and the superconductors are doped with interlayer impurities to make them metallic. The superconductive transition temperature can be maximized by varying the dopant
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
concentration. The simplest example is La2CuO4, which consist of alternating CuO2 and LaO layers which are insulating when pure. When 8% of the La is replaced by Sr, the latter act as dopants
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
, contributing holes to the CuO2 layers, and making the sample metallic. The Sr impurities also act as electronic bridges, enabling interlayer coupling. Proceeding from this picture, some theories argue that the basic pairing interaction is still interaction with phonons
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phonon is an excited state in the quantum mechanical ...
, as in the conventional superconductors with Cooper pairs
In condensed matter physics, a Cooper pair or BCS pair (Bardeen–Cooper–Schrieffer pair) is a pair of electrons (or other fermions) bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper ...
. While the undoped materials are antiferromagnetic, even a few percent of impurity dopants introduce a smaller pseudogap in the CuO2 planes which is also caused by phonons
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phonon is an excited state in the quantum mechanical ...
. The gap decreases with increasing charge carriers, and as it nears the superconductive gap, the latter reaches its maximum. The reason for the high transition temperature is then argued to be due to the percolating behaviour of the carriersthe carriers follow zig-zag percolative paths, largely in metallic domains in the CuO2 planes, until blocked by charge density wave domain walls
A domain wall is a type of topological soliton that occurs whenever a discrete symmetry is spontaneously broken. Domain walls are also sometimes called kinks in analogy with closely related kink solution of the sine-Gordon model or models with pol ...
, where they use dopant bridges to cross over to a metallic domain of an adjacent CuO2 plane. The transition temperature maxima are reached when the host lattice has weak bond-bending forces, which produce strong electron-phonon interactions at the interlayer dopants.[
]
D symmetry in YBCO
An experiment based on flux quantization of a three-grain ring of YBa2Cu3O7 (YBCO) was proposed to test the symmetry of the order parameter in the HTS. The symmetry of the order parameter could best be probed at the junction interface as the Cooper pairs tunnel across a Josephson junction
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
or weak link.
It was expected that a half-integer flux, that is, a spontaneous magnetization could only occur for a junction of ''d'' symmetry superconductors. But, even if the junction experiment is the strongest method to determine the symmetry of the HTS order parameter, the results have been ambiguous. John R. Kirtley
John Robert Kirtley (born August 27, 1949) is an American condensed matter physicist and a Consulting Professor at the Center for Probing the Nanoscale in the Department of Applied Physics at Stanford University. He shared the 1998 Oliver E. Buckl ...
and C. C. Tsuei thought that the ambiguous results came from the defects inside the HTS, so that they designed an experiment where both clean limit (no defects) and dirty limit (maximal defects) were considered simultaneously.[
] In the experiment, the spontaneous magnetization was clearly observed in YBCO, which supported the ''d'' symmetry of the order parameter in YBCO. But, since YBCO is orthorhombic, it might inherently have an admixture of ''s'' symmetry. So, by tuning their technique further, they found that there was an admixture of ''s'' symmetry in YBCO within about 3%.[
] Also, they found that there was a pure ''d''x2-y2 order parameter symmetry in the tetragonal Tl2Ba2CuO6.[
]
Spin-fluctuation mechanism
Despite all these years, the mechanism of high-c superconductivity is still highly controversial, mostly due to the lack of exact theoretical computations on such strongly interacting electron systems. However, most rigorous theoretical calculations, including phenomenological and diagrammatic approaches, converge on magnetic fluctuations as the pairing mechanism for these systems. The qualitative explanation is as follows:
In a superconductor, the flow of electrons cannot be resolved into individual electrons, but instead consists of many pairs of bound electrons, called Cooper pairs. In conventional superconductors, these pairs are formed when an electron moving through the material distorts the surrounding crystal lattice, which in turn attracts another electron and forms a bound pair. This is sometimes called the "water bed" effect. Each Cooper pair requires a certain minimum energy to be displaced, and if the thermal fluctuations in the crystal lattice are smaller than this energy the pair can flow without dissipating energy. This ability of the electrons to flow without resistance leads to superconductivity.
In a high-c superconductor, the mechanism is extremely similar to a conventional superconductor, except, in this case, phonons virtually play no role and their role is replaced by spin-density waves. Just as all known conventional superconductors are strong phonon systems, all known high-c superconductors are strong spin-density wave systems, within close vicinity of a magnetic transition to, for example, an antiferromagnet. When an electron moves in a high-c superconductor, its spin creates a spin-density wave around it. This spin-density wave in turn causes a nearby electron to fall into the spin depression created by the first electron (water-bed effect again). Hence, again, a Cooper pair is formed. When the system temperature is lowered, more spin density waves and Cooper pairs are created, eventually leading to superconductivity. Note that in high-c systems, as these systems are magnetic systems due to the Coulomb interaction, there is a strong Coulomb repulsion between electrons. This Coulomb repulsion prevents pairing of the Cooper pairs on the same lattice site. The pairing of the electrons occur at near-neighbor lattice sites as a result. This is the so-called ''d''-wave pairing, where the pairing state has a node (zero) at the origin.
Examples
Examples of high-c cuprate superconductors include YBCO
Yttrium barium copper oxide (YBCO) is a family of crystalline chemical compounds that display high-temperature superconductivity; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen ...
and BSCCO
Bismuth strontium calcium copper oxide (BSCCO, pronounced ''bisko''), is a type of cuprate superconductor having the generalized chemical formula Bi2 Sr2 Ca''n''−1 Cu''n'' O2''n''+4+''x'', with ''n'' = 2 being the most commonly stud ...
, which are the most known materials that achieve superconductivity above the boiling point of liquid nitrogen.
See also
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References
External links
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High-temperature superconductors
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