In
superconductivity
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
, 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 formation of
magnetic field vortices with an applied external
magnetic field.
This occurs above a certain critical field strength ''H
c1''. The vortex density increases with increasing field strength. At a higher critical field ''H
c2'', superconductivity is destroyed. Type-II superconductors do not exhibit a complete
Meissner effect.
History
In 1935, Rjabinin and
Shubnikov Shubnikov (russian: Шубников) is a Russian surname. Notable people with the surname include:
* Alexei Vasilievich Shubnikov
Alexei Vasilievich Shubnikov (russian: Алексей Васильевич Шубников; 29March 1887 – ...
experimentally discovered the Type-II superconductors. In 1950, the theory of the two types of
superconductors was further developed by
Lev Landau and
Vitaly Ginzburg in their paper on
Ginzburg–Landau theory. In their argument, a
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 into ...
had positive
free energy of the superconductor-normal metal boundary. Ginzburg and Landau pointed out the possibility of type-II superconductors that should form inhomogeneous state in strong magnetic fields. However, at that time, all known superconductors were type-I, and they commented that there was no experimental motivation to consider precise structure of type-II superconducting state. The theory for the behavior of the Type-II superconducting state in magnetic field was greatly improved by
Alexei Alexeyevich Abrikosov, who was elaborating on the ideas by
Lars Onsager and
Richard Feynman of quantum vortices in
superfluids. Quantum vortex solution in a superconductor is also very closely related to
Fritz London's work on
magnetic flux quantization in superconductors. The
Nobel Prize in Physics was awarded for the theory of Type-II superconductivity in 2003.
[A. A. Abrikosov]
"Type II superconductors and the vortex lattice"
Nobel Lecture, December 8, 2003
Vortex state
Ginzburg–Landau theory introduced the
superconducting coherence length ξ in addition to
London magnetic field penetration depth λ. According to Ginzburg-Landau theory, in a type-II superconductor
. Ginzburg and Landau showed that this leads to negative energy of the interface between superconducting and normal phases. The existence of the negative interface energy was also known since the mid-1930s from the early works by the London brothers. A negative
interface energy
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...
suggests that the system should be unstable against maximizing the number of such interfaces. This instability was not observed until the experiments of Shubnikov in 1936 where two critical fields were found.
In 1952 an observation of type-II superconductivity was also reported by Zavaritskii.
Fritz London demonstrated
that a magnetic flux can penetrate a superconductor via a topological defect that has integer phase winding and carries quantized magnetic flux. Onsager and Feynman demonstrated that quantum vortices should form in superfluids.
A 1957 paper by
A. A. Abrikosov, generalizes these ideas. In the limit of very short coherence length the vortex solution is identical to London's fluxoid,
where the vortex core is approximated by a sharp cutoff rather than a gradual vanishing of superconducting condensate near the vortex center. Abrikosov found that the vortices arrange themselves into a regular array known as a ''vortex lattice''.
[ Near a so-called upper critical magnetic field, the problem of a superconductor in an external field is equivalent to the problem of vortex state in a rotating superfluid, discussed by Lars Onsager and Richard Feynman.
]
Flux pinning
In the vortex state, a phenomenon known as flux pinning
Flux pinning is a phenomenon that occurs when flux vortices in a type-II superconductor are prevented from moving within the bulk of the superconductor, so that the magnetic field lines are "pinned" to those locations. The superconductor must be a ...
becomes possible. This is not possible with 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 into ...
s, since they cannot be penetrated by magnetic fields.[Rosen, J., Ph.D., & Quinn, L. "Superconductivity". In K. Cullen (ed.), ''Encyclopedia of physical science''.]
If a superconductor is cooled in a field, the field can be trapped, which can allow the superconductor to be suspended over a magnet, with the potential for a frictionless joint or bearing. The worth of flux pinning is seen through many implementations such as lifts, frictionless joints, and transportation. The thinner the superconducting layer, the stronger the pinning that occurs when exposed to magnetic fields.
Materials
Type-II superconductors are usually made of metal alloys
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
or complex oxide
A complex oxide is a chemical compound that contains oxygen and at least two other elements (or oxygen and just one other element that's in at least two oxidation states). Complex oxide materials are notable for their wide range of magnetic and el ...
ceramics. All high temperature superconductors
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. The adjective "high temperature" is only in respect to previo ...
are type-II superconductors. While most elemental superconductors are type-I, niobium, vanadium, and technetium are elemental type-II superconductors. Boron-doped diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...
and silicon are also type-II superconductors. Metal alloy superconductors can also exhibit type-II behavior (''e.g.'' niobium-titanium, one of the most common superconductors in applied superconductivity), as well as intermetallic compounds like niobium-tin.
Other type-II examples are the cuprate-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 a ...
ceramic materials which have achieved the highest superconducting critical temperatures. These include La1.85Ba0.15CuO4, 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 ...
, and YBCO ( Yttrium- Barium-Copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
- Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen (77 K). Due to strong vortex
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 ...
pinning, the cuprates are close to ideally hard superconductors.
Important uses
Strong superconducting electromagnets (used in MRI scanners, NMR machines, and particle accelerators) often use coils wound of niobium-titanium wires or, for higher fields, niobium-tin wires. These materials are type-II superconductors with substantial upper critical field ''Hc2'', and in contrast to, for example, the cuprate superconductors with even higher ''Hc2'', they can be easily machined into wires. Recently, however, 2nd generation superconducting tapes are allowing replacement of cheaper niobium-based wires with much more expensive, but superconductive at much higher temperatures and magnetic fields "2nd generation" tapes.
See also
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References
Superconductivity