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Unconventional superconductor
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Unconventional superconductor
Unconventional superconductors are materials that display superconductivity which is not explained by the usual BCS theory or its extension, the Eliashberg theory. The pairing in unconventional superconductors may originate from some other mechanism than the electron–phonon interaction. Alternatively, a superconductor is unconventional if the superconducting order parameter transforms according to a non-trivial irreducible representation of the point group or space group of the system. Per definition, superconductors that break additional symmetries to U (1) symmetry are known as unconventional superconductors.
The superconducting properties of CeCu2Si2, a type of heavy fermion material, were reported in 1979 by Frank Steglich. For a long time it was believed that CeCu2Si2 was a singlet d-wave superconductor, but since the mid-2010s, this notion has been strongly contested. In the early eighties, many more unconventional, heavy fermion superconductors were discovered, including UBe13, UPt3 and URu2Si2. In each of these materials, the anisotropic nature of the pairing was implicated by the power-law dependence of the nuclear magnetic resonance (NMR) relaxation rate and specific heat capacity on temperature. The presence of nodes in the superconducting gap of UPt3 was confirmed in 1986 from the polarization dependence of the ultrasound attenuation.
The first unconventional triplet superconductor, organic material (TMTSF)2PF6, was discovered by Denis Jerome, Klaus Bechgaard and coworkers in 1980 (TMTSF = Tetramethyltetraselenafulvalenium, see Fulvalene). Experimental works by Paul Chaikin's and Michael Naughton's groups as well as theoretical analysis of their data by Andrei Lebed have firmly confirmed unconventional nature of superconducting pairing in (TMTSF)2X (X=PF6, ClO4, etc.) organic materials.
High-temperature singlet d-wave superconductivity was discovered by J.G. Bednorz and K.A. Müller in 1986, who also discovered that the lanthanum-based cuprate perovskite material LaBaCuO4 develops superconductivity at a critical temperature (Tc) of approximately 35 K (-238 degrees Celsius). This was well above the highest critical temperature known at the time (Tc = 23 K), and thus the new family of materials was called high-temperature superconductors. Bednorz and Müller received the Nobel Prize in Physics for this discovery in 1987. Since then, many other high-temperature superconductors have been synthesized.
LSCO (La2−xSrxCuO4) was discovered the same year (1986). Soon after, in January 1987, yttrium barium copper oxide (YBCO) was discovered to have a Tc of 90 K, the first material to achieve superconductivity above the boiling point of liquid nitrogen (77 K). This was highly significant from the point of view of the technological applications of superconductivity because liquid nitrogen is far less expensive than liquid helium, which is required to cool conventional superconductors down to their critical temperature. In 1988 bismuth strontium calcium copper oxide (BSCCO) with Tc up to 107 K, and thallium barium calcium copper oxide (TBCCO) (T=thallium) with Tc of 125 K were discovered. The current record critical temperature is about Tc = 133 K (−140 °C) at standard pressure, and somewhat higher critical temperatures can be achieved at high pressure. Nevertheless, at present it is considered unlikely that cuprate perovskite materials will achieve room-temperature superconductivity.
On the other hand, other unconventional superconductors have been discovered. These include some that do not superconduct at high temperatures, such as strontium ruthenate Sr2RuO4, but that, like high-temperature superconductors, are unconventional in other ways. (For example, the origin of the attractive force leading to the formation of Cooper pairs may be different from the one postulated in BCS theory.) In addition to this, superconductors that have unusually high values of Tc but that are not cuprate perovskites have been discovered. Some of them may be extreme examples of conventional superconductors (this is suspected of magnesium diboride, MgB2, with Tc = 39 K). Others could display more unconventional features.
In 2008 a new class that does not include copper (layered oxypnictide superconductors), for example LaOFeAs, was discovered. An oxypnictide of samarium seemed to have a Tc of about 43 K, which was higher than predicted by BCS theory. Tests at up to 45 T suggested the upper critical field of LaFeAsO0.89F0.11 to be around 64 T. Some other iron-based superconductors do not contain oxygen.
As of 2009[update], the highest-temperature superconductor (at ambient pressure) is mercury barium calcium copper oxide (HgBa2Ca2Cu3Ox), at 138 K and is held by a cuprate-perovskite material, possibly 164 K under high pressure.
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Unconventional superconductor
Unconventional superconductors are materials that display superconductivity which is not explained by the usual BCS theory or its extension, the Eliashberg theory. The pairing in unconventional superconductors may originate from some other mechanism than the electron–phonon interaction. Alternatively, a superconductor is unconventional if the superconducting order parameter transforms according to a non-trivial irreducible representation of the point group or space group of the system. Per definition, superconductors that break additional symmetries to U (1) symmetry are known as unconventional superconductors.
The superconducting properties of CeCu2Si2, a type of heavy fermion material, were reported in 1979 by Frank Steglich. For a long time it was believed that CeCu2Si2 was a singlet d-wave superconductor, but since the mid-2010s, this notion has been strongly contested. In the early eighties, many more unconventional, heavy fermion superconductors were discovered, including UBe13, UPt3 and URu2Si2. In each of these materials, the anisotropic nature of the pairing was implicated by the power-law dependence of the nuclear magnetic resonance (NMR) relaxation rate and specific heat capacity on temperature. The presence of nodes in the superconducting gap of UPt3 was confirmed in 1986 from the polarization dependence of the ultrasound attenuation.
The first unconventional triplet superconductor, organic material (TMTSF)2PF6, was discovered by Denis Jerome, Klaus Bechgaard and coworkers in 1980 (TMTSF = Tetramethyltetraselenafulvalenium, see Fulvalene). Experimental works by Paul Chaikin's and Michael Naughton's groups as well as theoretical analysis of their data by Andrei Lebed have firmly confirmed unconventional nature of superconducting pairing in (TMTSF)2X (X=PF6, ClO4, etc.) organic materials.
High-temperature singlet d-wave superconductivity was discovered by J.G. Bednorz and K.A. Müller in 1986, who also discovered that the lanthanum-based cuprate perovskite material LaBaCuO4 develops superconductivity at a critical temperature (Tc) of approximately 35 K (-238 degrees Celsius). This was well above the highest critical temperature known at the time (Tc = 23 K), and thus the new family of materials was called high-temperature superconductors. Bednorz and Müller received the Nobel Prize in Physics for this discovery in 1987. Since then, many other high-temperature superconductors have been synthesized.
LSCO (La2−xSrxCuO4) was discovered the same year (1986). Soon after, in January 1987, yttrium barium copper oxide (YBCO) was discovered to have a Tc of 90 K, the first material to achieve superconductivity above the boiling point of liquid nitrogen (77 K). This was highly significant from the point of view of the technological applications of superconductivity because liquid nitrogen is far less expensive than liquid helium, which is required to cool conventional superconductors down to their critical temperature. In 1988 bismuth strontium calcium copper oxide (BSCCO) with Tc up to 107 K, and thallium barium calcium copper oxide (TBCCO) (T=thallium) with Tc of 125 K were discovered. The current record critical temperature is about Tc = 133 K (−140 °C) at standard pressure, and somewhat higher critical temperatures can be achieved at high pressure. Nevertheless, at present it is considered unlikely that cuprate perovskite materials will achieve room-temperature superconductivity.
On the other hand, other unconventional superconductors have been discovered. These include some that do not superconduct at high temperatures, such as strontium ruthenate Sr2RuO4, but that, like high-temperature superconductors, are unconventional in other ways. (For example, the origin of the attractive force leading to the formation of Cooper pairs may be different from the one postulated in BCS theory.) In addition to this, superconductors that have unusually high values of Tc but that are not cuprate perovskites have been discovered. Some of them may be extreme examples of conventional superconductors (this is suspected of magnesium diboride, MgB2, with Tc = 39 K). Others could display more unconventional features.
In 2008 a new class that does not include copper (layered oxypnictide superconductors), for example LaOFeAs, was discovered. An oxypnictide of samarium seemed to have a Tc of about 43 K, which was higher than predicted by BCS theory. Tests at up to 45 T suggested the upper critical field of LaFeAsO0.89F0.11 to be around 64 T. Some other iron-based superconductors do not contain oxygen.
As of 2009[update], the highest-temperature superconductor (at ambient pressure) is mercury barium calcium copper oxide (HgBa2Ca2Cu3Ox), at 138 K and is held by a cuprate-perovskite material, possibly 164 K under high pressure.