Covalent superconductors are

superconducting Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases g ...

materials where the atoms are linked by

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

s. The first such material was boron-doped

synthetic diamond A synthetic diamond or laboratory-grown diamond (LGD), also called a lab-grown, laboratory-created, man-made, artisan-created, artificial, or cultured diamond, is a diamond that is produced in a controlled technological process, in contrast to ...

grown by the

high-pressure In science and engineering the study of high pressure examines its effects on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure. ''High pressure'' usually means pressures of thousan ...

high-temperature (HPHT) method.
L. Boeri, J. Kortus and O. K. Anderse
2-Type Superconductivity in Hole-Doped Diamond"">"Three-Dimensional MgB₂-Type Superconductivity in Hole-Doped Diamond"

K.-W. Lee and W. E. Picket
"Superconductivity in Boron-Doped Diamond"

X. Blase, Ch. Adessi and D. Connetabl
"Role of the Dopant in the Superconductivity of Diamond"

E. Bustarret et al
"Dependence of the Superconducting Transition Temperature on the Doping Level in Single-Crystalline Diamond Films"
– free download The discovery had no practical importance, but surprised most scientists as superconductivity had not been observed in covalent semiconductors, including diamond and silicon.

History

The priority of many discoveries in science is vigorously disputed (see, e.g.,

Nobel Prize controversies Since the first award in 1901, conferment of the Nobel Prize has engendered criticismSumio Iijima has "discovered"

carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range ( nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''S ...

s in 1991, many scientists have pointed out that carbon nanofibers were actually observed decades earlier. The same could be said about superconductivity in covalent semiconductors. Superconductivity in

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

and silicon-germanium was predicted theoretically as early as in the 1960s. Shortly after, superconductivity was experimentally detected in germanium telluride. In 1976, superconductivity with ''T''_c = 3.5 K was observed experimentally in

implanted with copper ions; it was experimentally demonstrated that amorphization was essential for the superconductivity (in Ge), and the superconductivity was assigned to Ge itself, not copper.

Diamond

Superconductivity in diamond was achieved through heavy p-type doping by boron such that the individual doping atoms started interacting and formed an "impurity band". The superconductivity was of type-II with the critical temperature ''T''_c = 4 K and critical magnetic field ''B''_c = 4 T. Later, ''T''_c ≈ 11 K has been achieved in homoepitaxial CVD films. Regarding the origin of superconductivity in diamond, three alternative theories were suggested: conventional

BCS theory In physics, the Bardeen–Cooper–Schrieffer (BCS) theory (named after John Bardeen, Leon Cooper, and John Robert Schrieffer) is the first microscopic theory of superconductivity since Heike Kamerlingh Onnes's 1911 discovery. The theory descr ...

based on phonon-mediated pairing, correlated impurity band theory and spin-flip-driven pairing of holes weakly localized in the vicinity of the Fermi level. Experiments on diamonds enriched with ¹²C, ¹³C, ¹⁰B or ¹¹B isotopes revealed a clear ''T''_c shift, and its magnitude confirms the BCS mechanism of superconductivity in bulk polycrystalline diamond.

Carbon nanotubes

While there have been reports of intrinsic superconductivity in

carbon nanotubes A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''SWC ...

, many other experiments found no evidence of superconductivity, and the validity of these results remains a subject of debate. Note, however, a crucial difference between nanotubes and diamond: Although nanotubes contain covalently bonded carbon atoms, they are closer in properties to graphite than diamond, and can be metallic without doping. Meanwhile, undoped diamond is an insulator.

Intercalated graphite

When metal atoms are inserted (intercalated) between the graphite planes, several superconductors are created with the following transition temperatures:

Silicon

It was suggested that "Si and Ge, which also form in the diamond structure, may similarly exhibit superconductivity under the appropriate conditions", and indeed, discoveries of superconductivity in heavily boron doped Si (Si:B) and SiC:B have quickly followed. Similar to diamond, Si:B is type-II superconductor, but it has much smaller values of ''T''_c = 0.4 K and ''B''_c = 0.4 T. Superconductivity in Si:B was achieved by heavy doping (above 8 at.%), realized through a special non-equilibrium technique of gas immersion laser doping.

Silicon carbide

Superconductivity in

SiC The Latin adverb ''sic'' (; ''thus'', ''so'', and ''in this manner'') inserted after a quotation indicates that the quoted matter has been transcribed or translated as found in the source text, including erroneous, archaic, or unusual spelling ...

was achieved by heavy doping with boron or aluminum. Both the cubic (3C-SiC) and hexagonal (6H-SiC) phases are superconducting and show a very similar ''T''_c of 1.5 K. A crucial difference is however observed for the magnetic field behavior between aluminum and boron doping: SiC:Al is type-II, same as Si:B. On the contrary, SiC:B is type-I. In attempt to explain this difference, it was noted that Si sites are more important than carbon sites for superconductivity in SiC. Whereas boron substitutes carbon in SiC, Al substitutes Si sites. Therefore, Al and B "see" different environment that might explain different properties of SiC:Al and SiC:B.

Hydrogen sulfide

At pressures above 90 GPa (

gigapascal The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is an SI ...

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

becomes a metallic conductor of electricity. When cooled below a

critical temperature Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine *Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing in ...

its high-pressure phase exhibits

superconductivity Superconductivity is a set of physical properties observed in superconductors: materials where Electrical resistance and conductance, electrical resistance vanishes and Magnetic field, magnetic fields are expelled from the material. Unlike an ord ...

. The critical temperature increases with pressure, ranging from 23 K at 100 GPa to 150 K at 200 GPa. If hydrogen sulfide is pressurized at higher temperatures, then cooled, the critical temperature reaches , the highest accepted superconducting critical temperature as of 2015. By substituting a small part of sulfur with phosphorus and using even higher pressures, it has been predicted that it may be possible to raise the critical temperature to above and achieve room-temperature superconductivity.

References

External links

International Workshop on superconductivity in Diamond and Related Materials
2005
International Workshop on Superconductivity in Diamond and Related Materials
2008

* ttp://www.nims.go.jp/NFM/paper1/SuperconductingDiamond/SuperconductingDiamond.html Some papers on superconducting diamond {{DEFAULTSORT:Covalent Superconductor Superconductivity Superconductors