Covalent superconductor

Covalent superconductors are superconducting materials where the atoms are linked by covalent bonds. The first such material was boron-doped synthetic diamond grown by the high-pressure high-temperature (HPHT) method. The discovery had no practical importance, but surprised most scientists as superconductivity had not been observed in covalent semiconductors, including diamond and silicon.

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 Tc = 4 K and critical magnetic field Hc = 4 T. Later, Tc ~ 11K has been achieved in homoepitaxial CVD films.

Regarding the origin of superconductivity in diamond, three alternative theories exist at the moment: conventional BCS theory 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. Whereas there is no solid experimental support for either model, recent accurate measurements of isotopic shift of the transition temperature Tc upon boron and carbon isotopic substitutions favor the BCS theory.

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 Tc = 0.4 K and Hc = 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 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 Tc 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.

Carbon nanotubes

While there have been reports of intrinsic superconductivity in carbon nanotubes, 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

Main article: Graphite intercalation compound

Structure of CaC₆

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

Material	CaC6	Li3Ca2C6	YbC6	SrC6	KC8	RbC8	NaC3	KC3	LiC3	NaC2	LiC2
Tc (K)	11.5	11.15	6.5	1.65	0.14	0.025	2.3-3.8	3.0	<0.35	5.0	1.9

History

The priority of many discoveries in science is vigorously disputed (see, e.g., Nobel Prize controversies). Another example, after Sumio Iijima has "discovered" carbon nanotubes 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 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 Tc = 3.5 K was observed experimentally in germanium 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.

See also

• Conventional superconductor
• List of superconductors
• High-temperature superconductivity
• Room temperature superconductor
• Superconductivity
• Superconductor classification
• Technological applications of superconductivity
• Timeline of low-temperature technology
• Type-I superconductor
• Type-II superconductor
• Unconventional superconductor
• Silicon
• Silicon carbide
• Synthetic diamond

External links

• International Workshop on superconductivity in Diamond and Related Materials 2005
• International Workshop on Superconductivity in Diamond and Related Materials 2008
• New Diamond and Frontier Carbon Technology Volume 17, No.1 Special Issue on Superconductivity in CVD Diamond
• Some papers on superconducting diamond

• 
  L. Boeri, J. Kortus and O. K. Andersen "Three-Dimensional MgB2-Type Superconductivity in Hole-Doped Diamond",
  K.-W. Lee and W. E. Pickett "Superconductivity in Boron-Doped Diamond",
  X. Blase, Ch. Adessi and D. Connetable "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