28.4 Electrical Resistance in Solids 1183
Superconductivity
Superconductivity was discovered in 1911 by Kamerlingh Onnes,^14 who found that
the conductivity of solid mercury suddenly rose to a value at least as large as
1015 ohm−^1 m−^1 (and possibly infinite) when the mercury was cooled below 4.2 K.
Onnes coined the name “superconductivity” for the phenomenon. Since that time, other
substances have been found to exhibit superconductivity, generally with a transition
temperature below 23 K. Silicon has recently been found to exhibit super conductivity
when doped with boron.^15 It has a transition temperature of 0.35 K.Heike Kamerlingh Onnes,1853–1926,
wasaDutch physicist workingat the
University of Leidenwhenhe
discoveredsuperconductivity. He
essentially inventedlow-temperature
physics,andreceivedthe 1913 Nobel
Prize inphysics for this work.
A superconducting material rejects a magnetic field. That is, within the surface of
the sample of material that is exposed to a magnetic field, a compensating magnetic
field is generated that exactly cancels the magnetic field within the material, except for
a surface layer of thickness 10 to 100 nm. This effect is called theMeissner effect, and
can cause the levitation of a magnet above a superconductor. However, if the magnetic
field is increased above a certain critical value, which depends on temperature and on
the substance, the superconductivity disappears.The Meissner effect isnamedforWalter
Meissner,1882–1974,who withRobert
Ochsenfelddiscoveredthe effect in
- There is a generally accepted theory of superconductivity in metals. (^16) This theory
is based on the notion that under certain conditions the electrons interact with the
lattice of the solid in such a way that two electrons form a pair with opposite spins
having a lower energy than two single uncorrelated electrons. The pair of electrons
is called aCooper pair. Unless the pair is broken up, it is not possible for one of
the electrons to be scattered by a nucleus. Below a temperature called thetransition
temperature, there is not enough thermal energy to break up the pair, so that scattering
does not occur and an electrical current can flow without observable resistance.
Beginning in 1986, a number of ceramic compounds were discovered that exhibit
superconductivity with transition temperatures as high at 138 K (− 135 ◦C).^17 These
“high-temperature” semiconductors are oxides, containing copper along with two or
three other metals, such as barium and yttrium, or thallium, barium, and calcium. The
first substance to exhibit a transition temperature above that of boiling liquid nitrogen
(77 K) was YBa 2 Cu 3 Ox, wherexranges from 6.5 to 7.2. Ifxwere equal to 9, the
substance could have the crystal structure of a perovskite, with six oxygens surrounding
each copper and the copper atoms in layers.^18 The crystal has four oxygen atoms around
each copper atom, all in the plane of the copper atoms.^19
In 1988, Tl 2 Ca 2 Ba 2 Cu 3 O 10 +y, wherey<1, was found to have a transition tem-
perature of 125 K. The crystal structure of this compound was determined, and found
to have planes containing copper and oxygen atoms, as well as planes containing
thallium and oxygen atoms.^20 The theory of Cooper pairs does not seem adequate
to explain the superconductivity of the high-temperature superconductors, and the
relationship of their structures to their superconductivity is apparently not yet clear.
Blackstead and Dow have proposed a theory according to which the oxygen atom
layer is the superconducting layer, with the oxygens in an undercharged state and com-
pressed into a quasi-metallic state.^21 A convenient website about superconductivity is
(^14) H. Kamerlingh Onnes,Akad. van Wetenschappen (Amsterdam), 14 , 113 (1911).
(^15) E. Bustarret,et al.,Nature, 444 , 465 (2006).
(^16) J. Bardeen, L. N. Cooper, and J. R. Schrieffer,Phys. Rev., 108 , 1175 (1957). This theory is known
as the BCS theory.
(^17) A. M. Thayer,Chem. Eng. News, 67 (48), 9 (Nov. 27, 1989).
(^18) See the website http://www.superconductors.org/.
(^19) R. Dagani,Chem. Eng. News, 65 (19), 7 (May 11, 1987).
(^20) S. S. P. Parkin,Phys. Rev. Letters, 61 , 750 (1988).
(^21) H.A. Blackstead and J.D. Dow,Phys. Rev. B, 51 , 11830 (1995).