16 Superconductivity
Literature
- Rickayzen,Theory of Superconductivity, Interscience Publishers, 1965.
- Buckel, Kleiner,Supraleitung - Grundlagen und Anwendungen, Wiley, 2007.
- Ibach, Lüth,Solid State Physics - An Introduction to Principles of Material Science, Springer,
2002. - Kittel,Introduction to Solid State Physics, Wiley, 2005.
- Lecture Notes byE. SchachingerandP. Hadley.
16.1 Introduction
Superconductivity is the phenomenon of unmeasurable low resistance below a critical (or transition)
temperatureTc. It is observed in a wide class of materials, however, the theoretical description includes
numerous subtleties. Today the phenomenon is (basically) well understood but the development of
appropriate theoretical models and the corresponding experimental inspections display many aspects
of the development of physics in the 20th century.
In 1911 KamerlinghOnnes, three years after he first liquefied helium, observed that the electrical
resistance of mercury drops to an unmeasurable low value below the critical temperature ofTc≈ 4. 2
K. In fact it is fundamentally impossible to show that the resistance is exactly zero, but it is possible
to give an upper limit. K.Onnesclassified the first superconducting state with an upper limit to
the resistance of 10 −^5 Ω. However, the more surprising fact was that the resistance did not increase
with an increase of impurities in the mercury sample. Moreover, the resistance drops discontinuously
within a temperature range of less than 0. 01 K, see Fig. 157.
An elementary property of superconductivity as observed by K.Onnesis the experimental fact,
that the superconducting state may be destroyed by placing the sample in a strong magnetic field.
Furthermore, superconductivity reappears when the field is removed and the field strength required
to destroy the superconducting state depends on the orientation of the specimen within the field.
Nowadays, many different materials are known to show such behavior. Amongst non transition metals,
such as Be or Al, the phenomenon is also observed in samples of transition metals, such as Nb, Mo or
Zn, and even, when first exposed to high pressure (transition to the metallic phase) and then cooled
to very low temperatures (transition to superconducting state), in semiconductors, such as Si, Ge,
Se, Te. Typical transition temperatures for such materials are in the range of liquefied helium (∼ 4
K). The transition temperature may depend on the crystal structure of the material, such as in Bi,
where different modifications show different transition temperatures, but the crystal structure is not
a necessary condition for superconductivity. Moreover, a wide range of alloys show superconducting
behavior. For instance, the alloy Nb 3 Al 0. 75 Ge 0. 25 was for a long period the material with the highest
yet observed transition temperature (Tc= 20. 3 K). Amongst the most fascinating materials today are
high temperature superconductors such as Y-Ba-Cu-O alloys with transition temperatures up to 100
K (e.g. YBa 2 Cu 3 O 7 −x) or more exotic alloys, such as (Sn 5 In)Ba 4 Ca 2 Cu 10 OxwithTc= 212K.