Figure 157: Resistance as a function of temperature for a mercury sample as investigated by K.Onnes.
The history of the theoretical investigation of superconductivity includes numerous aspects of 20th
century theoretical physics. It took approximately 50 years until the first microscopic consistent
explanation was presented byBardeen, CooperandSchrieffer, today known as the BCS theory of
superconductivity. This theory required the development of quantum mechanics (1925), the discovery
of quantum field theories in the late 1940s and finally the application of quantum field theoretical
methods to solid state environments in the early 1950s. It turned out that the superconducting state
is a quantum many particle effect of macroscopic size. Note that the effect of zero resistance is not
describable within the framework of one electron dynamics. In a real crystal finite conductivity is the
result of interactions of electrons (or holes) with crystal defects or phonons. Even if this interaction
could be suppressed, electron-electron scattering would still cause a finite conductivity. In contrast to
the simplified one electron picture, the BCS theory explains the superconducting state as a superfluid
ofCooperpairs, which are quasiparticles consisting of two electrons which interact through phonons.
The BCS theory was the basis for many different explanations of new phenomena in many particle
systems.
Today’s applications of superconductors range from superconducting magnets, used in NMR spectrom-
eters, mass spectrometers or particle accelerators, to SQUIDs, which exploit theJosephsoneffect.
Several researches were awarded theNobelprize of Physics for achievements in the field of super-
conductivity:OnnesonMatter at low temperature(1913),Bardeen,CooperandSchriefferon
Theory of superconductivity(1972),Esaki,GiaeverandJosephsononTunneling in superconduc-
tors(1973),BednorzandMülleronHigh-temperature superconductivity(1987) andAbrikosov,
GinzburgandLeggettonPioneering contributions to the theory of superconductors and superfluids
(2003).
It is the aim of this section to give a brief introduction into some aspects of superconductivity. This
short introductory subsection is followed by a review of the fundamental experimental observations
in the second subsection. Phenomenological descriptions, i.e. thermodynamic considerations, the