2 Ë 1 Fundamentals of superconductivity
upper critical fieldHc2, between which they allow partial penetration of the magnetic
field (mixed state).
In the past few decades, superconductivity has been described in detail in many
works. This chapter only briefly introduces the essential features that directly relate
to the theme of this book, high-temperature superconducting magnetic levitation
(Maglev), in order to provide the readers with some necessary background knowledge
for later chapters. For further details about superconductivity, related bibliographies
are listed at the end of this chapter.
1.2 Discovery of superconductivity
In 1908, Heike Kamerlingh Onnes found the way to liquefy helium and to reach
temperatures as low as 4 K. Superconductivity was discovered first in mercury by
Onnes in 1911, [1] in the University of Leiden, Netherlands. The experiment was
performed on October 26, 1911, with the historic plot showing that the resistance
dropped abruptly to zero when the material was cooled below its critical temperature
at 4.2 K. [7] A type I superconductor is a material that exhibits two features, namely
zero electrical resistance and perfect diamagnetism, when it is cooled below critical
temperatureTc. The feature of the zero electrical resistance is that the electrical
resistance of the material suddenly drops to zero as the temperature decreases. The
ratio of the resistances between normal state and the superconducting state was
measured to exceed 10^14. In 1933, Meissner and Ochsenfeld [8] found that when a
superconducting sphere was cooled below its transition temperature in an applied
magnetic field, it excluded the magnetic flux. This perfect diamagnetic phenomenon
is called the Meissner-Ochsenfeld effect.
In 1913, the superconductivity of lead was found at 7 K. In 1937, at Kharkov,
Shubnikov et al. [9] experimentally discovered the phenomenon of type II supercon-
ductivity in single-crystal, single-phase superconducting alloys. In 1941, the super-
conductivity of niobium nitride was found at 16 K. However, even though thousands
of superconductors were discovered for many years, the highest critical tempera-
ture remained below 20 K, generally defined as low-temperature superconductor
(LTSC). Among LTSCs, only Nb-Ti and Nb 3 Sn are deemed to have promising engi-
neering applications, but rather low operating temperatures (4.2 K, liquid helium)
are required. After the discovery of Nb 3 Ge (Tc= 23 .2 K) in 1973, the critical tem-
perature of LTSC did not increase for more than 10 years. Nearly 30 years later, in
2001, magnesium diboride (MgB 2 ) was discovered. which has a transition tempe-
rature of 39 K. The MgB 2 has the highest critical temperature among conventional
superconductors.
In 1986, Bednorz and Müller, [10] of IBM Research Laboratory near Zurich,
Switzerland, discovered superconductivity in a lanthanum-based cuprate perovskite
material (La 2 −xBaxCuO 4 ) with a transition temperature of 35 K.