In 1911, Dutch physicist Heike Kamerlingh Onnes discovered that the resistance of
mercury absolutely disappears at temperatures below about 4K. This phenomenon is called
superconductivity; correspondingly, materials which have this property would be called
superconductors. Because of this great discovery, in 1913, he won a Nobel Prize in physics for
his research in this area.
The technological development of superconductivity was hampered by the cost of
producing the extremely low temperatures required to achieve the effect. Until 1986, new
superconducting ceramic materials were discovered which have considerably higher critical
transition temperatures. Gradually, as more and more ...view middle of the document...
When the external magnetic field exceeds a critical value
), the superconductor returns to its normal state. Hc is the only critical field value that type-I
superconductors possess. Most superconducting metals and metalloids are type-I
superconductors except vanadium, niobium, and ruthenium.
Figure 1. Type-I superconductors: average magnetization versus external field value.
Except for the elements vanadium, technetium and niobium, the type-II category of
superconductors is comprised of metallic compounds and alloys, such as Nb
As shown in Figure 2, the superconductor is in the Meissner state with full expulsion of
magnetic flux from its interior when it is below a lower critical field H
, acting as a perfect
superconductor. For an applied field above an upper critical field H
, magnetic flux fully
penetrates the type-II material and returns it to its normal state. If the applied field lies between
, there is a partial penetration of flux into the sample leading to regions in the interior
which are superconducting and others which are in the normal state; this is often referred to as
the mixed state.
Figure 2. Type-II superconductors: average magnetization versus external field value.
3.1 Zero Electrical Direct Current Resistance
The phenomenon of superconductivity is of vast potential importance in technology
because superconductivity means that charge can flow through a superconductor without
losing its energy to thermal energy. This indicates that currents created by electromagnetic
induction in a superconducting circuit can be maintained for several years without loss; the
electrons making up the current require a force and a source of energy at start-up time but not
thereafter. This "continuous current" has been repeatedly observed in numerous experiments.
The normal metallic conductors, such as copper and silver, always show some resistance
at all temperature, no matter how high their qualities are. Zero resistance phenomenon of
normal conductors can only occur under the ideal condition that the movement of electrons is
absolutely unimpeded. However, zero resistance phenomenon of a superconductor is
substantially different. The phenomenon can actually occur below a certain temperature (the
critical temperature). Figure 3 shows the comparison between a normal conductor and a
Figure 3. The relationship between resistance and temperature.
The scientific theories surrounding superconductivity are still under review; many of
them are phenomenological, explaining only what can be observed of superconductors.
3.2 Meissner Effect
The Meissner effect was discovered by German physicists Walther Meissner and Robert
Ochsenfeld in 1933 by measuring the magnetic field distribution outside superconducting tin
and lead spheres.
Figure 4. Diagram of the Meissner effect.
The Meissner effect can be...