SUPERCONDUCTORS

Basic Conditions of a Superconductor

Superconductivity can be destroyed if a sufficiently strong magnetic field is applied. A metal in this state has very unique magnetic properties that are unlike those at normal temperatures. A superconductor is often referred to as the perfect diamagnetic.  Diamagnetic, ideally, are a class of materials that do not conserve magnetic flux, but expel it.  A superconductor is classified as a perfect diamagnetic because by all measurable standards the magnetic flux within the material is zero.

Electrons have a wave-like nature so an electron moving through a metal can be represented by a plane wave progressing in the same direction. A metal has a crystalline structure with the atoms lying on a repetitive lattice; a plane wave can pass through a perfectly periodic structure without being scattered into other directions. An electron is able to pass through a perfect crystal without any loss of momentum of its original direction. That is why it is important for superconductors to have very low impurities; any fault in the periodicity of the crystal will scatter the electron wave and introduce some resistance. This is called the residual resistance and it is independent of the temperature.

Thermal vibrations also increase the resistance so when the temperature is lowered, the thermal vibrations of the atoms decrease and so the electrons are less frequently scattered. In short, the resistance of a metal is dependent on the purity of a metal and its temperature: metals with few impurities reach a superconducting state at low temperatures.

The superconductivity state of a metal exists only in a certain range of temperature and field strength. As stated earlier, a large magnetic field can destroy a superconductor. The condition for the superconducting state to exist in the metal is that of some combination for the superconducting state to exist in the metal is that of some combination of the temperature and the magnetic field strength. The critical values of the magnetic field strength and temperature are related by the following equation:

where H(c) is the critical field strength at the temperature T, H (0) is the maximum critical field strength occurring at absolute zero, T(c) is the critical temperature – the highest temperature for superconductivity.
There are also other basic conditions for superconductors and they can be explained by BCS Theory.

BCS Theory

John Bardeen, Leon Cooper, and Robert Schreiffer proposed the first major microscopic theory in 1957, now known as the BCS theory .   Using this theory, one starts with the concept that superconductors, in a non-superconducting state, possess electrons that even under normal conditions are not susceptible to resistivity.  Under the condition of low temperature and sometimes high pressure, these electrons tend to pair together to form Cooper pairs, often referred to as superelectrons.  These superelectrons, therefore have a both a charge and mass that is twice that of a normal electron.  It is these superelectrons that possess the perfect conduction observed in superconductivity.  According to the BCS theory, as the temperature of certain materials decreases, the density of these pairs increases.  This in turn, will increase the amount of superelectrons ready to conduct a perfect current.