Whenever, it comes to classifying the materials on the basis of their electrical properties, we usually classify them into three categories, i.e. Insulators, Semi-conductors and Conductors. But you know what, apart from these three basic categories, we also have an another type of materials known as superconductors.
As the name suggests, these are the materials which posses superconducting properties. Hence, superconductivity is a phenomenon in which the electrical resistivity suddenly drops to zero at its transition temperature ( temperature at which a normal conductor transforms to a super conductor ). Derivatives of niobium, stannum, mercury etc. and many more fall under this category of conductors.
Discovery of Superconductivity
Superconductivity was discovered by a Dutch physicist Heike Kamerlingh Onnes on April 8, 1911, in Leiden. By using liquid helium as a refrigerant, Onnes observed that the resistivity of mercury disappeared abruptly at a temperature of 4.19 K. Onnes disclosed his research in 1911, in a paper titled “On the Sudden Rate at Which the Resistance of Mercury Disappears.”
Onnes stated in that the “specific resistance” became thousands of times less in amount relative to the best conductor at ordinary temperature. Onnes later reversed the process and found that at 4.2 K, the resistance returned to the material. Initially, Onnes called the phenomenon “supraconductivity” and later, the term “superconductivity” was adopted.
Onnes was awarded the Nobel Prize in Physics in 1913 for his remarkable discovery of superconductivity.
Explanations for Superconductivity
After Onnes contribution, Great efforts were devoted to find out that how and why
superconductivity works. Many theories were put forward to explain this phenomenon, however the most accepted one till date is the BCS theory. BCS theory was the first microscopic theory of superconductivity which described superconductivity as a microscopic effect caused by a condensation of Cooper pairs into a boson-like state.
It was proposed by Bardeen, Cooper, and Schrieffer in 1957, for which they received the 1972 Nobel Prize in Physics.
Some Peculiar Properties of Superconductors
Time and again, superconductors have been observed to posses several peculiar
electrodynamic and thermodynamic properties. Superconductors possess almost zero resistance below their transition temperature, this means that electrons in case of superconductors are more ordered than in case of normal conductors. Since, entropy is the measure of randomness in a system, hence entropy of superconductors is less than the normal conductors. Talking about another important thermodynamic property, i.e. specific heat capacity, the specific heat capacity of superconductors is more than that of normal conductors, as more heat needs to be supplied to more ordered system to increase its temperature.
The electrodynamics of superconductors can be explained by London’s equations, where London’s first equation states that current density in case of superconductors is constant even at zero applied electric field, contrary to normal conductors where it is zero at zero electric field. London’s second equation is a mathematical representation of the Meissner’s effect, a phenomenon of utmost importance in field of superconductors.
The Meissner effect
The phenomenon of Meissner’s effect refers to the expulsion of magnetic field lines from a superconductor when it is placed in a magnetic field. This phenomenon was discovered by two German physicists Walther Meissner and Robert Ochsenfeld in 1933.This property of superconductors makes them analogous to diamagnetic substances.
We basically have two types of superconductors, i.e. type I and type II. Type I superconductors transform abruptly from their normal state to superconducting state and vice versa at the transition temperature, and hence show complete Meissner’s effect below their transition temperature. However, on the other hand,
type II superconductors do not show abrupt change, rather they first show partial Meissner’s effect in between two critical values of applied magnetic field and show complete Meissner’s effect only after showing partial effect. Meissner’s effect has wide practical applications, especially relating to levitation phenomenon. Even the fastest trains we have today are working on this principle.
Other Applications of Superconductors
MRI also uses superconducting metals to produce high magnetic field, as in case of
superconductors, high current can be introduced without generating much heat losses. Superconducting state of a metal can be easily destroyed by applying high magnetic field or high temperatures beyond the critical limit. Superconductors are also employed to form superconducting junctions, which lead to super currents which can flow for decades without any hindrance and without any external voltage supply. Only a slight voltage push is needed initially to start the process.
Till date, five Nobel Prizes in Physics have been awarded for research in superconductivity. Discovery of superconductivity has definitely opened new horizons of research in field of conductivity. Nowadays, focus has been slightly shifted from the available extremely low temperature to high temperature superconductors. This is because the extremely low temperatures are very difficult to achieve and moreover, more than difficult to maintain. Hence these days, researchers are working extensively to find high temperature Superconductors, so that the beneficial properties of these magical substances can be utilized to the fullest.