Chapter 4
Superconductivity

What is superconductivity? Superconductivity is a phenomenon in which materials conduct electricity without any resistance.

Electrons
Electrons can detach itself from an atom and be independent of itself. An atom can only be stable if it has the same number of positive and negative charges. When an electron, which has a negative charge, detaches from an atom, the atom becomes unstable. Since there are more positive charges than negative charges, the atom becomes positive. The positive atom is now called an ion.

Lattices
If you would be able to look at a metal conductor with a very powerful microscope you would be able to see framework in neat patterns of ions. This is called a "lattice." In the lattice, the ions are jiggling and the electrons are moving in random directions. If you were to lower the temperature the movement of the ions and electrons will begin to slow down.

Resistance
If you were to apply pressure to a metal conductor of electricity the electrons would move in the direction pushed. The electrons will eventually hit the ions; making the electrons ricochet off of the ions.

When the electrons collide with the ions it creates heat. Heat is something we do not want in transferring electricity. Energy is wasted by the collision of electrons. This is called electrical resistance.

The Discovery of Superconductivity
To reduce energy lost we can lower the temperature because as the temperature is lowered the atoms and electrons slow down, reducing the chance of the electrons hitting the atoms.

In April 1911, Heike Kamerlingh Onnes was experimenting with the resistance of mercury at temperatures close to absolute zero. He noticed that not only did the electrical resistance begin to lessen, but at 4.15K mercury had lost all resistance. The temperature at which materials become superconductive is called the critical temperature. Common elements are classified as type I superconductors.

The table below shows the critical temperatures of some type I superconductors.

Critical Magnetic Field
Kamerlingh Onnes discovered that when elements are put in a magnetic field the critical temperature lowers. When a magnetic field is increased to a certain value, or critical magnetic field, superconductivity completely goes away. The critical magnetic field of mercury is 0.041 Teslas and the critical magnetic field of lead is 0.08 Teslas. (A Tesla is a unit if measurement to measure the strength of magnetic fields). For comparison, the magnetic critical field of the Earth is 0.00005 Teslas.

This discovery discouraged scientists because it is impossible to avoid magnetic fields. If a magnetic field is strong enough it can destroy a material’s superconductive state. This is called the critical current.

Type II Superconductors
The 1950s and 1960s was a time when scientists searched for superconductors with higher critical temperatures. A new group of superconductors were found which had the element Niobium in it. Superconductors that have more than one element in it is called intermetallic superconductors. Intermetallic superconductors are classified as type II superconductors.

The list below tells the critical temperature of some type II superconductors.

The Meissner Effect
In 1933 Walther Hans Meissner and Robert Ochsefeld discovered that when type I superconductors are lowered below their critical temperature all interior magnetic fields are gone. The expulsion of all magnetic fields from the interior to the exterior of the superconductor is called the Meissner effect. An example of the Meissner effect is the demonstration in which a magnet is repelled by a superconductor.

The BCS Theory
In 1957 John Bardeen, Leon Cooper, and John Robert Schrieffer explained why superconductivity loses all resistance, known as the BCS theory. For their findings they shared the Nobel Prize in physics in 1972.

The BCS theory states that single electrons do not carry an electric current, but paired electrons do. These pairs are called Cooper pairs.

How can two negative electrons attract to each other? The reason is because when an electron collides with a positive atom it deforms it resulting with a small concentration of positive charge on the electron. Eventually an electron with some positive charge on it is attracted to another electron, forming a Cooper pair. The Cooper pairs flow around impurities and imperfections in the lattice. This results in no electrical resistance.

"Superconductivity is perhaps the most remarkable physical property in the universe"
- David Pines, University of Illinois Urbana-Champaign Center for Advanced Study Professor of physics and electrical and computing engineering