Answer to Question #105250 in Electric Circuits for Bryan

Question

Super conductors use very low temperature in order to decrease the

Answer 1

Most metals and substances with metallic properties reduce their resistivity to electric current when the temperature decreases. But some of them at a certain temperature called the critical temperature $T_c$ lose it completely. The first observation of superconductivity was made by Kamerlingh Onnes in Leiden in 1911. In fig.1 we can see the one of the first observation the temperature dependence of the resistance of mercury as it becomes superconducting [1].

Quite soon after this discovery, it became clear that the phenomenon of superconductivity is a purely quantum state of an electron gas that is easily destroyed when the temperature increases. However, a detailed theory of the superconducting state was not developed for a long time. Only in 1957 It was proposed by Bardeen, Cooper, and Schrieffer; they received the Nobel Prize in Physics for this theory in 1972 [2].

The essence of this theory is that electrons in a metal can exchange sound quanta - phonons and due to this interaction form bound pairs. In this case, a pair of electrons moving in concert with each other can not lose the moment of motion and conducts a current without damping. As the temperature increases, the energy of phonons and the thermal electron energy increases and becomes greater than the energy of combining electrons in pair. Thus it occurs the destruction of superconductivity and the transition of the metal to a normal state with a finite resistance.

Recently, a large number of superconductors with a strong interaction between electrons have been discovered. These superconductors can maintain a vanishingly small resistance at a sufficiently high temperature ($T_c>100 K$). It is possible that connections with room critical temperature will be opened soon. Without a doubt, this will lead to a revolution in all our technology.

[1] Frank J. Owens, Charles P. Poole, Jr.,'The new superconductors', 2002, Kluwer Academic Publishers, New York.

[2] https://en.wikipedia.org/wiki/BCS_theory