30.20: Superconductor

A substance that reaches superconductivity, a state in which magnetic fields cannot penetrate, and there is no electrical resistance, is referred to as a superconductor. In 1911, Heike Kamerlingh Onnes of Leiden University, a Dutch physicist, observed a relation between the temperature and the resistance of the element mercury. The mercury sample was then cooled in liquid helium to study the linear dependence of resistance on temperature. It was observed that, as the temperature decreased, the resistance dropped. Kamerlingh Onnes continued to cool the mercury sample further, and as the temperature approached 4.2 K, the resistance abruptly went to zero. This is the critical temperature for mercury, where the mercury sample enters into a phase with absolute zero resistance, and this phenomenon is known as superconductivity. In the case of conductors, the resistance is not equal to zero but is less than 0.01 Ω. There are various methods to measure very small resistances, such as the four-point method, but an ohmmeter is not acceptable for testing superconductivity resistance.

Several other materials have been discovered in the superconducting phase when the temperature reaches near absolute zero. In 1941, an alloy of niobium-nitride was found to become superconducting at 16 kelvin; similarly, in 1953, vanadium-silicon was found to become superconductive at 17.5 kelvin. The temperatures for the transition into superconductivity were slowly creeping higher. Surprisingly, many good conductors, like copper, silver, and gold, do not exhibit superconductivity.

A superconductor is a material that offers zero resistance to the flow of electrons with an efficient transfer of electrical energy and no heat loss.

In 1911, Heike Kamerlingh Onnes observed that resistance goes to zero when mercury is cooled below 4.2 Kelvin. This is known as the critical temperature for mercury, and the phenomenon is known as superconductivity.

When the temperature increases above the critical temperature, the resistance changes linearly with it.

Another aspect of a superconductor's magnetic behavior is observed when a homogeneous superconducting material is placed in an external uniform magnetic field.

When the temperature is greater than the critical temperature, the field inside the material is equal to the external uniform magnetic field. If it is lower than the critical temperature, the magnetic flux is expelled from the material, and the field inside it is zero. This phenomenon is known as the Meissner effect.

The superconductivity of a material below the critical temperature can be eliminated by applying the minimum magnitude of the magnetic field, known as the critical magnetic field.