Superconductivity is phenomena when materials are under certain conditions all of their electrical resistances disappear and perfect dia-magnetism by repelling magnetic fields occurs. Superconductor was named after super conductor since their electrical resistance is zero and electrical conducting property of the material is superior than that of other materials. The conditions to become superconductor are temperature, magnetic field and current. When a material that is not superconductor at room temperature changes to superconductor under certain conditions, it is called transition from normal state to superconducting state. Especially the highest temperature that a superconductor shows superconductivity without external magnetic field and current in thermodynamically superconducting state is called critical transition temperature. Superconducting materials have their own unique critical transition temperatures depending on materials. In general, low temperature superconductor (LTS) shows superconductivity below ~20K (-253℃, where k is abbreviation of Kelvin and means absolute temperature). The conditions that occur superconductivity phenomena are dependent on the 3 basic conditions which are temperature, strength of magnetic field and current density.

The most typical characteristic of superconductor is non of electrical resistance. Conductor is becoming a superconducting material that has the highest conductivity amongst conductors by completely disappearing all of its electrical resistances under certain conditions. In general when current is flowing in certain material Joule heat which is a product of current square and resistance is generated and thus energy loss is large. However much larger amount of current can be sent away without loss of energy in superconductor. For example, when current is flowing in ring typed superconductor the current is flowing forever without energy loss because of non of electrical resistance, as shown in Figure.

Another property of superconducting materials is the Meissner Effect. As a magnet is brought near a conductor, magnetic fields generated from a magnet approach to the conductor and the magnetic fields penetrate into the inside of a conductor. However a material became a superconductor under a certain temperature (critical temperature) the superconductor completely expels the magnetic field and the magnet encounters a repulsive force and is able to float on while maintaining a distance from the superconductor. At this time flux density (B) of the superconductor is zero and the superconductor behaves as a perfect diamagnet. This phenomenon is called Meissner Effect. When surrounding temperature is increased and over critical temperature, the superconductor is loosing superconducting phenomena (it is called 'quench' phenomena) and the magnet is no longer able to float on. This Meissner Effect is produced by making pole as opposed to external magnets, when current (shielding current) to offset the external magnetic field flows in the superconductor. This property has implications for making high speed, magnetically-levitated trains, and superconductivity bearing.

One other property of superconductors is that when two of them are joined by a thin, insulating layer, it is easier for the electron pairs (cooper pair) to pass from one superconductor to another without resistance . This is called the Josephson Effect. B.D. Josephson was prophetic and P.W. Anderson verified the effect experimentally. The electrons in semi-conductors and ordinary metals act independently. However the electrons joined by Josephson effect are moving in parallel and their shapes are described as a coherent wave. When DC flows through Josephson joins, DC flows without generating potential differences between superconductors despite the insulating layers up to current limit. It is called as DC Josephson effect. At this time, AC frequency is proportional to the DC voltage and an interference phenomenon occurs when external electromagnetic wave (microwave) is irradiated and DC current generates when AC frequency is equal to the electromagnetic wave frequency. This effect has implications for super fast electrical switches that can be used to make small, high-speed computers. Examples of applications are as following. Highly sensitive measuring devices such as magnetic fields, current and voltage by applying the effect that Josephson current is sensitive to external magnetic fields and detectors by applying the interference with AC Josephson effect and external electromagnetic waves. In addition, it can also be applied to manufacture calculating elements of computer. International voltage standard was changed to a method by applying the Josephson effect from a conventional standard voltage from January 1976.