Superconductors and Superconductivity :: physics science superconductor

Before Kamerlign Onnes, in 1908, was able to liquefy helium and bring its temperature down to about 1K, it had been known that the resistance of a metal falls when cooled below room temperature. However, it was not known what value the resistance would approach if the temperature was reduced towards 0K until Onnes, while experimenting with platinum, discovered that, its resistance fell when cooled to a very low value that depended on the metal’s purity. As the temperature of mercury was reduced toward 0K, the value of the resistance would fall smoothly until the resistance fell extremely suddenly at about 4K. Below 4K, mercury passed into a new state with electrical properties unlike those previously known: this new state that mercury had entered was called the â€Å"superconducting state.† Superconductivity can be destroyed if a sufficiently strong magnetic field is applied. A metal in this state has very unique magnetic properties that are unlike those at normal temperatures. A superconductor is often referred to as the perfect diamagnetic. Diamagnetic, ideally, are a class of materials that do not conserve magnetic flux, but expel it. A superconductor is classified as a perfect diamagnetic because by all measurable standards the magnetic flux within the material is zero. Electrons have a wave-like nature so an electron moving through a metal can be represented by a plane wave progressing in the same direction. A metal has a crystalline structure with the atoms lying on a repetitive lattice; a plane wave can pass through a perfectly periodic structure without being scattered into other directions. An electron is able to pass through a perfect crystal without any loss of momentum of its original direction. That is why it is important for superconductors to have very low impurities; any fault in the periodicity of the crystal will scatter the electron wave and introduce some resistance. This is called the residual resistance and it is independent of the temperature. Thermal vibrations also increase the resistance so when the temperature is lowered, the thermal vibrations of the atoms decrease and so the electrons are less frequently scattered. In short, the resistance of a metal is dependent on the purity of a metal and its temperature: metals with few impurities reach a superconducting state at low temperatures. The superconductivity state of a metal exists only in a certain range of temperature and field strength.