Super conductivity

Introduction Super conductivity was discovered by H.K.

Onnes in 1911. This new state was first first discovered in mercury when cooled below 4.2K.

Super conductivity is a phenomenon in which certain metals and alloys and ceramics conduct electricity without resistance when it is cooled below certain temperature called critical temperature(Tc).

Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon

Properties Electrical resistance:

Electrical resistance of a super conductor is very low and is the order of 10-7 ohm meter.

Effect of impurities:

When impurities are added, superconducting property is not lost but Tc value is lowered.

Isotope effect:

The Tc is inversely proportional to the square root of isotopic mass of single superconductor.

Magnetic field effect:

The minimum magnetic field required to destroy the superconducting state is called the critical magnetic field (Hc).

Hc=H0[1-(T/Tc )2]

Persistent current:

When current is made to flow through a superconducting ring which is at a temperature either equal to its Tc value or less than its Tc value. It was observed that the current was flowing through it without any potential deriving. It is called a persistent current.

Meissner effect:

The complete expulsion of all the magnetic field by a super conducting material is called meissner effect.

classification By their response to a magnetic field: they can be Type I, meaning they have a single

critical field, above which all superconductivity is lost; or they can be Type II, meaning they have two critical fields, between which they allow partial penetration of the magnetic field.

By the theory to explain them: they can be conventional (if they are explained by the BCS theory or its derivatives) or unconventional (if not).

By their critical temperature: they can be high temperature (generally considered if they reach the superconducting state by just cooling them with liquid nitrogen, that is, if Tc > 77 K), or low temperature (generally if they need other techniques to be cooled under their critical temperature).

By material: they can be chemical elements (as mercury or lead), alloys (as niobium-titanium or germanium-niobium or niobium nitride), ceramics (as YBCO or the magnesium diboride), or organic superconductors (as fullerenes or carbon nanotubes, though these examples technically might be included among the chemical elements as they are composed entirely of carbon).

High temperature superconductors Until 1986, physicists had believed that BCS theory forbade superconductivity at

temperatures above about 30 K. In that year, Bednorz and Müller discovered superconductivity in a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987).[2] It was soon found that replacing the lanthanum with yttrium (i.e., making YBCO) raised the critical temperature to 92 K.[30]

Since about 1993, the highest temperature superconductor was a ceramic material consisting of mercury, barium, calcium, copper and oxygen (HgBa2Ca2Cu3O8+δ) with Tc = 133–138 K.[34][35] The latter experiment (138 K) still awaits experimental confirmation, however.

In February 2008, an iron-based family of high-temperature superconductors was discovered.[36][37] Hideo Hosono, of the Tokyo Institute of Technology, and colleagues found lanthanum oxygen fluorine iron arsenide (LaO1-xFxFeAs), an oxypnictide that superconducts below 26 K. Replacing the lanthanum in LaO1−xFxFeAs with samarium leads to superconductors that work at 55 K.[38]

Applications Superconductors are used to build Josephson junctions which are the

building blocks of SQUIDs (superconducting quantum interference devices), the most sensitive magnetometers known. SQUIDs are used in scanning SQUID microscopes and magnetoencephalography. Series of Josephson devices are used to realize the SI volt. Depending on the particular mode of operation, a superconductor-insulator-superconductor Josephson junction can be used as a photon detector or as a mixer. The large resistance change at the transition from the normal- to the superconducting state is used to build thermometers in cryogenic micro-calorimeter photon detectors. The same effect is used in ultrasensitive bolometers made from superconducting materials.

Promising future applications include high-performance smart grid, electric power transmission, transformers, power storage devices, electric motors (e.g. for vehicle propulsion, as in vactrains or maglev trains), magnetic levitation devices, fault current limiters, nanoscopic materials such as buckyballs, nanotubes, composite materials and superconducting magnetic refrigeration. However, superconductivity is sensitive to moving magnetic fields so applications that use alternating current (e.g. transformers) will be more difficult to develop than those that rely upon direct current.

Magnetic levitation

Josephson effect, SQUID

CRYOTRON