Superconductivity was discovered in 1911 by H.K. Onnes when mercury was cooled below 4.2K. Superconductors conduct electricity without resistance below a critical temperature called Tc. They expel all magnetic fields (Meissner effect) and allow persistent currents to flow without resistance. Superconductors are classified by their response to magnetic fields, theoretical explanations, critical temperatures, and materials. High temperature superconductors with Tc above 77K were discovered in 1986 and have Tc as high as 138K. Applications include SQUIDs, Josephson junctions, and future uses such as power grids, motors, and maglev trains.

2. 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

3. 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]

4.  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.

5. 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).

7. 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]

9. 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.

10.  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.

11. Magnetic levitation

12. Josephson effect, SQUID

13. CRYOTRON