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QUANTUM LEVITATION (Superconductors)Presented By : ABHIK DEBNATH 15PH40002

Definitions

Concepts associated with Superconductors

Description of Quantum locking

Applications and Future of Superconductors

OUTLINE

IT ALL STARTS WITH SUPERCONDUCTIVITY!Superconductivityis a phenomenon of exactly zeroelectrical resistanceand expulsion ofmagnetic flux fieldsoccurring in certain materials whencooledbelow a characteristiccritical temperature. It was discovered by Dutch physicistH.K. Onneson April8, 1911. Likeferromagnetismandatomic spectral lines, superconductivity is aquantum mechanicalphenomenon. It is characterized by theMeissner effect, the complete ejection ofmagnetic field linesfrom the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as theidealizationofperfect conductivityinclassical physics.

What is a Superconductor made of?Almost any material, if cooled enough, can be made into a superconductor. Even many materials which are insulators at room temperature can be superconductors when cooled to extremely low temperatures.Elements : Al, Sn, Hg, PbAlloys : Mercury or Yttrium basedOrganics : Carbon nanotubesCeramics : LBCO, YBCO, TBCCOResistance vs. absolute temperature curve by Onnes which marked the discovery of superconductivity.

Classification of SuperconductorsBy their magnetic properties:Type I superconductors: those having just onecritical field,Hc, and changing abruptly from one state to the other when it is reached.Type II superconductors: having two critical fields,Hc1andHc2, being a perfect superconductor under thelower critical field(Hc1) and leaving completely the superconducting state above theupper critical field(Hc2), being in a mixed state when between the critical fields.By their critical temperature:Low-temperature superconductors, or LTS: those whose critical temperature is below 30 K.High-temperature superconductors, or HTS: those whose critical temperature is above 30 K.Nowadays, 77 K is used as the split to emphasize whether or not we can cool the sample withliquid nitrogen(whoseboiling pointis 77K), which is much more feasible thanliquid helium(the alternative to achieve the temperatures needed to get low-temperature superconductors).

By material: Superconductor material classes includechemical elements(e.g.mercuryorlead),alloys(such asniobium-titanium,germanium-niobium, andniobium nitride),ceramics (YBCOandmagnesium diboride),superconducting pnictides(like fluorine-doped LaOFeAs) ororganic superconductors(fullerenesandcarbon nanotubes; though perhaps these examples should be included among the chemical elements, as they are composed entirely ofcarbon).

Expulsion of magnetic fields The Meissner Effect:

The magnetic properties of superconductors are as dramatic as their complete lack of resistance. In 1933, Hans Meissner and Robert Ochsenfeld studied the magnetic behaviour of superconductors and found that when certain ones are cooled below their critical temperatures, they have an interesting property of expelling a magnetic field. They discovered that a superconductor will not allow a magnetic field to penetrate its interior. It achieves this by producing a magnetic mirror surface currents which produce a magnetic field that exactly counters the external field. The phenomenon of the expulsion of magnetic fields from the interior of a superconductor is known as the Meissner effect.

A good comparison to electricity is that a good conductor expels static electric fields by moving charges to its surface. In effect, the surface charges produce an electric field that exactly cancels the externally applied field inside the conductor. In a similar manner, a superconductor expels magnetic fields by forming surface currents. At ordinary temperatures, these currents decay almost instantaneously due to the finite resistivity of the conductor. However, when cooling the superconductors below Tc, persistent surface currents are induced and produce a magnetic field that exactly cancels the externally applied field inside the superconductor.

Superconductor in the presence of an external magnetic field. (a) At temperatures above Tc, the field lines penetrate the sample because it is in its normal state. (b) When the rod is cooled to T