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1. Zero electrical resistance
the first characteristic property of asuperconductor is its electrical resistance. The electricalresistance of the superconductor is ro below the transitiontemperature (7). It is the ‘quickest test’ to prove thesuperconductivity.This property of zero electrical resistance isknown as ‘defining property’ of a superconductor.
The minimum magnetic field to destroythe superconducting property any temperature is known as criticalmagnetic field (Hc).The critical magnetic field (Hc) depends upon thetemperature of the superconducting material.
where
H0 is critical magnetic field at absolute zero temperature1
Tc is superconducting trarsition temperature of the material.
T is the temperature below T of the superconducting material.
The application of very high electricalcurrent to a superconducting material destroys its superconductingproperty. Consider a wire made up of a superconductor as shown infig. Let ‘i’ be the current flowing through the wire.The flow ofhigh current induces a magnetic field. This induced magnetic field inthe conductor destroys the superconducting property. The criticalcurrent required to destroy the superconducting property is given by
Ic=2πrHc
When a superctnductor in the form of aring is placed in a magnetic field, then the current is induced in itby electromagnetic induction.Persistent current a steady currentwhich flows through a superconducting ring without any decrease inits strength as long as the material is in superconducting state iscalled persistent current.The current persists even after the removalof themagnetic field.
When a bulk sample of placed in auniform magnetic the magnetic lines of force penetratematerial.However, when cooled to superconducting state belowtransition temperature the magnetic flux lines are pushed out fromspecimen
By applying very high pressures, we canbring the of a material nearer to room temperature, i.e., T isdirectly proportional to pressure at very high pressures.
• Entropy and specific heat decreasesat transition temperature.
• Thermo-electric effect disappearsin the super conducting state.
• There is no change in the crystalstructure in the superconducting state as revealed by X-raydiffraction studies. This suggests that superconductivity is moreconcerned with conduction electrons than with the atoms themselves.
• There is no change in elastic andphoto-electric properties.
• In the absence of magnetic field,there is no change in volume at the transition temperature
11. Frequency
At very high frequencies, the zeroresistarce of a superconductor is modified. The transitiontemperature is not affected by the frequency variation.
The general properties especially themagnetic property of superconducting state are modified by theaddition of impurities.
If the size of the specimen is reducedbelow 10^-4 cm, the properties of superconducting state are
14. Josephson effect
DC Josephson effect : The tunneling ofsuperconducting electron pairs through Josephson junction leads tothe flow of current without a voltage drop. This phenomenon is knownas ‘DC Josephson effect’.
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2. Describe the BCS theory ofsuperconductmg.
(A. U Tirunelveli May 2009)
Consider the model in fig. (a), inwhich two electrons propagate along a single lattice row. Eachelectron experiences an attraction towards its nearest positive ion.When the electrons get very close to each other in the region betweenions, they repel each other due to their mutual coulomb force.In anequilibrium condition, a balance between attraction and repulsion isestablihed and the two electrons combine to form cooper pair. ‘l’hecollection Of such coopel pair (hosons) in a bulk sample condenses toform super conducting state.
In order to explain the zeroresistivity exhibited by the superconductors, consider one of theelectrons of
the cooper pair propagating through thelattice as shown in fig. (b). The coulomb attraction between theelectron and ions deforms the lattice which is propagated along withthe electron. This propagating wave is associated with phonontransmission and the electron-phonon resonance allows the electronalong with its pair elsewhere in the lattice to movt withoutresistance
Predictions of UCS Theory
observed phenonmenon of isotope effectand the variation
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(ii) BCS theory explains the existenceof an energy
gap between the ground state(superconducting state)
and first excited state. The energy gaprepresents the
energy required to break a cooper pair.Hence, larger
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energy gaps correspond to more stablesuperconductoT’S