Properties of Superconductors
1. Zero electrical resistance
the first characteristic property of a superconductor is its electrical resistance. The electrical resistance of the superconductor is ro below the transition temperature (7). It is the ‘quickest test’ to prove the superconductivity.This property of zero electrical resistance is known as ‘defining property’ of a superconductor.
2. Effect of magnetic field
The minimum magnetic field to destroy the superconducting property any temperature is known as critical magnetic field (Hc).The critical magnetic field (Hc) depends upon the temperature of the superconducting material.
H0 is critical magnetic field at absolute zero temperature1
(OK) of the material.
Tc is superconducting trarsition temperature of the material.
T is the temperature below T of the superconducting material.
3. Effect of electrical current
The application of very high electrical current to a superconducting material destroys its superconducting property. Consider a wire made up of a superconductor as shown in fig. Let ‘i’ be the current flowing through the wire.The flow of high current induces a magnetic field. This induced magnetic field in the conductor destroys the superconducting property. The critical current required to destroy the superconducting property is given by
4. Persistent Current :
When a superctnductor in the form of a ring is placed in a magnetic field, then the current is induced in it by electromagnetic induction.Persistent current a steady current which flows through a superconducting ring without any decrease in its strength as long as the material is in superconducting state is called persistent current.The current persists even after the removal of themagnetic field.
5. Meissner effect
When a bulk sample of placed in a uniform magnetic the magnetic lines of force penetrate material.However, when cooled to superconducting state below transition temperature the magnetic flux lines are pushed out from specimen
7.Effect of pressure
By applying very high pressures, we can bring the of a material nearer to room temperature, i.e., T is directly proportional to pressure at very high pressures.
8. Thermal properties
• Entropy and specific heat decreases at transition temperature.
• Thermo-electric effect disappears in the super conducting state.
9. General properties
• There is no change in the crystal structure in the superconducting state as revealed by X-ray diffraction studies. This suggests that superconductivity is more concerned with conduction electrons than with the atoms themselves.
• There is no change in elastic and photo-electric properties.
• In the absence of magnetic field, there is no change in volume at the transition temperature
At very high frequencies, the zero resistarce of a superconductor is modified. The transition temperature is not affected by the frequency variation.
The general properties especially the magnetic property of superconducting state are modified by the addition of impurities.
If the size of the specimen is reduced below 10^-4 cm, the properties of superconducting state are
14. Josephson effect
DC Josephson effect : The tunneling of superconducting electron pairs through Josephson junction leads to the flow of current without a voltage drop. This phenomenon is known as ‘DC Josephson effect’.
2. Describe the BCS theory of superconductmg.
(A. U Tirunelveli May 2009)
Consider the model in fig. (a), in which two electrons propagate along a single lattice row. Each electron experiences an attraction towards its nearest positive ion. When the electrons get very close to each other in the region between ions, they repel each other due to their mutual coulomb force.In an equilibrium condition, a balance between attraction and repulsion is establihed and the two electrons combine to form cooper pair. ‘l’he collection Of such coopel pair (hosons) in a bulk sample condenses to form super conducting state.
In order to explain the zero resistivity exhibited by the superconductors, consider one of the electrons of
the cooper pair propagating through the lattice as shown in fig. (b). The coulomb attraction between the electron and ions deforms the lattice which is propagated along with the electron. This propagating wave is associated with phonon transmission and the electron-phonon resonance allows the electron along with its pair elsewhere in the lattice to movt without resistance
Predictions of UCS Theory
(i) BCS theory could successfully predict the
observed phenonmenon of isotope effect and the variation
of critical magnetic field with temperate
(ii) BCS theory explains the existence of an energy
gap between the ground state (superconducting state)
and first excited state. The energy gap represents the
energy required to break a cooper pair. Hence, larger
energy gaps correspond to more stable superconductoT’S