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.
Hc
=
H0[
1
– (T/Tc)2
]
where
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
Ic=2πr
Hc
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
io. Stress
11. Frequency
At very high frequencies, the zero
resistarce of a superconductor is modified. The transition
temperature is not affected by the frequency variation.
12. Impurity
The general properties especially the
magnetic property of superconducting state are modified by the
addition of impurities.
13. Size
If the size of the specimen is reduced
below 10^-4 cm, the properties of superconducting state are
modified.
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.
(8 Marks)
(A. U Tirunelveli May 2009)
BCS Theory
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
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