Download Type II Superconductor

SUPERCONDUCTIVITY
Definition:
Transition (or) Critical temperature (Tc):
The temperature at which the transition from normal state to
superconducting state takes place on cooling in the absence of magnetic field is called “Transition (or) Critical
temperature (Tc).”
General Properties:
1. The transition temperature is different for different substances.
2. For a chemically pure and structurally perfect material, the superconducting transition is very
sharp .For chemically impure and structurally imperfect material, the superconducting
transition is very broad.
3. The current in a superconducting ring persists for a long time.
4. Materials having high normal resistivity exhibit superconductivity.
5. Superconducting elements, in general, lie in the inner columns of the periodic table.
6. For elements in a given row in the periodic table, Tc versus Z2 gives straight line.
7. Transition metals having odd number of valence electrons are favourable to exhibit
superconductivity while metals having even number of valence electrons are infavourable.
8. Materials for which Zρ > 106 show superconductivity.(where Z = number of valence electrons
and ρ is resistivity.)
9. Superconductivity is found to occur in metallic elements in which the number of valence
electron lies between 2 and 8.
10. Ferromagnetic and antiferromagnetic materials are not superconductors.
11. In addition to the drop in electrical resistivity to zero when cooled to transition temperature
the following changes also occur:
i. The magnetic flux lines are expelled out of the superconductor. This
property is called as Meissner effect.
ii. There is a discontinuous change in specific heat.
iii. There are small changes in thermal conductivity and the volume of the
material.
Isotopic Effect:
The transition temperature is directly proportional to isotopic mass of a material.
TC α M- α (α = 0.5)
TCM- α = constant
Effect of Magnetic Field:
Meissner Effect:
When a weak magnetic field is applied to a superconducting specimen at a
temperature below transition temperature Tc, the magnetic flux lines are expelled out. This effect is
called “Meissner Effect.”
The magnetic induction inside the specimen is given by,
𝐵 = 𝜇0 (𝐻 + 𝑀) ----- (1)
Where H is the external magnetic field and M is the magnetization produced inside
the specimen.
According to the Meissner effect, the magnetic induction in superconductor is B = 0,
∴ 𝜇0 (𝐻 + 𝑀) = 𝑜
H = -M
Magnetic susceptibility can be expressed as,
𝝌=
𝑴
𝑯
= -1
Thus the material is perfectly diamagnetic.
Types of Superconductors:
Based on exhibiting Meissner effect superconductors can be classified as type
I and type II superconductors.
A. Type I (or) Soft Superconductors:
Superconductors exhibiting a complete Meissner
effect are called “type I (or) Soft Superconductors.”
When magnetic field is gradually increased from its initial value H < HC,
at HC the diamagnetism is abruptly disappear and the transition from superconducting state to
normal state is sharp as shown in figure.
Pure specimens of Al, Zn, Hg and Sn are some examples of type I
superconductors.
B. Type II (or) Hard Superconductors:
Superconductors which have two critical fields
are called “type II (or) Hard Superconductors.”
In type II superconductors up to lower critical
field HC1 the specimen is in a pure superconducting state and magnetic flux lines are rejected. When
the field is increased beyond HC1, the magnetic flux lines start penetrating through the specimen and
it is in mixed state between HC1 and HC2. Above upper critical field HC2, the specimen is in a normal
state. This means that the Meissner effect is incomplete in the region between HC1 and HC2. This
region is called as “Mixed state”. Type II superconductors are also called as hard Superconductors.
Zr and Nb are some examples for type II superconductors.
High TC Superconductors:
The superconductors whose transition temperature is greater than 35K are
called “High Tc Superconductors”. The five chemical systems of high TC superconductors so far
developed are,
1. BaPb1-xBixo3
2. La2-xMxCuO4-x (M = Ba, Sr)
3. Ba2MCu3O7-x (M = rare earth metals such as Gd, Eu etc.)
4. Ba2-xLa1+xCu3O
5. Bi2CaSr2Cu2O
Applications of Superconductors:
1. Electric generators:
Superconducting generators are very smaller in size and
weight when compared with conventional generators.
2. Low Loss transmission lines and transformers:
Since the resistance is almost zero at superconducting state, the
power loss during transmission is negligible.
3. Magnetic Levitation:
A superconducting material can be suspended in air
against the repulsive force from a permanent magnet. This is called “Magnetic Levitation” and used
in high speed transportation.
4. Generation Of High Magnetic Fields:
Superconducting materials are used for producing very
high magnetic fields of the order of 50T.
5. Fast Electrical Switching:
Superconducting material can change to normal state by
application of a magnetic field and this principle is used in developing a switching element.
6. Logic and Storage Functions in Computers:
Superconductors are used to perform logic and storage
functions in computers.
7. SQUIDS: ( Superconducting Quantum Interference Devices)
SQUIDS are used study small magnetic signals from the brain and
heart, SQUIDS magnetometers used to detect the paramagnetic response of liver.