Aswathy A K

VAST, Thrissur
Superconductivity was discovered by the Dutch physicist
Kammerlingh Onnes in 1911 with mercury. While he was studying
the properties of mercury at low temperatures he observed that as
purified mercury is cooled, its resistivity vanished at 4.2K.
Above this temperature the resistivity is small but finite while the
below this point resistivity is so small that it is zero. Hg is converted
into a super conductor at 4.2K.

This phenomenon of complete disappearance of electrical

resistance below certain temperature is known superconductivity
and the temperature at which this transition takes place is called
transition temperature / Critical temperature (Tc).
(or)
The ability of certain substances to conduct electricity without any
resistance when they are cooled below a particular temperature is
called superconductivity.
Aswathy A K
VAST, Thrissur
Transition Temperature/ Critical temperature (Tc)
It is defined as the Temperature at which the resistivity of a material suddenly
falls to zero or it is the temperature at which normal conductor is converted
into a superconductor .

Superconducting transition is reversible also. When a sample is heated, its

normal resistivity is recovered at transition temperature.

The relation between temperature and resistivity for a superconductor is

shown in the figure

From this figure it is clear that a normal conductor has some resistivity at very
low temperature. But for superconductor, it suddenly falls to zero. Aswathy A K
VAST, Thrissur
PROPERTIES OF SUPERCONDUCTORS
1. Resistance is zero
2. Conductivity is infinite
3. Effect of current : when current is passed through a superconductor, it produces a
magnetic filed that changes it from SC state to normal state.
The maximum current that can the passed through a superconducting material without
destroying its superconducting property is called critical current Ic.

4. Applying stress or pressure ,changes Tc

5. Isotope effect:
The variation of transition temperature with isotopic mass is called isotope effect.

1
Tc ∝
𝑀

where M is the isotopic mass

6. Effect of magnetic field:
• The minimum magnetic field required to destroy superconductivity is called critical field Hc at a
temperature below Tc
When the applied field is below Hc – the material will be in superconducting state.
When the applied field exceeds Hc – the material gets converted to its normal state.

Below is the graph representing the variation of critical field Hc with temperature
As temperature increases the critical field decreases.
Hc falls to zero at Tc

H𝒄 (0)
MEISSNER EFFECT
 W. Meissner and Robert found that when certain substances are
cooled below their critical temperature in the presence of
magnetic field, the magnetic flux lines is expelled from the interior
of the specimen and becomes perfect diamagnet.

 That means when a normal conducting material is placed in a

magnetic field H, the lines of force penetrate through the material.
If the material is cooled below their critical temperature, the
magnetic lines of force are expelled from the material. This
phenomenon is called Meissner effect.
The complete expulsion of magnetic flux lines from the interior of a
superconductor during the transition from normal state to
superconducting state is called Meissner effect
Aswathy A K
VAST, Thrissur
Superconducting
material
-} M-e,,t ,(~ U- et(Je<l"
°" tvtaiuJ
t, &evted ~w ~41 l%'k aJ
-t~e. ,f k, k ~A~l i ~ e ) ,·l ffdJ' {e._<i,
~elic ffet"1.- R:lw:> '°'1tfdl /:f;£ ~Uw~ t€J f&a

~~meu ·
1W IJ , / g ::; o,
~et"l e-c«)I, dUtlnl~ {g F C/./-rM)j - (!)
~e, {w f€J- tvJ R.f),,},; '1 f {vu !fa v~ .
gra
l,.) 7

J-,{ --t .lk f't f 11,.e,k~• 0/11. x:,idu.f_ er) X ie Yl'J{'t,.

A~,, T .1.. 'c.-

1!> :::=- 0

,..) tq~ lit [j) ~ ~(, 't) l 1-1 <r M)

t-1+ M s:: D
M -H
•
. . J.,,{~d't MA, ~urt-A1ri'l1~ / x :: !i _1
tt -=-
,·. !1RA--~~du~ hertowt~ rdlut t i : ~ a1 ~12,
i~ke,~ -v -u. ·
- M6~net u Abl~J,~
w~ wllWAi {JJ ~ttJ-eJ I ~Q 1 ,. / tr
,lot&? &J~l
,,t-1( ~,e/1 ~ i i t ,•e, f-dd
!);yie,o f #l.~ft,<Qj~ ~cJ . .
 TYPES OF SUPERCONDUCTORS

 The superconductors are classified into two

according to their behaviour in the external
magnetic field .
 Type 1 superconductors Type 2 superconductors

Aswathy A K
VAST, Thrissur
APPLICATIONS OF SUPERCONDUCTIVITY
1. Electric motors and generators can be manufactured with superconducting windings.
Superconducting generators are very smaller in size and have very high efficiency compared
to conventional generators.
2. Superconductors can be used to produce very high magnetic fields of the order of 50T.
3. Superconductors can be used for electric power transmission.
4. Superconductors can be used in particle accelerators, cyclotrons etc.
5. Superconductors are used in Magnetic levitation (Maglev) trains. Such trains can flow over
the track with speed of 500 km/hr.
6. Superconductors are used for MRI (Magnetic Resonance Imaging ).
7. Superconductors are used in Superconducting Quantum Interference Device (SQUID). It is a
superconducting device used to detect very weak magnetic fields . It can be used as a
magnetometer, used to detect presence of submarines.
8. Superconductors are used to develop high capacity and high speed computer chips.
BCS THEORY
 Explanation for the occurrence of superconductivity was given by BCS theory
and it was put forward by three scientists named Bardeen, Cooper and
Schrieffer.
 According to BCS theory, the superconductivity is due to the interaction of
electrons and phonons. That means when an electron approaches a lattice of
positive ions, the electron is attracted by the neighbouring positive ions,and
it forms a positive ion core .
 Due to the attraction between the electron and the positive ion core, the
lattice gets deformed.

Aswathy A K
VAST, Thrissur
• When another electron passes by the side of the assembly of the
electron and the ion core, it gets attracted towards the assembly.
Thus a pair of electron is formed, called 'cooper pair‘
• This type of interaction is called electron phonon electron
interaction.
• At low temperature T<Tc, the attractive force between the to
electrons is greater than the repulsive force between them.
• At very low temperatures, pairs of electron will smoothly sail over
the lattice points without any collision.
• That is zero resistance and thus favours superconductivity.
HIGH TEMPERATURE
SUPERCONDUCTORS(HTSC)
 Superconductors can be grouped into low and high Tc
superconductors based on their transition temperature.
 Substances having Tc below 24K are considered as low Tc
superconductors .
 In low Tc , it is very difficult and expensive to maintain the low
temperature for a long period. So scientists made a lot of
research works to produce superconductors with high transition
temperature .
 Such with high transition temperature are called high
temperature superconductors. That has transition temperature
greater than 40K.
 Superconductor with transition temperature above 77K was
remarkable development

Aswathy A K
VAST, Thrissur
EXAMPLES OF HTSC
 The first high Tc superconductor was discovered in 1986
by IBM researchers Karl Muller and Johannes Bednorz for
which they were awarded Nobel Prize. They developed
La‐Ba‐CuO system of ceramic superconductor with a Tc of
34K
 In certain ceramic superconductors, the cell contains 1
atom of rare earth metal, 2 barium atoms, 3 copper
atoms and 7 oxygen atoms. Since the numbers of atoms
in the metal are in the ratio 1:2:3, such ceramic
superconductors are called 1‐2‐3 superconductors. The
transition temperature of 1‐2‐3 is 90K.

Aswathy A K
VAST, Thrissur
EXAMPLES OF HTSC

 Bi – Sr – Ca – Cu – O (BSCCO) with Tc = 107 K

 Tl – Ba – Ca – Cu – O (TBCCO) with Tc = 125K

Mercury Thallium Barium Calcium Copper Oxide

with Tc = 138K

Aswathy A K
VAST, Thrissur