2. DISCOVERY:
• Superconductivity was discovered on
april 8,1911 by Heike Kamerlingh
Onnes , who was studying the
resistance of solid mercury at
cryogenic temperatures using the
recently-discovered liquid helium as
a refrigerant .
• At the temperature of 4.2 K, he
observed that the resistance abruptly
disappeared. In subsequent decades,
superconductivity was found in
several other materials.
3. WHAT IS SUPERCONDUCTIVITY ?
Superconductivity is a phenomenon occurring in
certain materials generally at very low
temperatures , characterized by exactly zero
electrical resistance and the exclusion of the
interior magnetic field .
In simple words For some materials, the
resistivity vanishes at some low temperature: they
become superconducting ,such materials are
called superconductors .
4. Critical temperature
The temperature at which electrical resistance is
zero is called the Critical temperature (Tc)
The cooling of the material can be achieved
using liquid nitrogen or liquid helium for even
more lower temperature.
5.
6. BCS THEORY
• The complete microscopic theory of
superconductivity was finally proposed in 1957
by Bardeen , Cooper , and Schrieffer . This BCS
theory explained the superconducting current as
a super fluid of Cooper pairs , pairs of electrons
interacting through the exchange of phonons.
For this work, the authors were awarded the
Nobel Prize in 1972.
7. BCS – COOPER PAIRS
• A Cooper pair is the name given to electrons that
are bound together in a certain manner first
described by Leon Cooper . In normal
superconductors, the attraction is due to the
electron interaction.
• The Cooper Pair state forms the basis of the BCS
theory of superconductivity developed by John
Bardeen , John Schrieffer and Leon Cooper for
which they shared the 1972 Nobel Prize .
8. Formation of cooper pairs
Cooper pairs are formed by an attractive
force between electrons from the
exchange of phonon.
(phonon in acoustics are analogus to
photons in electromagnetic)
The energy of phonon is usually less
than 0.1eV
9. • When a metal is cooled to the critical temperature,
electrons in the metal form Cooper Pairs.
• Cooper Pairs are electrons which exchange
phonons and become bound together
• Bound electrons behave like bosons. Their
wavefunctions don’t obeyPauli exclusion rule and
thus they can all occupy the same quantum state.
• The BCS theory of Superconductivity states that
bound photons have slightly lower
• energy, which prevents lattice collisions and thus
eliminates resistance.
• As long as kT < binding energy, then a current can
flow without dissipation
13. Meissner effect
• In 1920 meissner discovered not only did
superconductors exibit zero resistance but also
spontaneous expell all magnetic flux when
cooled through the superconducting transition ,
that is they are also perfect dimagnets .
• We call this the meissner effect .
14.
15. Josephson effect
• When two superconductors are joined by a thin
,insulating layer ,it is easier for the electron pairs
to pass from one superconductor to another
without resistance .
• This is called the josephson effect
• This effect has applications for superfast
electrical switches that can be used to make smal
,high speed computers.
16. Specific heat
• A finite jump in d specific heat is observed at the
critical temperature.
• In a superconducting phase, the electron
resistance changes with the jump , while the
energy undergoes a continous variation .
• When the substance is cooled its specific heat
typically decreases but at the critical
temperature, it increases suddenly
17. Super fluidity
• This phenomenon was first observed in Helium
at a temp below 2.17 K. helium at this temp flow
quite freely without any friction , through any
gaps and even small capillery tubes.
• Once it is set in motion it will keep on flowing
forever-if there are no external forces acting
upon it.
• Unlike all chemicals helium does not solidify
when cooled down near absolute zero.
18. Thermal conductivity
• In an ideal superconductor , there is a marked
drop in the thermal conductivity when
superconductivity sets in .
• In non ideal superconuctors an increase in
thermal conductivity on becoming super
conducting has been observed in few specimens
.
19. Isotope effect
• It has been observed that critical temperature
varies with isotopic mass.
• i.e critical temperature is inversly propertional
to the square root of mass of the isotope
20. Field intensity
• Removal of the superconductivity state does not
only occus by raising the temoerature , but also
by subjecting a material to a magnetic field .
• The critical value of magnetic field for the
distruction of superconductivity , Hc is function
of temperature , at T=Tc , Hc =0 .
• With only small deviations, the critical field Hc
varies with the temperature according to the
parabolic law .
21. H =H0 [ 1 – (T/TC )2 ]
Hc = the critical field at which material goes into super
conducting state
T= operating temperature in 0K
Tc = critical temperature in 0K
H0= Value of magnetic field develop in material at 0
0K
22. • The magnetic field which causes a superconductor to
become normal from superconducting state is not
neccesarily an external applied field , it may rise as a
result of electric current flow in the conductor
• In a long superconductor wire of radius r,the super
conductivity may be destroyed when a current I
exceeds a critical current value Ic, which at the
surface of wire produce a critical field H is given by Ic
= 2 *3.14*rHc
called Silsbee’s rule
24. Classification of superconductors is
based on :
I. Resonance to magnetic field
II. By theory of operation
III. By critical temperature
IV. By materials
25. I. Resonance to magnetic feild
a. Type l :it has single critical field above which
all the superconductivity is lost.
b. Type ll :it has two critical field between which
it allows partial penetration of magnetic field .
26. ll.By theory of operation
• It is conventional if it can be explained by BCS
theory or non – concventional otherwise .
27. lll. By critical temperature
• A superconductor is consider high temperatur
ewhen it reaches a superconducting state when
cool using liquid nitrogen(i.e at Tc = 77 K)
• Or low temperature if aggresive cooling
techniques are required .
29. Type l Superconductors Type ll Superconductors
• These are pure and soft metals
like lead and indium.
• The critical field of these
conductors are low
• Hence these materials are not
suitable for high field
applications.
• They are called soft
superconductors (or ideal)
• They have low melting point .
• These are alloy and hard
metals like lead and indium.
• The critical field of these
conductors are high
• Hence these materials are
suitable for high field
applications.
• They are called hard
superconductors
• They have high melting point .
30. Type l Superconductors Type ll Superconductors
• These materials obeys silsbee’s
rule
• These materials shows
meissner effect .the transition
from normal to
superconducting state is sharp.
• They have low value of Hc and
Tc .
• These materials breaks
silsbee’s rule
• These materials shows
incomplete meissner effect ,
has broad transition region.
• They have high value of Hc and
Tc .
35. Applications
Superconductors are used as a powerfull
electromagnets in following areas :
• Magnets used for nuclear fusion
• Magnets used for high energy physics .
• They are used in generators and motors .
• Magnitically leviated transportation
• Magnetic resonance imaging and other
application .
36. • Superconductors are used to build josephson
junction which are the building block of SQUIDs
( superconducting quantum interference device)
the most sensitive magnetometers known.
SQUIDs are used in scanning SQUID
microscope .
37. Superconductors can be used in application like
high performance smart grid , electric power
transmission transformers ,powr storage device
,electric motars , fault current relays ,nanotubes
composite materials etc .
