A presentaion about theory of Superconductivity.

1. A presentation about
SUPERCONDUCTIVITY
By
Aqeel A. Al-Taie
May, 2016

2. Outline:
• Definition of Superconductivity
• Discovery
• The Meissner Effect
• BCS Theory
• Properties of Superconductivity
• Applications of Superconductivity
• Conclusions

3. Definition of 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. Discovery
• Superconductivity was discovered on 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 suddenly disappeared. In
subsequent decades, superconductivity was
found in several other materials.

5. 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.

7. The Meissner Effect
The limit of external magnetic field strength at
which a superconductor can exclude the field is
known as the critical field strength, Hc.
In 1920 Meissner discovered not only did
superconductors exhibit zero resistance but also
spontaneous expel all magnetic flux when cooled
through the superconducting transition , that is
they are also perfect dimagnets .
We call this the Meissner effect .

8. Normal state Superconducting state
H H

9. 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.

10. 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 .

11. Formation of cooper pairs
Cooper pairs are formed by
an attractive force
between electrons from
the exchange of phonon.
The energy of phonon is
usually less than 0.1eV

12. • 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 obey Pauli 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. PROPERTIES :
Josephson effect
Specific heat
Super fluidity
Thermal conductivity
Isotope effect
Field intensity
Persistent current

14. 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
small ,high speed computers.

15. Specific heat
• A finite jump in the specific heat is observed at
the critical temperature.
• In a superconducting phase, the electron
resistance changes with the jump , while the
energy undergoes a continuous variation .
• When the substance is cooled its specific heat
typically decreases but at the critical temperature,
it increases suddenly.

16. 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 capillary 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.

17. Thermal conductivity
• In an ideal superconductor , there is a marked
drop in the thermal conductivity when
superconductivity sets in .
• In non ideal superconductors an increase in
thermal conductivity on becoming super
conducting has been observed in few specimens.

18. Isotope effect
• It has been observed that critical temperature
varies with isotopic mass.
• i.e., critical temperature is inversely
proportional to the square root of mass of the
isotope

19. Field intensity
• Removal of the superconductivity state does not
only occur by raising the temperature , but also
by subjecting a material to a magnetic field .
• The critical value of magnetic field for the
destruction 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,
Hc =H0 [ 1 – (T/Tc )2 ]

20. • The magnetic field which causes a superconductor to
become normal from superconducting state is not
necessarily 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

21. Persistent current
• Superconductors are having property to
maintain a current without application of
voltage .

22. Applications of Superconductivity
Some of them are:
- Magnetic levitation(Maglev)
- Magnetic Field Detection
- (SQUID)
- Power Transmission
- Superconducting Generators

23. Conlusions
• That superconductivity is one of the important
discoveries in the field of modern physics.
• The mentioned properties predicts that the
superconductivity can be classified into different
types.
• The advantage of superconductors is their
capability of carrying enormous amount of power
without loss under critical temperature, so
superconductors can save a lot of energy.

24. References
• Concepts of modern Physics- Arthur
Beiser.
• Introduction to superconductivity-Michael
Tinkham.
• www.superconductors.org
• www.chavaenergy.com

25. Thanks for Listening