Superconductivity: Discovery of Superconductivity. Explanation of terms: Superconductivity, Transition temperature and Meissner effect, Types.ppsx

Discovery of Superconductivity. Explanation of terms: Superconductivity, Transition temperature and Meissner effect. Different types of superconductors viz, conventional superconductors, organic superconductors, alkali metal fullerides and high temperature superconductors.

1. Superconductivity
Discovery of Superconductivity. Explanation of terms: Superconductivity, Transition temperature and
Meissner effect. Different types of superconductors viz, conventional superconductors, organic
superconductors, alkali metal fullerides and high temperature superconductors.

2. Discovery of Superconductivity
•Dutch scientists Holst and
Onnes at Leiden University
in 1911.
•Mercury at liquid helium
temperatures (4.2 K)
Heike Kamerlingh
Onnes
Gilles Holst
Dutch physicist Heike Kamerlingh Onnes was the first to prepare liquid helium on July
10, 1908

3. Electrical resistance in metals and superconductors
T(K)
R
(
m
Ω
)
M
etals
Superconductors
Electrical resistance increases with temperature in
metals

4. Electrical resistance in Metals
Electrical resistance increases
with temperature in metals
because higher temperatures
cause the metal atoms to vibrate
more vigorously, leading to more
frequent collisions between the
moving electrons and the
vibrating atoms.
Phonons-electron interaction
High temperatures
Low temperatures

5. Superconductivity
Superconductivity is the
property of certain materials
to exhibit zero electrical
resistance.
10
20
30
40
20 40 60 80100 140
T(K)
R
(
m
Ω
)
Superconducting
state
Metallic
state
T
c
YBa
2
Cu
3
O
7

6. Transition Temperature (T
c
)
The transition temperature,
or critical temperature (Tc), of
a superconductor is the
temperature below which it
loses all electrical resistance
and exhibits superconductivity
10
20
30
40
20 40 60 80100 140
T(K)
R
(
m
Ω
)
Superconducting
state
Metallic
state
T
c

7. Insulators-Semiconductors-Conductors
Insulators have extremely high
resistance, effectively blocking
current. Semiconductors offer
moderate resistance that can be
controlled, and their resistance
decreases with rising
temperature, unlike conductors
T(K)
R
(
m
Ω
)
Insulators
S
e
m
ic
o
n
d
u
c
t
o
r
s
Conductors
Valance Band Valance Band Valance Band
Conduction Band
Conduction Band
Conduction Band
Band Gap > 5eVBand Gap <5 eVNo Band Gap

8. Meissner effect
Superconducting materials
exhibit ‘perfect diamagnetism’
and expel a magnetic field,
provided that the field is below
the critical field strength, Hc.
This was discovered by Meissner
and Ochsenfeld in 1933.
H
T>T
c T<T
c

9. Cooper pairs
Cooper pairs are two electrons bound together in a superconductor at low temperatures,
forming a composite boson that condenses into the material's lowest energy state, resulting
in zero electrical resistance. This pairing is mediated by lattice vibrations (phonons), where a
moving electron creates a temporary region of positive charge in the lattice that attracts a
second electron. As Cooper pairs behave as bosons, they can all occupy the same lowest
energy state.

10. Broad properties of superconductors
1.Zero electrical resistance – Below Tc, superconductors allow current to flow without any energy loss.
2.Meissner effect – Superconductors expel magnetic flux lines, showing perfect diamagnetism.
3.Critical temperature (Tc) – The temperature below which a material becomes superconducting.
4.Critical magnetic field (Hc) – The maximum magnetic field strength a superconductor can withstand
before losing superconductivity.
5.Critical current density (Jc) – The maximum current density a superconductor can carry without
becoming normal.
6.Energy gap – A small gap forms at the Fermi level due to Cooper pair formation, explaining zero
resistance.
7.Isotope effect – Tc varies inversely with the square root of the isotopic mass, showing phonon
involvement.
8.Persistent currents – Superconductors can maintain current indefinitely without any applied voltage.

11. Different types of
superconductors
1.Conventional
2.Organic
3.Alkali Metal Fullerides
4.High Temperature

12. Conventional Superconductors
•Metals & alloys (e.g., Pb, Hg, NbTi).
•Exhibit superconductivity at very low Tc (below
~30 K).
•Explained by BCS theory (Cooper pairs via
phonons).
The BCS (Bardeen-Cooper-Schrieffer) theory is the first successful microscopic theory of
superconductivity, proposed in 1957 by John Bardeen, Leon Cooper, and John Robert Schrieffer. It
explains how electrons in a material can overcome their mutual repulsion and form weakly bound
pairs at very low temperatures, allowing for the flow of electric current without resistance.
Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize in Physics in 1972.
Zinc – 0.85K
Mercury –
4.15K
Tin – 7.72K

13. Organic Superconductors
Carbon-based molecular crystals (e.g., (TMTSF)₂PF₆).
Weak van der Waals bonding, highly anisotropic.
Low Tc, but important for studying electron
correlations.
Anisotropy
Properties vary with direction.

14. Alkali Metal Fullerides
•Compounds with C₆₀
(buckminsterfullerene) doped by alkali
metals (e.g., K₃C₆₀, Rb₃C₆₀).
•Show superconductivity around 20–40 K.
•Combination of molecular orbitals &
electron-phonon interactions..

15. High Temperature
ali Metal Fullerides
•Mostly copper oxides (cuprates) and iron-based compounds.
•Tc above liquid nitrogen temperature (77 K), e.g., YBa₂Cu₃O₇
(~92 K).
•Mechanism not fully explained; beyond BCS theory.

16. Applications
•MRI machines (NbTi, conventional).
•NMR spectrometer
•Superconducting Quantum Interference
Devices (SQUIDs) -high sensitivity
magnetometers.
•Power cables, levitation, maglev trains
(HTSCs).