Post on 03-Apr-2018
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Superconductivity
o Zero resistivity
o Meissner effect
o Magnetic effects
o Type I & II
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Introduction
Zero electrical resistance
◦ Superconductors carry current without energy loss
Perfect diamagnetism
◦ Superconductors float (levitate) above magnetic fields
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History of superconductors
1911: Onnes finds that at 4.2K the
resistance of mercury suddenlydrops to zero. He called this effect
superconductivity and the
temperature at which this occurs,
critical temperature Tc
1933: Walter Meissner and Robert
Ochsenfeld discover that a
superconducting material repels a
magnetic field (Meissner effect)
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1957: First widely-accepted theory by John Bardeen, Leon
Cooper, and John Schrieffer (BCS theory)
1962: Brian D. Josephson predicts that electrical current
would flow between two superconducting materials - even
when they are separated by a non-superconductor or
insulator. “Josephson effect”.
1986: Alex Müller and Georg Bednorz created the first
superconducting cuprate: La2-xBaxCuO4 (Tc =30 K). Got
Nobel in 1987. “High Tc superconductivity”
1987: Discovery of YBa2Cu3O6+ (YBCO) a material thatsuperconducts at temperatures above the temperature of
liquid nitrogen - a commonly available coolant
The current world record Tc of 138 K is held by
Hg0.8Tl0.2Ba2Ca2Cu3O8.33
History contd.
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Perfect diamagnet & superconductor
Perfect diamagnet
If a conductor already had a
steady magnetic field through itand was then cooled through the
transition to a zero resistance
state, becoming a perfect
diamagnet, the magnetic field
would be expected to stay thesame.
Superconductor
Remarkably, the magnetic
behavior of a superconductor isdistinct from perfect
diamagnetism. It will actively
exclude any magnetic field
present when it makes the phase
change to the superconductingstate.
Two mutually independent properties defining SC are r = 0 and B
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Effect of magnetization
Superconductivity can be destroyed also by an
external magnetic field H c which is also called thecritical one
Phase
diagra
m
2
2( 0) 1
C C
C
T H H T
T
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There are two types of superconductors, Type I and Type
II, according to their behaviour in a magnetic field
Type I superconductors are pure metals and alloys
Type I
superconducting state
normal state
This transition is
abrupt
Types
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superconducting normal state is gradual
Type II
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Types I & II comparison
The Type II superconductors
have much higher critical
magnetic fields than Type I, but
for most of that field range they
are mixtures of normal and
superconducting.
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Thermodynamic properties
E n t r o p
y
T
Al
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BCS Theory (1957) deals with the behaviour of electrons in
superconducting materials at very low temperatures
Low temperatures minimize the vibrational energy of individual
atoms in the crystal lattice
An electron moving freely through
the material encounters lessimpedance due to vibrational
distortions of the lattice at low
temperatures
The Coulomb attraction betweenthe passing electron and the
positive ion distorts the crystal
structure
BCS Theory
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- -2 1
The region of increased positive charge density propagates
through the crystal as a quantized sound wave called a phonon
The passing electron has emitted a phonon
A second electron experiences a Coulomb attraction from the
increased region of positive charge density created by the first
electron
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Electrons are said to pair into Cooper pairs through interaction with
the crystal lattice (indicated by isotope effect where TC is different for
different isotopes)
Cooper pairs are formed by two electrons, which overcome their
Coulomb repulsion and experience an attraction through phonon
exchanges
The electrons in a Cooper Pair possess antiparallel spin, resulting in
a total spin of zero for the pair
Cooper Electron Pairs act like single particles (BOSONS)
Since the Cooper Pair has zero spin, the pair is not required to obey
the Pauli exclusion principle
Bosons are particles which have integer spin and their energy
distribution is described by Bose-Einstein statistics
BCS Theory contd.
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Cooper pairs condense into a highly ordered ground state
Condensation: At low temperatures, bosons collect into the same
energy state
The pairs retain this ordered structure while moving through the
crystal lattice
Each pair becomes locked into its position with others pairs, and asa result no random scattering of electron pairs may occur
Zero resistivity may be defined as the absence of electron
scattering; hence, the superconductor now demonstrates zero
resistivity
The binding energy of a Cooper pair at absolute zero is about 3KT C
As the temp rises the binding energy is reduced and goes to zero
when T=Tc
. Above T=Tc
a Cooper pair is not bound.
Fi di f BCS h
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Findings of BCS theory
The binding energy of Cooper pair gives arise to
energy gap of the order of 10-3 eV
Eg(T = 0) = 3.53 kT C
A li ti
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MRI Exploits the high
magnetic fields expelled
by superconducting wires
for medical applications
The wide applicability of
superconductors is due to
Since the superconducting coils are capable of producing very stable,
large magnetic field strengths, they generate high quality images.
Medical Industry
• Diamagnetism
• Zero resistance
• Higher current
Applications
T t ti I d t
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Superconductor coils create strong magnetic fields that produce
the effect of levitation by repulsion
As a result, high speeds of up to 500 miles per hour are
possible with only a small consumption of energy
Maglev trains hover above a magnetic field without any
contact with the tracks
Transportation Industry
El t i i d t
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High temperature superconductors (HTS) can be used in the
production of more cost effective motors and generators
HTS power cables can carry
two to ten times more power
in equally or smaller sized
cables
Electric power industry
Superconducting cyclotron
(MSU)
R f
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References
A. Beiser – “Concepts of Modern Physics”, 6 Ed., Tata
McGraw-Hill (New Delhi, 2003)
Charles Kittel – “Introduction to Solid State Physics”, 7
Ed., John Wiley and Sons (New York, 1996)
www.wikipedia.org
http://hyperphysics.phy-
astr.gsu.edu/hbase/hframe.html