Superconductivity

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Superconductivity
Superconductivity is perhaps the most remarkable
physical property in the Universe
David Pines
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Discovery of Superconductivity
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Superconductors
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The breakthrough -1986
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Superconductivity in alloys and oxides
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Superconductor Types
The Type 1 category of superconductors is mainly
comprised of metals and metalloids that show some
conductivity at room temperature. They require
incredible cold to slow down molecular vibrations
sufficiently to facilitate unimpeded electron
flow in accordance with what is known as BCS
theory.
The Type 2 category of superconductors is
comprised of metallic compounds and alloys. The
recently-discovered superconducting "perovskites"
(metal-oxide ceramics that normally have a ratio
of 2 metal atoms to every 3 oxygen atoms) belong
to this Type 2 group. They achieve higher Tc's
than Type 1 superconductors by a mechanism that
is still not completely understood. Conventional
wisdom holds that it relates to the planar
layering within the crystalline structure
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Bardeen Cooper Schreiffer Theory
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Bardeen Cooper Schreiffer Theory
BCS theory requires
(a) low temperatures - to minimise the number of
random (thermal) phonons (ie those associated
with electron-ion interactions must dominate)
(b) a large density of electron states just
below EF (the electrons associated with these
states are those that are energetically suited
to form pairs)
(c) strong electron phonon coupling
BCS theory is an effective, all encompassing
microscopic theory of superconductivity from
which all of the experimentally observed results
emerge naturally
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Electron-Lattice Interactions

• Electron moving throughlattice exerts an
  attractiveforce on nearby ions.
• Causes a lattice deformation local
  concentration of charge.

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THE BCS THEORY OF SUPERCONDUCTIVITY
BCS Theory suggests that superconductors have
zero electrical resistance below their critical
temperatures because at such temperatures the
electrons pass unimpeded through the crystal
lattice and therefore lose no energy.  The theory
states that the supercurrent in a superconductor
is carried by many millions of bound electron
pairs, called Cooper pairs. 
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THE BCS THEORY
According to the theory, as one negatively
charged electron passes by positively charged
ions in the lattice of the superconductor, the
lattice distorts. This in turn causes phonons to
be emitted which forms a trough of positive
charges around the electron. Before the electron
passes by and before the lattice springs back to
its normal position, a second electron is drawn
into the trough. It is through this process that
two electrons, which should repel one another,
link up. The forces exerted by the phonons
overcome the electrons' natural repulsion. The
electron pairs are coherent with one another as
they pass through the conductor in unison. The
electrons are screened by the phonons and are
separated by some distance. When one of the
electrons that make up a Cooper pair and passes
close to an ion in the crystal lattice, the
attraction between the negative electron and the
positive ion cause a vibration to pass from ion
to ion until the other electron of the pair
absorbs the vibration. The net effect is that the
electron has emitted a phonon and the other
electron has absorbed the phonon. It is this
exchange that keeps the Cooper pairs together. It
is important to understand, however, that the
pairs are constantly breaking and reforming.
Because electrons are indistinguishable
particles, it is easier to think of them as
permanently paired.
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The Meissner Effect
So far everything we have discussed is equally
true for a perfect conductor as well as a
superconductor
In 1933 Meissner and Oschenfeld made a discovery
which distinguished between the two
A superconductor excludes all magnetic flux from
its interior
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Magnetic Levitation
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Levitation of a magnet above a superconductor
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Meissner e?ect
A superconductor in magnetic ?eld
Superconductor always expels the magnetic ?ux
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Superconductivity and magnetism are natural
enemies
When a superconducting material is cooled below
its critical temperature in the presence of an
applied magnetic field, it expels all magnetic
flux from its interior. Supercurrents induced by
the magnet flow through the superconductor and
produce a magnetic field that exactly cancels out
the magnets own field.  
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Principle of Maglev
The principal of a Magnet train is that floats on
a magnetic field and is propelled by a linear
induction motor. They follow guidance tracks with
magnets. These trains are often refered to as
Magnetically Levitated trains which is
abbreviated to MagLev. Although maglevs don't use
steel wheel on steel rail usually associated with
trains, the dictionary definition of a train is a
long line of vehicles travelling in the same
direction - it is a train.
A maglev train floats about 10mm above the
guidway on a magnetic field. It is propelled by
the guidway itself rather than an onboard engine
by changing magnetic fields. Once the train is
pulled into the next section the magnetism
switches so that the train is pulled on again.
The Electro-magnets run the length of the
guideway.
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Maglev Train
The train runs in a concrete guide way on sides
of which there are three systems of copper coils.
One system serves for the train levitation,
another one for the train propulsion, and the
third one for lateral stability in the guideway.
The left figure demonstrates the principle of the
train levitation. The superconducting coils on
the cars produce high magnetic field of about 5
Tesla. At sufficiently high speed (above 130
km/h) this field induces magnetic field in the
stable copper coils on the bed sides that is high
enough to keep the train safely above the bottom.
Below the critical speed the train is driven by a
conventional electrical motor and runs on rubber
wheels. Electric current passing through the
copper coils on the ground produce alternating
magnetic field that attracts the superconducting
magnets of the train and propells the train
forward.
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Maglev Train
The first MagLev train was developed in Japan in
1972 and Japan has been the leaders in levitated
transport since. In 1990, the Yamanashi MagLev
test line opened and has been operating ever
since. The test line is an 18.4 km stretch of
track that runs solely on the technology of
superconductors. The MagLev trains are much
safer, faster and environmentally friendly than
their traditional counterparts. Japan is leading
the way, continually investing more money into
the further research of levitated vehicles. The
MagLev trains that run on the Yamanashi test line
have been clocked at speeds up to 581 km h-1.
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