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Magnetization
• magnetization or magnetic
polarization is the vector field that
expresses the density of permanent
or induced magnetic dipole
moments in a magnetic material.
• Net magnetization results from the
response of a material to an
external magnetic field.
• Magnetization is not always uniform
within a body, but rather varies
between different points.
BCS THEORY
• BCS theory is the first microscopic theory of
superconductivity since its discovery in 1911.
It explains,
• The interaction of phonons and electrons
• Cooper pairs : A key conceptual element in this theory is the
pairing of electrons close to the Fermi level into Cooper
pairs through interaction with the crystal lattice.
• Superconducting Energy Gap
Phonon
• A phonon is a quantum mechanical description
of an elementary vibration motion in which
a lattice of atoms or molecules uniformly
oscillates at a single frequency.
• When an electron is advancing through a path
in between two sets of ions in the lattice.
• Electron attracts the nearby +ve charges and
the lattice get distorted.
• This distortion produce some density variation
in the crystal.
• It will propagate like a wave
Cooper Pairs
• Cooper pair is a pair of electrons (or fermions) bound together
at low temperatures in a certain manner first described in 1956
by American physicist Leon Cooper
• An electron in a metal normally behaves as a free particle. The
electron is repelled from other electrons due to their
negative charge, but it also attracts the positive ions that make up
the rigid lattice of the metal.
• This attraction distorts the ion lattice, moving the ions slightly
toward the electron, increasing the positive charge density of the
lattice in the vicinity.
Cooper Pairs
• This positive charge can attract other
electrons. At long distances, this
attraction between electrons due to the
displaced ions can overcome the
electrons' repulsion due to their
negative charge, and cause them to
pair up.
• The rigorous quantum mechanical
explanation shows that the effect is
due to electron–phonon interactions.
Superconducting Energy Gap
• Minimum energy required to break up cooper pairs
or
the energy required to separate the electrons apart is
called superconducting energy gap.
• When the temperature increases, energy gap decreases.
• When T = Tc , this energy gap become zero.
TYPES OF SUPERCONDUCTORS
TYPE 1 : Lead, Tin, Hg
TYPE 2: Niobium, Germanium
Josephson Junction
• A Josephson junction is made up of
two superconductors, separated by a non
superconducting layer so thin that
electrons can cross through the insulating
barrier.
• The flow of current between the
superconductors in the absence of an
applied voltage is called a Josephson
current.
• The movement of electrons across the
barrier is known as Josephson
tunneling.
SQUID
SQUID
• A superconducting quantum interference device
(SQUID) is a mechanism used to measure extremely
weak signals.
• Using a device called a Josephson junction, a SQUID can
detect a change of energy as much as 100 billion times
weaker than the electromagnetic energy that moves a
compass needle.
• SQUIDs have been used for a variety of testing purposes
that demand extreme sensitivity, including engineering,
medical, and geological equipment.
• Because they measure changes in a magnetic field with
such sensitivity, they do not have to come in contact
with a system that they are testing
SQUID
• Super current changes periodically with magnetic
flux.
• Junction region have much lower value than the
rest of the superconducting ring.
• When the current in the junction exceeds the
critical value, the junction become normal, then
the fluxons penetrate through the link.
• So the current falls to critical value and then the
link reverts to superconducting state.
• Then the junction act as a gate.
• It is prepared in such a way that it allows only
single fluxons.
SQUID
HTSC
• High-temperature superconductors are materials that
behave as superconductors at unusually high
temperatures.
• Whereas "ordinary" or metallic superconductors
usually have transition temperatures below 30 K
(−243.2 °C), and must be cooled using liquid helium in
order to achieve superconductivity.
• HTS have been observed with transition temperatures
as high as 138 K (−135 °C), and can be cooled to
superconductivity using liquid nitrogen.
• Eg: 1yttrium 2barium 3copper 7oxide, bismuth
strontium calcium copper oxide
• 1-2-3 super conductors: (no. atoms of metals 1:2:3)
Applications of superconductors
• medical
magnetic resonance imaging
biotechnical engineering
• electronics
SQUIDs
transistors
Josephson Junction devices
circuitry connections
particle accelerators
sensors
• Industrial separation
magnets
sensors and transducers
magnetic shielding
Power Generation Motors
Transmission
• Transportation:
Magnetically levitated vehicles
Marine propulsion