Superconductivity / Superconductors
• When the electrical resistance of a substance
drops suddenly to zero on cooling it below a
certain temperature then this phenomenon is
known as superconductivity.
The substances showing this property are called superconductors
e.g. lead, gallium, silver, indium etc.
Properties:
1. Zero Electrical resistance
2. Perfect Diamagnetics
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Superconductivity
• The phenomenon of superconductivity was
discovered by Kammerling Onnes in 1911.
• Certain materials and alloys show a remarkable
behavior when they are cooled to a sufficiently
low temperature near absolute zero.
• But when the temperature reaches few degrees
above absolute zero, the electrical resistivity
falls sharply and become almost zero (i.e. electrical
conductivity becomes almost infinite). Now the substance
is said to be in superconducting state.
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• In 1911, Onnes discovered that at about 4 K, the
resistivity of metallic mercury (melting point = 234
K) decreased suddenly to essentially zero, rather
than continuing to decrease only slowly with
decreasing temperature as expected
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Applications of Superconductors
• Superconducting magnets
• Cryotron (The cryotron is a switch that operates
using superconductivity)
• Magnetic Laviation
• Power Transformers
• SQUIDS i.e. Superconducting
Quantum Interference devices
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critical temperature or transition
temperature (Tc)
• The temperature at which a material loses its
electrical resistance is called the critical
temperature or transition temperature.
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Critical magnetic field (Magnetic Property)
• When very strong magnetic field applied to
superconducting material, it disappears super
conducting property that magnetic field is
called as critical magnetic field.
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• The superconducting metals lie in the middle of
the periodic table. They are not good conductors
at room temperature as normal metals.
• The conductors, which are not conductors in
ordinary sense, also show the property of
superconductivity at low temperature. An alloy
may be superconductor even if it is composed of
two metals which are not individually
superconductors such as Bi-Pd.
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Temperature dependence of resistivity in
superconducting materials
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Persistence current
• Persistence current is one of the most important
properties of a superconductor.
• When a current of large magnitude is induced in a
superconducting ring, the current persisted in the ring
even after the removal of the field at the temperature
below the critical temperature, such a current flows
without reducing its strength is known is persistent current
According to Silsbee’s rule, for a superconducting wire, the
induced current to destroy the superconducting property is
given by,
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Classification of superconductors
• Superconductors can be classified into the
following two categories on the basis of their
magnetic behavior
1. Type-I or soft superconductors
2. Type-II or hard superconductors
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Type-I or soft superconductors
characteristics
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Type-I or soft superconductors
characteristics
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Type-II or hard superconductors
• These superconductors have the magnetization
curve as shown in fig.
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Type-II or hard superconductors
characteristics
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Type-II or hard superconductors
characteristics
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Type-II or hard superconductors
characteristics
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BCS THEORY (Quantum Theory)
• In 1957, Bardeen, Cooper and Schriffer
developed a microscopic quantum theory of
superconductivity which accounts for the most
of the properties exhibited by the
superconductors.
• In an ordinary metal the electrical resistance
arises due to collisions of conduction electrons
with the vibrating ions in the crystal lattice.
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Electron-phonon-electron interaction (formation of
Cooper Pair)
• Normally the force between the electrons is repulsive. In superconducting
force between the two electrons becomes attractive due to the formation of
Cooper pair.
• When a current flows through a superconductor and an electron i.e. negative
charge comes near the positive ion core of the lattice then the electron
experiences an attractive force. Now due to the interaction between electron
and ion core, the ion core is set into motion i.e. slightly displaced and as a
result, a distortion is produced in the lattice. This distortion now travels away
as a mechanical wave i.e. phonons.
• These waves are quantized in the same way as electromagnetic waves. In
electromagnetic radiation such quanta of energy is known as photon. Similarly
in lattice vibrations each quanta of energy is called as phonon.
• The distortion in the lattice causes an increase in the density of ions in the
region of distortion. The higher density of ions in the distorted region attracts
another electron. In this way a free electron exerts a small attractive force on
another electron through phonons which are quanta of lattice vibrational
energy. This process is called as electron-lattice-electron interaction through
a phonon field (mechanical waves). 21
Cooper Pair
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High temperature superconductors
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Few examples are
[BPBO]
[LBCO]
[YBCO]
[TBCO]
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Characteristics which change in
superconducting transitions
• The thermal conductivity of superconductors undergoes a
continuous change between two phases. This is usually lower in
superconducting state as shown in fig.
• At transition the specific heat does not vary linearly with
temperature but it is found to vary exponentially with
temperature.
• All thermoelectric effects disappear in the superconducting
state.
• The magnetic characteristics of superconducting state are
modified by the addition of impurities.
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