Quantum Statistics
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of the energy gap is the reason why superconductivity is a low-temperature
phenomenon.
The attenuation of a current implies a change in the state of conducting
electrons. Since the ground state is separated by the energy gap ∆, the excitation
of the Cooper pairs is not possible at low velocity [corresponding to currents
less than I c (T)]. This implies motion without friction and therefore,
superconductivity of the metal (or alloy).
Magnetic Properties
The magnetic properties of superconductors are quite complicated. In the class
of superconductors known as type I superconductors (which includes most of
the elemental superconductors), the magnetic field is excluded from the body
of the superconductors for B < B c (T) showing perfect Meissner effect. However,
the Meissner effect disappears for B > B c. (T).
For type II superconductors, an example of which is lead-indium alloy,
perfect Meissner effect occurs for B < B 1 (T), but only a partial exclusion of the
field for B 1 (T) < B < B 2 (T), and a complete penetration of the field for B > B 2
(T). The reason for this behaviour is that for B (T) between B 1 (T) and B 2 (T), the
material is in a mixed state. A close investigation of the specimen shows the
presence of small circular regions in the normal state, called vortices or fluxoids.
They are surrounded by large regions which are in the superconducting state. It
is the presence of both the states which gives rise to partial penetration of the
field. Materials with high critical temperatures tend to fall in the class of the
type II superconductors.
Since usually B 2 (T) >> B c (T), carefully-prepared type II superconductors
are used for the manufacture of high-field magnets which require almost no
power input and little cooling. Technology based on superconductors would
receive a major boost if superconductivity could be produced at higher
temperatures, say at liquid nitrogen temperature (T b = 77.4 K). This possibility
has been considered recently.
There are two additional properties which are of interest, quantization of
flux enclosed by a superconductor and Josephson junctions, which are discussed
briefly.
Quantization of Flux
Consider a superconducting loop in which a current is circulating. The current
generates a magnetic field whose flux across the area enclosed by the loop is
quantized.
241
of the energy gap is the reason why superconductivity is a low-temperature
phenomenon.
The attenuation of a current implies a change in the state of conducting
electrons. Since the ground state is separated by the energy gap ∆, the excitation
of the Cooper pairs is not possible at low velocity [corresponding to currents
less than I c (T)]. This implies motion without friction and therefore,
superconductivity of the metal (or alloy).
Magnetic Properties
The magnetic properties of superconductors are quite complicated. In the class
of superconductors known as type I superconductors (which includes most of
the elemental superconductors), the magnetic field is excluded from the body
of the superconductors for B < B c (T) showing perfect Meissner effect. However,
the Meissner effect disappears for B > B c. (T).
For type II superconductors, an example of which is lead-indium alloy,
perfect Meissner effect occurs for B < B 1 (T), but only a partial exclusion of the
field for B 1 (T) < B < B 2 (T), and a complete penetration of the field for B > B 2
(T). The reason for this behaviour is that for B (T) between B 1 (T) and B 2 (T), the
material is in a mixed state. A close investigation of the specimen shows the
presence of small circular regions in the normal state, called vortices or fluxoids.
They are surrounded by large regions which are in the superconducting state. It
is the presence of both the states which gives rise to partial penetration of the
field. Materials with high critical temperatures tend to fall in the class of the
type II superconductors.
Since usually B 2 (T) >> B c (T), carefully-prepared type II superconductors
are used for the manufacture of high-field magnets which require almost no
power input and little cooling. Technology based on superconductors would
receive a major boost if superconductivity could be produced at higher
temperatures, say at liquid nitrogen temperature (T b = 77.4 K). This possibility
has been considered recently.
There are two additional properties which are of interest, quantization of
flux enclosed by a superconductor and Josephson junctions, which are discussed
briefly.
Quantization of Flux
Consider a superconducting loop in which a current is circulating. The current
generates a magnetic field whose flux across the area enclosed by the loop is
quantized.