Elements of Modern Physics
238
though not always, smaller than the one predicted by Eq. (7.105), e.g. 0.4 × 10
6
for Cr, 0.30 × 10
6
for Ni, etc. in MKS units
7.6 SUPERCONDUCTIVITY
Superconductivity is an interesting phenomenon in which electrons, which are
fermions, behave like bosons. The reason for this is that under some special
conditions, pairs of electrons form weakly-bound states which exhibit properties
of Bose systems.
When the temperature of some metals, semiconductors and alloys is lowered
to a few degrees kelvin, the electrical resistance of the material suddenly drops
to zero [see Fig. 7.9 (a)]. The substance is then said to have become a
superconductor and the temperature T c at which the transition takes place is
known as the critical or transition temperature, e.g. T c = 0.015 K for tungsten,
3.72 K for tin, 9.3 K for niobium, and the highest known value 23.2 K for the
Nb 3 Ge alloy. The transition to a superconducting state is quite sharp for a pure
and physically perfect specimen. In some cases it has been observed to occur
within a temperature range of 10
–5
K. However, for impure or physically imperfect
specimens, the transition may be over a range as large as 0.1 K or more.
Superconductivity has not been observed in all substances. In particular, it
has not been detected in alkali metals, ferromagnetic substances, and relatively
good conductors of electricity such as Ag, Cu, Au. Matthias has pointed out
that superconductivity occurs only in substances which have an average of two
to eight valence electrons per atom. Also, a small atomic volume is favourable
for superconductivity. It is worth noting that an alloy may be a superconductor
even if it is composed of two metals which themselves are not superconductors,
e.g. Bi-Pd.
Some of the important properties of superconductors are the following:
1. The current in the superconductors persists for a very long time. This is
demonstrated by placing a loop of the superconductor in a magnetic
field, lowering its temperature below T c and then removing the field.
The current which is set up is found to persist over a period longer than
two years without any attenuation.
2. The magnetic field does not penetrate into the body of the superconductor
(permeability µ = 0). This property, known as the Meissner effect, is the
fundamental characterization of superconductivity. However, when the
magnetic field B is greater than a critical value B c (T) [see Fig. 7.9 (b)],
the superconductor becomes a normal conductor [B c (T) is zero at T = T c
and has the largest value at T = 0].
238
though not always, smaller than the one predicted by Eq. (7.105), e.g. 0.4 × 10
6
for Cr, 0.30 × 10
6
for Ni, etc. in MKS units
7.6 SUPERCONDUCTIVITY
Superconductivity is an interesting phenomenon in which electrons, which are
fermions, behave like bosons. The reason for this is that under some special
conditions, pairs of electrons form weakly-bound states which exhibit properties
of Bose systems.
When the temperature of some metals, semiconductors and alloys is lowered
to a few degrees kelvin, the electrical resistance of the material suddenly drops
to zero [see Fig. 7.9 (a)]. The substance is then said to have become a
superconductor and the temperature T c at which the transition takes place is
known as the critical or transition temperature, e.g. T c = 0.015 K for tungsten,
3.72 K for tin, 9.3 K for niobium, and the highest known value 23.2 K for the
Nb 3 Ge alloy. The transition to a superconducting state is quite sharp for a pure
and physically perfect specimen. In some cases it has been observed to occur
within a temperature range of 10
–5
K. However, for impure or physically imperfect
specimens, the transition may be over a range as large as 0.1 K or more.
Superconductivity has not been observed in all substances. In particular, it
has not been detected in alkali metals, ferromagnetic substances, and relatively
good conductors of electricity such as Ag, Cu, Au. Matthias has pointed out
that superconductivity occurs only in substances which have an average of two
to eight valence electrons per atom. Also, a small atomic volume is favourable
for superconductivity. It is worth noting that an alloy may be a superconductor
even if it is composed of two metals which themselves are not superconductors,
e.g. Bi-Pd.
Some of the important properties of superconductors are the following:
1. The current in the superconductors persists for a very long time. This is
demonstrated by placing a loop of the superconductor in a magnetic
field, lowering its temperature below T c and then removing the field.
The current which is set up is found to persist over a period longer than
two years without any attenuation.
2. The magnetic field does not penetrate into the body of the superconductor
(permeability µ = 0). This property, known as the Meissner effect, is the
fundamental characterization of superconductivity. However, when the
magnetic field B is greater than a critical value B c (T) [see Fig. 7.9 (b)],
the superconductor becomes a normal conductor [B c (T) is zero at T = T c
and has the largest value at T = 0].