WHICH ELEMENTS ARE LOW-TEMPERATURE SUPERCONDUCTORS?
It would be easy to imagine that all superconductors were metals, and
that a large number of different metals would superconduct given a
sufficiently low temperature. However, this is not the case.
Two common low-temperature superconductors are lead and
mercury, although an alloy of tin
and niobium works
at higher temperatures than these. It is interesting to note that these
metals (except niobium) are in very close proximity within the periodic
table, and are relatively poor conductors at room temperature: indeed,
there are good reasons for this.
In order for Cooper pairs to form, the interactions between the lattice
structure and the valence electrons within the metallic bond must be
of the correct strength. If they are too weak, then it is difficult for
the electrons
to exert any force on the surrounding metal ions. Conversely, if they
are too strong, the electrons cannot move very easily within the metal.
The reason why metals close together in the periodic table will
sometimes form a group of superconductors is that the key to
determining the strength of the interactions is the ratio of valence
electrons donated to the delocalised cloud against the atomic mass of
the element. Needless to say, this correct ratio will repeat
periodically as the number of valence electrons and the atomic mass
increment through the table.
This is also the reason why good conductors at room temperature which
are close to these in the periodic table--for example; copper, silver,
platinum, and gold--do not become superconductors at low temperatures:
the interactions between the lattice and the valence electrons are
simply too weak.
