High Tc Superconductivity Theory
With the discovery of materials that went superconducting at temperatures above the theoretical limit imposed by BCS theory (see History), there was (and still is) much debate as to what the mechanism of superconduction in these compounds might be.
One explanation involves the use of holes within the superconductor -
many high Tc superconductors are compounds such as YBa2Cu3O7-x
(see Making Your Own Superconductors) or La(2-x)SrxCuO4,
where the metal ion (in these cases, copper) will be partially oxidised;
obviously, each metal ion cannot be physically partially oxidised, rather,
the lattice will be comprised of a ratio of Cu2+ to Cu3+
ions, depending on x.
Although the positive 'holes' are usually stabilised by surrounding counterions
(such as oxygen in the cases above), the highly charged ions will still
ideally want to reduce (e.g. Cu3+ to Cu2+), however,
they cannot gain an electron from neighbouring (Cu2+) ions,
as this does not solve the problem.
The theory is supported by the fact that Tc varies with the
amount of doping (x) - too many or too few holes destroys the effect.