High T_c 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 T_c superconductors are compounds such as YBa₂Cu₃O_7-x (see Making Your Own Superconductors) or La_(2-x)Sr_xCuO₄, 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.
This means that there are 'holes' of positive charge (Cu³⁺ ions) within the lattice. This type of superconductor is hence referred to as a p-type superconductor; Compounds can also be doped to insert extra electrons into the lattice (i.e. a reduction), e.g. La₂CuO_(4+x) - this is called an n-type superconductor.

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.
When a current is applied to the superconductor, the electrons travel along the ion planes in the lattice. As an electron passes a hole in a neighbouring plane, it will push negative charge from orbitals on a reduced cation towards the hole (due to electrostatic repulsion), by distorting the lattice.
The oxidised cation (Cu³⁺) then reduces, and the reduced ion (Cu²⁺) oxidises - effectively, the hole moves backwards (as an electron moves forwards). This 'extra' current that is caused by the normal current is the supercurrent.

The theory is supported by the fact that T_c varies with the amount of doping (x) - too many or too few holes destroys the effect.
You can see the copper planes in YBa₂Cu₃O_7-x in the VRML strucutre included in the Uses section (on the Making Your Own Superconductors page).