A review of the size of the energy gap in superconducting oxides

An attempt is made to summarize a large body of data dealing with measurements of the energy gap in superconducting oxides, such as La(2-x)Sr(x)CuO(4-y), YBa2Cu3O(7-y), and related compounds. A plot of available data of the energy gap 2 Delta as a function of the superconducting transition temperature Tc reveals a large 'dispersion' of some data from 2 Delta/kBTc = 3.53 for the weak coupling BCS prediction. Nevertheless, it can be concluded that the size of the energy gap in these exotic materials is within the weak coupling BCS prediction, in view of some controversial issues that could be responsible for the dispersion, such as the gap anisotropy as well as identification of the energy gap.