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

An attempt has been made to summarize a large body of data dealing with measurements of the energy gap in superconducting oxides like La_2–xSr_xCuO_4–y, YBa₂Cu₃O_7–y, and related compounds. A plot of available data of the energy gap 2 as a function of the superconducting transition temperatureT_crevealed a large dispersion of some data from 2/k_BT_c=3.53 for the weak coupling BCS prediction. Nevertheless, we could conclude 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.