The energy gap in the electron excitation spectrum of oxide superconductors
Analysis of results of investigations of the energy gap $\Delta$ in oxide high-$T_c$ superconductors by the methods of the tunnel effect, electron photoemission, IR absorption, Raman scattering of light, and others, indicates satisfactory agreement of results obtained by different methods. For all the oxide superconductors with $T_c\ge$ 30 K the ratio $2\Delta/kT_c$ = 5--6 is significantly greater than the value predicted by the BCS theory. The deviations of the experimental data near the edge and the bottom of the gap from the values that follow from the traditional model for the distribution in energy of the excitations are within the limits of accuracy of the measurements. The data obtained do not contradict the ideas concerning the electron-phonon mechanism of superconductivity in the oxide superconductors that have been investigated.