Superconducting energy gap inMBa2Cu3O7−δ-type materials

We present a far-infrared study of ceramic superconductors of the $M{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ family with $M=\mathrm{Dy}$, ${\mathrm{Sm}}_{0.5}$ ${\mathrm{Ho}}_{0.5}$, and ${\mathrm{Sm}}_{0.5}$ ${\mathrm{Y}}_{0.5}$. Emphasis is placed on the determination of the position of the superconducting energy gap $E_g$ and its temperature dependence. We find that for all compositions studied $\frac{E_g}{k_B}{T}_c\approx 3.2\pm 0.3$. We also find indications of the temperature dependence of the gap and discuss the influence of the phonon features on the quantitative determination of the gap position.