Superconducting energy gap ofYBa2Cu3O9−δmeasured by far-infrared reflection

The superconducting energy gap of the high-$T_c$ superconductor $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$ ($T_c=90\mathrm{}$ K) has been measured by the far-infrared technique in the frequency range 30-150 ${\mathrm{cm}}^{-1\mathrm{}}$ and the temperature range 4.2-200 K. The frequency-dependent reflectances of two different samples show a common feature near ${\omega }_c\approx 125$ ${\mathrm{cm}}^{-1\mathrm{}}$ where the reflectance curves drop rapidly at very low temperature but are smooth at temperatures larger than 90 K. This indicates that the oxide superconductor $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$ has a superconducting energy gap $2{\Delta }_0=15.5\mathrm{}$ meV at very low temperatures and that the ratio $\frac{2{\Delta }_0}{k_B{T}_c}\approx 2.0$ is close to that of superconductors La-Ba-Cu-O and La-Sr-Cu-O, but smaller than the Bardeen-Cooper-Schrieffer theoretical value of 3.5. The temperature dependence of the superconducting energy gap $\Delta \mathrm{}\left(T\right)\mathrm{}$ is also obtained.