FAR INFRARED REFLECTION MEASUREMENT OF THE ENERGY GAP OF YBa_(2)Cu_(3)O_(9-δ)

The superconducting energy gap of YBa_(2)Cu_(3)O_(9-δ)(Tc=90K)has been measured by means of far infrared reflection spectra.The value of 2Δ/kTc is between 2.2 and 3.4.According to the analysis of the dielectric function derived from K-K relations,we suggest that the energy gap value would be less than 18.8meV(150cm-1).The temperature dependence of the energy gapΔ(T)is also obtained.