FAR INFRARED REFLECTION MEASUREMENT OF THE ENERGY GAP OF YBa₂Cu₃O_9-δ

The superconducting energy gap of YBa₂Cu₃O_9-δ(T_c=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 gap Δ(T) is also obtained.