Raman investigation of the superconducting gap of YBa₂Cu₃O_x for 6.85<x<7.0

The temperature dependences of the linewidth and frequency of the 340-cm^-1 Raman-active phonon have been measured in five lightly twinned crystals of YBa₂Cu₃O_x with different oxygen concentrations x. The observed changes in the linewidth and frequency that occur for temperatures below the critical temperature have been found to depend strongly on the value of x within the region 6.85≲x≲7.0. The linewidth and frequency dependences for T<T_c result in superconducting gap energy estimates which indicate that 2Δ decreases for oxygen concentration x>6.95 and remains aproximately constant for 6.85≲x≲6.95. Given the low level of impurities in these crystals these results are attributed to variations in the hole concentrations which arise primarily from variations in the oxygen content.