The polarized Raman spectra of underdoped YBa 2 Cu 3 O 6.5 (T c ϭ61 K͒ have been measured. The XY (B 2g ) spectrum exhibits a superconductivity-induced redistribution and a frequency dependence that suggests the presence of nodes near the ͑Ϯ1,Ϯ1͒ directions in k space. The XЈY Ј(B 1g ) spectrum is unaffected by the transition to the superconducting state and the intensity I(B 1g ) is significantly reduced relative to the level found in optimally doped compounds. This drop in intensity occurs

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... ause of a loss of spectral weight from the regions of the Brillouin zone near ͑,0͒, which is associated with the opening of a pseudogap in the underdoped compound.