Manifestation of a pseudogap in the Raman spectra of underdoped YBa2Cu3O6.5?

Raman scattering experiments have been carried out on single crystals of un derdoped YBa2Cu3O6.5 and optimally doped YBa2Cu3O6.95. The B-2g spectra of the underdoped samples are comparable in strength to those from optimally d oped compounds while the B-1g continua in the underdoped samples are much w eaker than they are in optimally doped compounds. The loss of spectral weig ht in the B-1g channel in the underdoped compounds may be associated with t he presence of a normal state pseudogap which masks the transition to the s uperconducting state. There is however, no definitive indication in the spe ctra of either an onset temperature T*, or a feature that could be used to estimate an upper energy limit, or gap energy, for the spectral weight depl etion. Assuming weighted distributions for the degree of depletion a simple model is used to calculate a schematic representation of the spectra in th e B-1g and B-2g channels. The results imply that the observed spectral weig ht depletion is localized to Fermi surface regions near the (pi, 0) points of the Brillouin zone. (C) 1998 Elsevier Science Ltd. All rights reserved.