Isotope shift of the 590-cm-1 Raman feature in underdoped Bi2Sr2CaCu2O8+d

Raman-scattering studies have been performed on underdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}.$ In single crystals underdoped by oxygen removal, a 590-${\mathrm{cm}}^{\ensuremath{-}1}$ peak is observed in the ${B}_{1g}$ spectrum. The feature is observed to soften in frequency by 3.8% with isotopic exchange of ${}^{16}\mathrm{O}$ by ${}^{18}\mathrm{O}.$ In contrast, the 590-${\mathrm{cm}}^{\ensuremath{-}1}$ peak is not observed in crystals underdoped by Y substitution which suggests that it is a vibrational mode activated by oxygen deficency. We have also found that underdoping leads to a depletion of low-energy spectral weight from regions of the Fermi surface located near the Brillouin-zone axes.