OXYGEN-ISOTOPE EFFECT ON THE VIBRATIONAL-MODES OF BI2SR2CACU2O8+DELTA

The O-16-O-18 oxygen isotope effect has been studied in the Raman spec tra of single crystals of the high-T-c superconductor Bi2Sr2CaCu2O8+de lta. It is found that the oxygen isotope shifts of all of the vibratio nal peaks fall into one of three narrow ranges. The absence of a signi ficant isotope shift for certain peaks indicates either that they corr espond to normal modes that involve only the motion of cations (peaks at and below 181 cm(-1)), or that they involve the vibration of oxygen on sites where isotope exchange proceeds extremely slowly (peaks at 3 10 cm(-1) and 661 cm(-1)). The isotope-shift data provide strong evide nce that, contrary to previous assignments, the A(1g) peaks at 287 cm( -1) and 350 cm(-1) correspond to vibrational modes of oxygen atoms in the copper layer. Large oxygen isotope shifts support the assignment o f the 386 cm(-1) peak to in-plane vibrations of oxygen atoms in the st rontium layer, and of the 458 cm(-1) peak to c-axis vibrations of thes e same atoms. The!! are inconsistent with an earlier suggestion that t he 458 cm(-1) peak corresponds to a vibrational mode of oxygen atoms i n the copper layer.