Abstract
We have measured the effect of oxygen isotope substitution on the -plane reflectance of underdoped . The frequency shift of the transverse optic phonons due to the substitution of by yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with . The reflectance shoulder at shows a much smaller exponent of in the normal state and in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recently.
- Received 31 October 2001
DOI:https://doi.org/10.1103/PhysRevLett.89.087003
©2002 American Physical Society