Optical study of the Zn-substitution effect on phonon anomalies and spin gap in underdoped YBa2Cu3Oy

Performing optical reflectivity measurements with the electric field vector parallel to the c-axis of Zn-substituted, oxygen deficient YBa₂Cu₃O_y single crystals, we find a strong Zn-substitution effect on the anomaly of the inplane oxygen bending mode at 320 cm^–1 and the new absorption band at 450 cm^–1, which are observed in underdoped, non-substituted crystals. Contrary to that, the low-frequency suppression of the conductivity is almost not affected. We conclude that the phonon anomaly and the absorption band are related to each other, but that the suppression of the conductivity is an independent phenomenon. Moreover, the present work could be strong evidence for a model where some phonons couple to antiferromagnetic fluctuations at the (, )-point leading to anomalous behaviour of these phonons when a spin gap opens.on leave from Institut für Technische Physik,Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe Germany