Detailed studies on the mid-infrared-active phonon spectra 01 YBa&u-,M,)30i-b; M = CO, Fe and Zn for 0 Q 1: Q IO%, have been carried out from 30 K to mom temperature. The phonon mode near 580 cm-' softens at the supermnducting transition temperature (Tc), as reponed earlier. The new observation is a marked difference in the teniperature dependence of the phonon in Co. and Fe-doped compounds as opposed to Zndoped and undoped material. We observe thal the phonon begins to soften at temperatures well obove 7;-in some cases N ZT, (= 180 K)-in the CO-and Fe-doped YBazCuaOi-8, whilst no such behaviour is detected in Zndoped and undoped samples We a s u e that this precursor phonon softening above T, in COand Fewbslituted samples is due to superconducting Huctuations. Our proposal is that the transition-metal-ion substitution affect3 the diniom'onalily of the system and the ramifications of this a# crucial for explaining the observed differences between Ztilundoped YBa2Cu301-6 and ColFedoped compounds. Our observations cast doubt on the idea that m superconductors like YBazCuaOi-6 show a Kosterlitz-i%ouless behaviour. Instead, it appears tllat T, -0 K and fluctuations are reduced for the fully decoupled layea.
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