EVIDENCE FOR SUPERCONDUCTING FLUCTUATIONS AT 180-K - INFLUENCE OF ZN, FE AND CO DOPING ON THE PHONON RENORMALIZATION OF YBA2CU3O7-DELTA

Detailed studies on the mid-infrared-active phonon spectra of YBa2(Cu1-xMx)3O7-delta; M = Co, Fe and Zn for 0 less-than-or-equal-to x less-than-or-equal-to 10%, have been carried out from 30 K to room temperature. The phonon mode near 580 cm-1 softens at the superconducting transition temperature (T(c)), as reported earlier. The new observation is a marked difference in the temperature dependence of the phonon in Co- and Fe-doped compounds as opposed to Zn-doped and undoped material. We observe that the phonon begins to soften at temperatures well above T(c)-in some cases approximately 2T(c) (almost-equal-to 180 K)-in the Co- and Fe-doped YBa2Cu3O7-delta, whilst no such behaviour is detected in Zn-doped and undoped samples. We argue that this precursor phonon softening above T(c) in Co- and Fe-substituted samples is due to superconducting fluctuations. Our proposal is that the transition-metal-ion substitution affects the dimensionality of the system and the ramifications of this are crucial for explaining the observed differences between Zn/undoped YBa2Cu3O7-delta and Co/Fe-doped compounds. Our observations cast doubt on the idea that 2D superconductors like YBa2Cu3O7-delta show a Kosterlitz-Thouless behaviour. Instead, it appears that T(c) --> 0 K and fluctuations are reduced for the fully decoupled layers.