Evidence for superconducting fluctuations at 180 K: influence of Zn, Fe and Co doping on the phonon renormalization of YBa2Cu3O7- delta

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Abstract
Detailed studies on the mid-infrared-active phonon spectra of YBa2(Cu1-xMx)3O7- delta ; M=Co, Fe and Zn for 0-1 softens at the superconducting transition temperature (Tc), 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. The authors observe that the phonon begins to soften at temperatures well above Tc-in some cases approximately 2Tc( approximately=180 K)-in the Co- and Fe-doped YBa2Cu3O7- delta , whilst no such behaviour is detected in Zn-doped and undoped samples. They argue that this precursor phonon softening above Tc in Co- and Fe-substituted samples is due to superconducting fluctuations. Their 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. Their observations cast doubt on the idea that 2D superconductors like YBa2Cu3O7- delta show a Kosterlitz-Thouless behaviour. Instead, it appears that Tc to 0 K and fluctuations are reduced for the fully decoupled layers.